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INTERNET-DRAFT                                          Clifford Neuman
Obsoletes: 1510                                                 USC-ISI
                                                                 Tom Yu
                                                            Sam Hartman
                                                            Ken Raeburn
                                                                    MIT
                                                      February 15, 2004
                                                Expires 15 August, 2004

            The Kerberos Network Authentication Service (V5)

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe),
   ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

   The distribution of this memo is unlimited. It is filed as draft-
   ietf-krb-wg-kerberos-clarifications-05.txt, and expires 15 August
   2004.  Please send comments to: ietf-krb-wg@anl.gov

ABSTRACT

   This document provides an overview and specification of Version 5 of
   the Kerberos protocol, and updates RFC1510 to clarify aspects of the
   protocol and its intended use that require more detailed or clearer
   explanation than was provided in RFC1510. This document is intended
   to provide a detailed description of the protocol, suitable for
   implementation, together with descriptions of the appropriate use of
   protocol messages and fields within those messages.



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OVERVIEW

   This document describes the concepts and model upon which the
   Kerberos network authentication system is based. It also specifies
   Version 5 of the Kerberos protocol.  The motivations, goals,
   assumptions, and rationale behind most design decisions are treated
   cursorily; they are more fully described in a paper available in IEEE
   communications [NT94] and earlier in the Kerberos portion of the
   Athena Technical Plan [MNSS87].

   This document is not intended to describe Kerberos to the end user,
   system administrator, or application developer. Higher level papers
   describing Version 5 of the Kerberos system [NT94] and documenting
   version 4 [SNS88], are available elsewhere.

BACKGROUND

   The Kerberos model is based in part on Needham and Schroeder's
   trusted third-party authentication protocol [NS78] and on
   modifications suggested by Denning and Sacco [DS81]. The original
   design and implementation of Kerberos Versions 1 through 4 was the
   work of two former Project Athena staff members, Steve Miller of
   Digital Equipment Corporation and Clifford Neuman (now at the
   Information Sciences Institute of the University of Southern
   California), along with Jerome Saltzer, Technical Director of Project
   Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other
   members of Project Athena have also contributed to the work on
   Kerberos.

   Version 5 of the Kerberos protocol (described in this document) has
   evolved from Version 4 based on new requirements and desires for
   features not available in Version 4. The design of Version 5 of the
   Kerberos protocol was led by Clifford Neuman and John Kohl with much
   input from the community. The development of the MIT reference
   implementation was led at MIT by John Kohl and Theodore Ts'o, with
   help and contributed code from many others. Since RFC1510 was issued,
   extensions and revisions to the protocol have been proposed by many
   individuals. Some of these proposals are reflected in this document.
   Where such changes involved significant effort, the document cites
   the contribution of the proposer.

   Reference implementations of both version 4 and version 5 of Kerberos
   are publicly available and commercial implementations have been
   developed and are widely used. Details on the differences between
   Kerberos Versions 4 and 5 can be found in [KNT94].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.


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                            Table of Contents



1. Introduction ...................................................    7
1.1. Cross-realm operation ........................................    9
1.2. Choosing a principal with which to communicate ...............   10
1.3. Authorization ................................................   11
1.4. Extending Kerberos Without Breaking Interoperability .........   12
1.4.1. Compatibility with RFC 1510 ................................   12
1.4.2. Sending Extensible Messages ................................   13
1.5. Environmental assumptions ....................................   14
1.6. Glossary of terms ............................................   14
2. Ticket flag uses and requests ..................................   17
2.1. Initial, pre-authenticated, and hardware authenticated
      tickets .....................................................   18
2.2. Invalid tickets ..............................................   18
2.3. Renewable tickets ............................................   18
2.4. Postdated tickets ............................................   19
2.5. Proxiable and proxy tickets ..................................   20
2.6. Forwardable tickets ..........................................   21
2.7. Transited Policy Checking ....................................   21
2.8. OK as Delegate ...............................................   22
2.9. Other KDC options ............................................   23
2.9.1. Renewable-OK ...............................................   23
2.9.2. ENC-TKT-IN-SKEY ............................................   23
2.9.3. Passwordless Hardware Authentication .......................   23
3. Message Exchanges ..............................................   23
3.1. The Authentication Service Exchange ..........................   23
3.1.1. Generation of KRB_AS_REQ message ...........................   25
3.1.2. Receipt of KRB_AS_REQ message ..............................   25
3.1.3. Generation of KRB_AS_REP message ...........................   25
3.1.4. Generation of KRB_ERROR message ............................   28
3.1.5. Receipt of KRB_AS_REP message ..............................   28
3.1.6. Receipt of KRB_ERROR message ...............................   29
3.2. The Client/Server Authentication Exchange ....................   30
3.2.1. The KRB_AP_REQ message .....................................   30
3.2.2. Generation of a KRB_AP_REQ message .........................   30
3.2.3. Receipt of KRB_AP_REQ message ..............................   31
3.2.4. Generation of a KRB_AP_REP message .........................   33
3.2.5. Receipt of KRB_AP_REP message ..............................   33
3.2.6. Using the encryption key ...................................   34
3.3. The Ticket-Granting Service (TGS) Exchange ...................   34
3.3.1. Generation of KRB_TGS_REQ message ..........................   36
3.3.2. Receipt of KRB_TGS_REQ message .............................   37



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3.3.3. Generation of KRB_TGS_REP message ..........................   38
3.3.3.1. Checking for revoked tickets .............................   40
3.3.3.2. Encoding the transited field .............................   41
3.3.4. Receipt of KRB_TGS_REP message .............................   42
3.4. The KRB_SAFE Exchange ........................................   43
3.4.1. Generation of a KRB_SAFE message ...........................   43
3.4.2. Receipt of KRB_SAFE message ................................   43
3.5. The KRB_PRIV Exchange ........................................   44
3.5.1. Generation of a KRB_PRIV message ...........................   45
3.5.2. Receipt of KRB_PRIV message ................................   45
3.6. The KRB_CRED Exchange ........................................   46
3.6.1. Generation of a KRB_CRED message ...........................   46
3.6.2. Receipt of KRB_CRED message ................................   47
3.7. User-to-User Authentication Exchanges ........................   47
4. Encryption and Checksum Specifications .........................   49
5. Message Specifications .........................................   50
5.1. Specific Compatibility Notes on ASN.1 ........................   52
5.1.1. ASN.1 Distinguished Encoding Rules .........................   52
5.1.2. Optional Integer Fields ....................................   52
5.1.3. Empty SEQUENCE OF Types ....................................   52
5.1.4. Unrecognized Tag Numbers ...................................   53
5.1.5. Tag Numbers Greater Than 30 ................................   53
5.2. Basic Kerberos Types .........................................   53
5.2.1. KerberosString .............................................   53
5.2.2. Realm and PrincipalName ....................................   55
5.2.3. KerberosTime ...............................................   56
5.2.4. Constrained Integer types ..................................   56
5.2.5. HostAddress and HostAddresses ..............................   57
5.2.6. AuthorizationData ..........................................   57
5.2.6.1. IF-RELEVANT ..............................................   59
5.2.6.2. KDCIssued ................................................   59
5.2.6.3. AND-OR ...................................................   60
5.2.6.4. MANDATORY-FOR-KDC ........................................   60
5.2.7. PA-DATA ....................................................   61
5.2.7.1. PA-TGS-REQ ...............................................   62
5.2.7.2. Encrypted Timestamp Pre-authentication ...................   62
5.2.7.3. PA-PW-SALT ...............................................   62
5.2.7.4. PA-ETYPE-INFO ............................................   63
5.2.7.5. PA-ETYPE-INFO2 ...........................................   63
5.2.8. KerberosFlags ..............................................   64
5.2.9. Cryptosystem-related Types .................................   65
5.3. Tickets ......................................................   67
5.4. Specifications for the AS and TGS exchanges ..................   74
5.4.1. KRB_KDC_REQ definition .....................................   74
5.4.2. KRB_KDC_REP definition .....................................   82
5.5. Client/Server (CS) message specifications ....................   85
5.5.1. KRB_AP_REQ definition ......................................   85
5.5.2. KRB_AP_REP definition ......................................   89



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5.5.3. Error message reply ........................................   90
5.6. KRB_SAFE message specification ...............................   90
5.6.1. KRB_SAFE definition ........................................   90
5.7. KRB_PRIV message specification ...............................   92
5.7.1. KRB_PRIV definition ........................................   92
5.8. KRB_CRED message specification ...............................   92
5.8.1. KRB_CRED definition ........................................   93
5.9. Error message specification ..................................   95
5.9.1. KRB_ERROR definition .......................................   95
5.10. Application Tag Numbers .....................................   97
6. Naming Constraints .............................................   98
6.1. Realm Names ..................................................   98
6.2. Principal Names ..............................................   99
6.2.1. Name of server principals ..................................  101
7. Constants and other defined values .............................  101
7.1. Host address types ...........................................  101
7.2. KDC messaging - IP Transports ................................  103
7.2.1. UDP/IP transport ...........................................  103
7.2.2. TCP/IP transport ...........................................  103
7.2.3. KDC Discovery on IP Networks ...............................  104
7.2.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names .......  105
7.2.3.2. Specifying KDC Location information with DNS SRV
      records .....................................................  105
7.2.3.3. KDC Discovery for Domain Style Realm Names on IP
      Networks ....................................................  106
7.3. Name of the TGS ..............................................  106
7.4. OID arc for KerberosV5 .......................................  106
7.5. Protocol constants and associated values .....................  106
7.5.1. Key usage numbers ..........................................  107
7.5.2. PreAuthentication Data Types
      .............................................................  108
7.5.3. Address Types
      .............................................................  109
7.5.4. Authorization Data Types
      .............................................................  109
7.5.5. Transited Encoding Types
      .............................................................  109
7.5.6. Protocol Version Number
      .............................................................  110
7.5.7. Kerberos Message Types
      .............................................................  110
7.5.8. Name Types
      .............................................................  110
7.5.9. Error Codes
      .............................................................  110
8. Interoperability requirements ..................................  112
8.1. Specification 2 ..............................................  112
8.2. Recommended KDC values .......................................  115



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9. IANA considerations ............................................  115
10. Security Considerations .......................................  116
11. Author's Addresses ............................................  120
12. Acknowledgements ..............................................  121
13. REFERENCES ....................................................  122
13.1 NORMATIVE REFERENCES .........................................  122
13.2 INFORMATIVE REFERENCES .......................................  123
14. Copyright Statement ...........................................  124
15. Intellectual Property .........................................  125
A. ASN.1 module ...................................................  125
B. Changes since RFC-1510 .........................................  133
END NOTES .........................................................  136







































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1. Introduction

      Kerberos provides a means of verifying the identities of
      principals, (e.g. a workstation user or a network server) on an
      open (unprotected) network. This is accomplished without relying
      on assertions by the host operating system, without basing trust
      on host addresses, without requiring physical security of all the
      hosts on the network, and under the assumption that packets
      traveling along the network can be read, modified, and inserted at
      will[1]. Kerberos performs authentication under these conditions
      as a trusted third-party authentication service by using
      conventional (shared secret key [2]) cryptography. Kerberos
      extensions (outside the scope of this document) can provide for
      the use of public key cryptography during certain phases of the
      authentication protocol [@RFCE: if PKINIT advances concurrently
      include reference to the RFC here]. Such extensions support
      Kerberos authentication for users registered with public key
      certification authorities and provide certain benefits of public
      key cryptography in situations where they are needed.

      The basic Kerberos authentication process proceeds as follows: A
      client sends a request to the authentication server (AS)
      requesting "credentials" for a given server. The AS responds with
      these credentials, encrypted in the client's key. The credentials
      consist of a "ticket" for the server and a temporary encryption
      key (often called a "session key"). The client transmits the
      ticket (which contains the client's identity and a copy of the
      session key, all encrypted in the server's key) to the server. The
      session key (now shared by the client and server) is used to
      authenticate the client, and may optionally be used to
      authenticate the server. It may also be used to encrypt further
      communication between the two parties or to exchange a separate
      sub-session key to be used to encrypt further communication.

      Implementation of the basic protocol consists of one or more
      authentication servers running on physically secure hosts. The
      authentication servers maintain a database of principals (i.e.,
      users and servers) and their secret keys. Code libraries provide
      encryption and implement the Kerberos protocol.  In order to add
      authentication to its transactions, a typical network application
      adds calls to the Kerberos library directly or through the Generic
      Security Services Application Programming Interface, GSSAPI,
      described in separate document [ref to GSSAPI RFC]. These calls
      result in the transmission of the necessary messages to achieve
      authentication.

      The Kerberos protocol consists of several sub-protocols (or
      exchanges).  There are two basic methods by which a client can ask



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      a Kerberos server for credentials. In the first approach, the
      client sends a cleartext request for a ticket for the desired
      server to the AS. The reply is sent encrypted in the client's
      secret key. Usually this request is for a ticket-granting ticket
      (TGT) which can later be used with the ticket-granting server
      (TGS).  In the second method, the client sends a request to the
      TGS. The client uses the TGT to authenticate itself to the TGS in
      the same manner as if it were contacting any other application
      server that requires Kerberos authentication. The reply is
      encrypted in the session key from the TGT.  Though the protocol
      specification describes the AS and the TGS as separate servers,
      they are implemented in practice as different protocol entry
      points within a single Kerberos server.

      Once obtained, credentials may be used to verify the identity of
      the principals in a transaction, to ensure the integrity of
      messages exchanged between them, or to preserve privacy of the
      messages. The application is free to choose whatever protection
      may be necessary.

      To verify the identities of the principals in a transaction, the
      client transmits the ticket to the application server. Since the
      ticket is sent "in the clear" (parts of it are encrypted, but this
      encryption doesn't thwart replay) and might be intercepted and
      reused by an attacker, additional information is sent to prove
      that the message originated with the principal to whom the ticket
      was issued. This information (called the authenticator) is
      encrypted in the session key, and includes a timestamp. The
      timestamp proves that the message was recently generated and is
      not a replay.  Encrypting the authenticator in the session key
      proves that it was generated by a party possessing the session
      key. Since no one except the requesting principal and the server
      know the session key (it is never sent over the network in the
      clear) this guarantees the identity of the client.

      The integrity of the messages exchanged between principals can
      also be guaranteed using the session key (passed in the ticket and
      contained in the credentials). This approach provides detection of
      both replay attacks and message stream modification attacks. It is
      accomplished by generating and transmitting a collision-proof
      checksum (elsewhere called a hash or digest function) of the
      client's message, keyed with the session key. Privacy and
      integrity of the messages exchanged between principals can be
      secured by encrypting the data to be passed using the session key
      contained in the ticket or the sub-session key found in the
      authenticator.

      The authentication exchanges mentioned above require read-only



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      access to the Kerberos database. Sometimes, however, the entries
      in the database must be modified, such as when adding new
      principals or changing a principal's key.  This is done using a
      protocol between a client and a third Kerberos server, the
      Kerberos Administration Server (KADM). There is also a protocol
      for maintaining multiple copies of the Kerberos database. Neither
      of these protocols are described in this document.

1.1. Cross-realm operation

      The Kerberos protocol is designed to operate across organizational
      boundaries. A client in one organization can be authenticated to a
      server in another. Each organization wishing to run a Kerberos
      server establishes its own "realm". The name of the realm in which
      a client is registered is part of the client's name, and can be
      used by the end-service to decide whether to honor a request.

      By establishing "inter-realm" keys, the administrators of two
      realms can allow a client authenticated in the local realm to
      prove its identity to servers in other realms[3]. The exchange of
      inter-realm keys (a separate key may be used for each direction)
      registers the ticket-granting service of each realm as a principal
      in the other realm. A client is then able to obtain a ticket-
      granting ticket for the remote realm's ticket-granting service
      from its local realm. When that ticket-granting ticket is used,
      the remote ticket-granting service uses the inter-realm key (which
      usually differs from its own normal TGS key) to decrypt the
      ticket-granting ticket, and is thus certain that it was issued by
      the client's own TGS. Tickets issued by the remote ticket-granting
      service will indicate to the end-service that the client was
      authenticated from another realm.

      A realm is said to communicate with another realm if the two
      realms share an inter-realm key, or if the local realm shares an
      inter-realm key with an intermediate realm that communicates with
      the remote realm. An authentication path is the sequence of
      intermediate realms that are transited in communicating from one
      realm to another.

      Realms may be organized hierarchically. Each realm shares a key
      with its parent and a different key with each child. If an inter-
      realm key is not directly shared by two realms, the hierarchical
      organization allows an authentication path to be easily
      constructed. If a hierarchical organization is not used, it may be
      necessary to consult a database in order to construct an
      authentication path between realms.

      Although realms are typically hierarchical, intermediate realms



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      may be bypassed to achieve cross-realm authentication through
      alternate authentication paths (these might be established to make
      communication between two realms more efficient). It is important
      for the end-service to know which realms were transited when
      deciding how much faith to place in the authentication process. To
      facilitate this decision, a field in each ticket contains the
      names of the realms that were involved in authenticating the
      client.

      The application server is ultimately responsible for accepting or
      rejecting authentication and SHOULD check the transited field. The
      application server may choose to rely on the KDC for the
      application server's realm to check the transited field. The
      application server's KDC will set the TRANSITED-POLICY-CHECKED
      flag in this case. The KDCs for intermediate realms may also check
      the transited field as they issue ticket-granting tickets for
      other realms, but they are encouraged not to do so. A client may
      request that the KDCs not check the transited field by setting the
      DISABLE-TRANSITED-CHECK flag. KDCs SHOULD honor this flag.

1.2. Choosing a principal with which to communicate

      The Kerberos protocol provides the means for verifying (subject to
      the assumptions in 1.5) that the entity with which one
      communicates is the same entity that was registered with the KDC
      using the claimed identity (principal name). It is still necessary
      to determine whether that identity corresponds to the entity with
      which one intends to communicate.

      When appropriate data has been exchanged in advance, this
      determination may be performed syntactically by the application
      based on the application protocol specification, information
      provided by the user, and configuration files. For example, the
      server principal name (including realm) for a telnet server might
      be derived from the user specified host name (from the telnet
      command line), the "host/" prefix specified in the application
      protocol specification, and a mapping to a Kerberos realm derived
      syntactically from the domain part of the specified hostname and
      information from the local Kerberos realms database.

      One can also rely on trusted third parties to make this
      determination, but only when the data obtained from the third
      party is suitably integrity protected while resident on the third
      party server and when transmitted.  Thus, for example, one should
      not rely on an unprotected domain name system record to map a host
      alias to the primary name of a server, accepting the primary name
      as the party one intends to contact, since an attacker can modify
      the mapping and impersonate the party with which one intended to



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      communicate.

      Implementations of Kerberos and protocols based on Kerberos MUST
      NOT use insecure DNS queries to canonicalize the hostname
      components of the service principal names (i.e. MUST NOT use
      insecure DNS queries to map one name to another to determine the
      host part of the principal name with which one is to communicate).
      In an environment without secure name service, application authors
      MAY append a statically configured domain name to unqualified
      hostnames before passing the name to the security mechanisms, but
      should do no more than that.  Secure name service facilities, if
      available, might be trusted for hostname canonicalization, but
      such canonicalization by the client SHOULD NOT be required by KDC
      implementations.

      Implementation note: Many current implementations do some degree
      of canonicalization of the provided service name, often using DNS
      even though it creates security problems. However there is no
      consistency among implementations about whether the service name
      is case folded to lower case or whether reverse resolution is
      used. To maximize interoperability and security, applications
      SHOULD provide security mechanisms with names which result from
      folding the user-entered name to lower case, without performing
      any other modifications or canonicalization.

1.3. Authorization

      As an authentication service, Kerberos provides a means of
      verifying the identity of principals on a network. Authentication
      is usually useful primarily as a first step in the process of
      authorization, determining whether a client may use a service,
      which objects the client is allowed to access, and the type of
      access allowed for each. Kerberos does not, by itself, provide
      authorization. Possession of a client ticket for a service
      provides only for authentication of the client to that service,
      and in the absence of a separate authorization procedure, it
      should not be considered by an application as authorizing the use
      of that service.

      Such separate authorization methods MAY be implemented as
      application specific access control functions and may utilize
      files on the application server, or on separately issued
      authorization credentials such as those based on proxies [Neu93],
      or on other authorization services. Separately authenticated
      authorization credentials MAY be embedded in a ticket's
      authorization data when encapsulated by the KDC-issued
      authorization data element.




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      Applications should not accept the mere issuance of a service
      ticket by the Kerberos server (even by a modified Kerberos server)
      as granting authority to use the service, since such applications
      may become vulnerable to the bypass of this authorization check in
      an environment if they interoperate with other KDCs or where other
      options for application authentication are provided.

1.4. Extending Kerberos Without Breaking Interoperability

      As the deployed base of Kerberos implementations grows, extending
      Kerberos becomes more important. Unfortunately some extensions to
      the existing Kerberos protocol create interoperability issues
      because of uncertainty regarding the treatment of certain
      extensibility options by some implementations. This section
      includes guidelines that will enable future implementations to
      maintain interoperability.

      Kerberos provides a general mechanism for protocol extensibility.
      Some protocol messages contain typed holes -- sub-messages that
      contain an octet-string along with an integer that defines how to
      interpret the octet-string. The integer types are registered
      centrally, but can be used both for vendor extensions and for
      extensions standardized through the IETF.

      In this document, the word "extension" means an extension by
      defining a new type to insert into an existing typed hole in a
      protocol message.  It does not mean extension by addition of new
      fields to ASN.1 types, unless explicitly indicated otherwise in
      the text.

1.4.1. Compatibility with RFC 1510

      It is important to note that existing Kerberos message formats can
      not be readily extended by adding fields to the ASN.1 types.
      Sending additional fields often results in the entire message
      being discarded without an error indication. Future versions of
      this specification will provide guidelines to ensure that ASN.1
      fields can be added without creating an interoperability problem.

      In the meantime, all new or modified implementations of Kerberos
      that receive an unknown message extension SHOULD preserve the
      encoding of the extension but otherwise ignore the presence of the
      extension. Recipients MUST NOT decline a request simply because an
      extension is present.

      There is one exception to this rule. If an unknown authorization
      data element type is received by a server other than the ticket
      granting service either in an AP-REQ or in a ticket contained in



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      an AP-REQ, then authentication MUST fail. One of the primary uses
      of authorization data is to restrict the use of the ticket. If the
      service cannot determine whether the restriction applies to that
      service then a security weakness may result if the ticket can be
      used for that service. Authorization elements that are optional
      SHOULD be enclosed in the AD-IF-RELEVANT element.

      The ticket granting service MUST ignore but propagate to
      derivative tickets any unknown authorization data types, unless
      those data types are embedded in a MANDATORY-FOR-KDC element, in
      which case the request will be rejected.  This behavior is
      appropriate because requiring that the ticket granting service
      understand unknown authorization data types would require that KDC
      software be upgraded to understand new application-level
      restrictions before applications used these restrictions,
      decreasing the utility of authorization data as a mechanism for
      restricting the use of tickets. No security problem is created
      because services to which the tickets are issued will verify the
      authorization data.

      Implementation note: Many RFC 1510 implementations ignore unknown
      authorization data elements. Depending on these implementations to
      honor authorization data restrictions may create a security
      weakness.

1.4.2. Sending Extensible Messages

      Care must be taken to ensure that old implementations can
      understand messages sent to them even if they do not understand an
      extension that is used. Unless the sender knows an extension is
      supported, the extension cannot change the semantics of the core
      message or previously defined extensions.

      For example, an extension including key information necessary to
      decrypt the encrypted part of a KDC-REP could only be used in
      situations where the recipient was known to support the extension.
      Thus when designing such extensions it is important to provide a
      way for the recipient to notify the sender of support for the
      extension. For example in the case of an extension that changes
      the KDC-REP reply key, the client could indicate support for the
      extension by including a padata element in the AS-REQ sequence.
      The KDC should only use the extension if this padata element is
      present in the AS-REQ. Even if policy requires the use of the
      extension, it is better to return an error indicating that the
      extension is required than to use the extension when the recipient
      may not support it; debugging why implementations do not
      interoperate is easier when errors are returned.




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1.5. Environmental assumptions

      Kerberos imposes a few assumptions on the environment in which it
      can properly function:

   *  "Denial of service" attacks are not solved with Kerberos. There
      are places in the protocols where an intruder can prevent an
      application from participating in the proper authentication steps.
      Detection and solution of such attacks (some of which can appear
      to be not-uncommon "normal" failure modes for the system) is
      usually best left to the human administrators and users.

   *  Principals MUST keep their secret keys secret. If an intruder
      somehow steals a principal's key, it will be able to masquerade as
      that principal or impersonate any server to the legitimate
      principal.

   *  "Password guessing" attacks are not solved by Kerberos. If a user
      chooses a poor password, it is possible for an attacker to
      successfully mount an offline dictionary attack by repeatedly
      attempting to decrypt, with successive entries from a dictionary,
      messages obtained which are encrypted under a key derived from the
      user's password.

   *  Each host on the network MUST have a clock which is "loosely
      synchronized" to the time of the other hosts; this synchronization
      is used to reduce the bookkeeping needs of application servers
      when they do replay detection. The degree of "looseness" can be
      configured on a per-server basis, but is typically on the order of
      5 minutes. If the clocks are synchronized over the network, the
      clock synchronization protocol MUST itself be secured from network
      attackers.

   *  Principal identifiers are not recycled on a short-term basis. A
      typical mode of access control will use access control lists
      (ACLs) to grant permissions to particular principals. If a stale
      ACL entry remains for a deleted principal and the principal
      identifier is reused, the new principal will inherit rights
      specified in the stale ACL entry. By not re-using principal
      identifiers, the danger of inadvertent access is removed.

1.6. Glossary of terms

      Below is a list of terms used throughout this document.

   Authentication
      Verifying the claimed identity of a principal.




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   Authentication header
      A record containing a Ticket and an Authenticator to be presented
      to a server as part of the authentication process.

   Authentication path
      A sequence of intermediate realms transited in the authentication
      process when communicating from one realm to another.

   Authenticator
      A record containing information that can be shown to have been
      recently generated using the session key known only by the client
      and server.

   Authorization
      The process of determining whether a client may use a service,
      which objects the client is allowed to access, and the type of
      access allowed for each.

   Capability
      A token that grants the bearer permission to access an object or
      service. In Kerberos, this might be a ticket whose use is
      restricted by the contents of the authorization data field, but
      which lists no network addresses, together with the session key
      necessary to use the ticket.

   Ciphertext
      The output of an encryption function. Encryption transforms
      plaintext into ciphertext.

   Client
      A process that makes use of a network service on behalf of a user.
      Note that in some cases a Server may itself be a client of some
      other server (e.g. a print server may be a client of a file
      server).

   Credentials
      A ticket plus the secret session key necessary to successfully use
      that ticket in an authentication exchange.

   Encryption Type (etype)
      When associated with encrypted data, an encryption type identifies
      the algorithm used to encrypt the data and is used to select the
      appropriate algorithm for decrypting the data.  Encryption type
      tags are communicated in other messages to enumerate algorithms
      that are desired, supported, preferred, or allowed to be used for
      encryption of data between parties.  This preference is combined
      with local information and policy to select an algorithm to be
      used.



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   KDC
      Key Distribution Center, a network service that supplies tickets
      and temporary session keys; or an instance of that service or the
      host on which it runs. The KDC services both initial ticket and
      ticket-granting ticket requests. The initial ticket portion is
      sometimes referred to as the Authentication Server (or service).
      The ticket-granting ticket portion is sometimes referred to as the
      ticket-granting server (or service).

   Kerberos
      The name given to the Project Athena's authentication service, the
      protocol used by that service, or the code used to implement the
      authentication service.  The name is adopted from the three-headed
      dog which guards Hades.

   Key Version Number (kvno)
      A tag associated with encrypted data identifies which key was used
      for encryption when a long lived key associated with a principal
      changes over time.  It is used during the transition to a new key
      so that the party decrypting a message can tell whether the data
      was encrypted using the old or the new key.

   Plaintext
      The input to an encryption function or the output of a decryption
      function. Decryption transforms ciphertext into plaintext.

   Principal
      A named client or server entity that participates in a network
      communication, with one name that is considered canonical.

   Principal identifier
      The canonical name used to uniquely identify each different
      principal.

   Seal
      To encipher a record containing several fields in such a way that
      the fields cannot be individually replaced without either
      knowledge of the encryption key or leaving evidence of tampering.

   Secret key
      An encryption key shared by a principal and the KDC, distributed
      outside the bounds of the system, with a long lifetime. In the
      case of a human user's principal, the secret key MAY be derived
      from a password.

   Server
      A particular Principal which provides a resource to network
      clients.  The server is sometimes referred to as the Application



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      Server.

   Service
      A resource provided to network clients; often provided by more
      than one server (for example, remote file service).

   Session key
      A temporary encryption key used between two principals, with a
      lifetime limited to the duration of a single login "session".  In
      the Kerberos system, a session key is generated by the KDC.  The
      session key is distinct from the sub-session key, described next..

   Sub-session key
      A temporary encryption key used between two principals, selected
      and exchanged by the principals using the session key, and with a
      lifetime limited to the duration of a single association. The sub-
      session key is also referred to as the subkey.

   Ticket
      A record that helps a client authenticate itself to a server; it
      contains the client's identity, a session key, a timestamp, and
      other information, all sealed using the server's secret key. It
      only serves to authenticate a client when presented along with a
      fresh Authenticator.


2. Ticket flag uses and requests

      Each Kerberos ticket contains a set of flags which are used to
      indicate attributes of that ticket. Most flags may be requested by
      a client when the ticket is obtained; some are automatically
      turned on and off by a Kerberos server as required. The following
      sections explain what the various flags mean and give examples of
      reasons to use them. With the exception of the INVALID flag
      clients MUST ignore ticket flags that are not recognized. KDCs
      MUST ignore KDC options that are not recognized. Some
      implementations of RFC 1510 are known to reject unknown KDC
      options, so clients may need to resend a request without new KDC
      options if the request was rejected when sent with options added
      since RFC 1510. Since new KDCs will ignore unknown options,
      clients MUST confirm that the ticket returned by the KDC meets
      their needs.

      Note that it is not, in general, possible to determine whether an
      option was not honored because it was not understood or because it
      was rejected either through configuration or policy. When adding a
      new option to the Kerberos protocol, designers should consider
      whether the distinction is important for their option. In cases



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      where it is, a mechanism for the KDC to return an indication that
      the option was understood but rejected needs to be provided in the
      specification of the option. Often in such cases, the mechanism
      needs to be broad enough to permit an error or reason to be
      returned.

2.1. Initial, pre-authenticated, and hardware authenticated tickets

      The INITIAL flag indicates that a ticket was issued using the AS
      protocol, rather than issued based on a ticket-granting ticket.
      Application servers that want to require the demonstrated
      knowledge of a client's secret key (e.g. a password-changing
      program) can insist that this flag be set in any tickets they
      accept, and thus be assured that the client's key was recently
      presented to the application client.

      The PRE-AUTHENT and HW-AUTHENT flags provide additional
      information about the initial authentication, regardless of
      whether the current ticket was issued directly (in which case
      INITIAL will also be set) or issued on the basis of a ticket-
      granting ticket (in which case the INITIAL flag is clear, but the
      PRE-AUTHENT and HW-AUTHENT flags are carried forward from the
      ticket-granting ticket).

2.2. Invalid tickets

      The INVALID flag indicates that a ticket is invalid. Application
      servers MUST reject tickets which have this flag set. A postdated
      ticket will be issued in this form. Invalid tickets MUST be
      validated by the KDC before use, by presenting them to the KDC in
      a TGS request with the VALIDATE option specified. The KDC will
      only validate tickets after their starttime has passed. The
      validation is required so that postdated tickets which have been
      stolen before their starttime can be rendered permanently invalid
      (through a hot-list mechanism) (see section 3.3.3.1).

2.3. Renewable tickets

      Applications may desire to hold tickets which can be valid for
      long periods of time. However, this can expose their credentials
      to potential theft for equally long periods, and those stolen
      credentials would be valid until the expiration time of the
      ticket(s). Simply using short-lived tickets and obtaining new ones
      periodically would require the client to have long-term access to
      its secret key, an even greater risk. Renewable tickets can be
      used to mitigate the consequences of theft. Renewable tickets have
      two "expiration times": the first is when the current instance of
      the ticket expires, and the second is the latest permissible value



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      for an individual expiration time. An application client must
      periodically (i.e. before it expires) present a renewable ticket
      to the KDC, with the RENEW option set in the KDC request. The KDC
      will issue a new ticket with a new session key and a later
      expiration time. All other fields of the ticket are left
      unmodified by the renewal process. When the latest permissible
      expiration time arrives, the ticket expires permanently. At each
      renewal, the KDC MAY consult a hot-list to determine if the ticket
      had been reported stolen since its last renewal; it will refuse to
      renew such stolen tickets, and thus the usable lifetime of stolen
      tickets is reduced.

      The RENEWABLE flag in a ticket is normally only interpreted by the
      ticket-granting service (discussed below in section 3.3). It can
      usually be ignored by application servers. However, some
      particularly careful application servers MAY disallow renewable
      tickets.

      If a renewable ticket is not renewed by its expiration time, the
      KDC will not renew the ticket. The RENEWABLE flag is reset by
      default, but a client MAY request it be set by setting the
      RENEWABLE option in the KRB_AS_REQ message. If it is set, then the
      renew-till field in the ticket contains the time after which the
      ticket may not be renewed.

2.4. Postdated tickets

      Applications may occasionally need to obtain tickets for use much
      later, e.g. a batch submission system would need tickets to be
      valid at the time the batch job is serviced. However, it is
      dangerous to hold valid tickets in a batch queue, since they will
      be on-line longer and more prone to theft.  Postdated tickets
      provide a way to obtain these tickets from the KDC at job
      submission time, but to leave them "dormant" until they are
      activated and validated by a further request of the KDC. If a
      ticket theft were reported in the interim, the KDC would refuse to
      validate the ticket, and the thief would be foiled.

      The MAY-POSTDATE flag in a ticket is normally only interpreted by
      the ticket-granting service. It can be ignored by application
      servers. This flag MUST be set in a ticket-granting ticket in
      order to issue a postdated ticket based on the presented ticket.
      It is reset by default; it MAY be requested by a client by setting
      the ALLOW-POSTDATE option in the KRB_AS_REQ message.  This flag
      does not allow a client to obtain a postdated ticket-granting
      ticket; postdated ticket-granting tickets can only by obtained by
      requesting the postdating in the KRB_AS_REQ message. The life
      (endtime-starttime) of a postdated ticket will be the remaining



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      life of the ticket-granting ticket at the time of the request,
      unless the RENEWABLE option is also set, in which case it can be
      the full life (endtime-starttime) of the ticket-granting ticket.
      The KDC MAY limit how far in the future a ticket may be postdated.

      The POSTDATED flag indicates that a ticket has been postdated. The
      application server can check the authtime field in the ticket to
      see when the original authentication occurred. Some services MAY
      choose to reject postdated tickets, or they may only accept them
      within a certain period after the original authentication. When
      the KDC issues a POSTDATED ticket, it will also be marked as
      INVALID, so that the application client MUST present the ticket to
      the KDC to be validated before use.

2.5. Proxiable and proxy tickets

      At times it may be necessary for a principal to allow a service to
      perform an operation on its behalf. The service must be able to
      take on the identity of the client, but only for a particular
      purpose. A principal can allow a service to take on the
      principal's identity for a particular purpose by granting it a
      proxy.

      The process of granting a proxy using the proxy and proxiable
      flags is used to provide credentials for use with specific
      services. Though conceptually also a proxy, users wishing to
      delegate their identity in a form usable for all purpose MUST use
      the ticket forwarding mechanism described in the next section to
      forward a ticket-granting ticket.

      The PROXIABLE flag in a ticket is normally only interpreted by the
      ticket-granting service. It can be ignored by application servers.
      When set, this flag tells the ticket-granting server that it is OK
      to issue a new ticket (but not a ticket-granting ticket) with a
      different network address based on this ticket. This flag is set
      if requested by the client on initial authentication. By default,
      the client will request that it be set when requesting a ticket-
      granting ticket, and reset when requesting any other ticket.

      This flag allows a client to pass a proxy to a server to perform a
      remote request on its behalf (e.g. a print service client can give
      the print server a proxy to access the client's files on a
      particular file server in order to satisfy a print request).

      In order to complicate the use of stolen credentials, Kerberos
      tickets are usually valid from only those network addresses
      specifically included in the ticket[4]. When granting a proxy, the
      client MUST specify the new network address from which the proxy



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      is to be used, or indicate that the proxy is to be issued for use
      from any address.

      The PROXY flag is set in a ticket by the TGS when it issues a
      proxy ticket.  Application servers MAY check this flag and at
      their option they MAY require additional authentication from the
      agent presenting the proxy in order to provide an audit trail.

2.6. Forwardable tickets

      Authentication forwarding is an instance of a proxy where the
      service that is granted is complete use of the client's identity.
      An example where it might be used is when a user logs in to a
      remote system and wants authentication to work from that system as
      if the login were local.

      The FORWARDABLE flag in a ticket is normally only interpreted by
      the ticket-granting service. It can be ignored by application
      servers. The FORWARDABLE flag has an interpretation similar to
      that of the PROXIABLE flag, except ticket-granting tickets may
      also be issued with different network addresses. This flag is
      reset by default, but users MAY request that it be set by setting
      the FORWARDABLE option in the AS request when they request their
      initial ticket-granting ticket.

      This flag allows for authentication forwarding without requiring
      the user to enter a password again. If the flag is not set, then
      authentication forwarding is not permitted, but the same result
      can still be achieved if the user engages in the AS exchange
      specifying the requested network addresses and supplies a
      password.

      The FORWARDED flag is set by the TGS when a client presents a
      ticket with the FORWARDABLE flag set and requests a forwarded
      ticket by specifying the FORWARDED KDC option and supplying a set
      of addresses for the new ticket. It is also set in all tickets
      issued based on tickets with the FORWARDED flag set. Application
      servers may choose to process FORWARDED tickets differently than
      non-FORWARDED tickets.

      If addressless tickets are forwarded from one system to another,
      clients SHOULD still use this option to obtain a new TGT in order
      to have different session keys on the different systems.

2.7. Transited Policy Checking

      In Kerberos, the application server is ultimately responsible for
      accepting or rejecting authentication and SHOULD check that only



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      suitably trusted KDCs are relied upon to authenticate a principal.
      The transited field in the ticket identifies which realms (and
      thus which KDCs) were involved in the authentication process and
      an application server would normally check this field. If any of
      these are untrusted to authenticate the indicated client principal
      (probably determined by a realm-based policy), the authentication
      attempt MUST be rejected. The presence of trusted KDCs in this
      list does not provide any guarantee; an untrusted KDC may have
      fabricated the list.

      While the end server ultimately decides whether authentication is
      valid, the KDC for the end server's realm MAY apply a realm
      specific policy for validating the transited field and accepting
      credentials for cross-realm authentication. When the KDC applies
      such checks and accepts such cross-realm authentication it will
      set the TRANSITED-POLICY-CHECKED flag in the service tickets it
      issues based on the cross-realm TGT. A client MAY request that the
      KDCs not check the transited field by setting the DISABLE-
      TRANSITED-CHECK flag. KDCs are encouraged but not required to
      honor this flag.

      Application servers MUST either do the transited-realm checks
      themselves, or reject cross-realm tickets without TRANSITED-
      POLICY-CHECKED set.

2.8. OK as Delegate

      For some applications a client may need to delegate authority to a
      server to act on its behalf in contacting other services.  This
      requires that the client forward credentials to an intermediate
      server.  The ability for a client to obtain a service ticket to a
      server conveys no information to the client about whether the
      server should be trusted to accept delegated credentials.  The OK-
      AS-DELEGATE provides a way for a KDC to communicate local realm
      policy to a client regarding whether an intermediate server is
      trusted to accept such credentials.

      The copy of the ticket flags in the encrypted part of the KDC
      reply may have the OK-AS-DELEGATE flag set to indicates to the
      client that the server specified in the ticket has been determined
      by policy of the realm to be a suitable recipient of delegation.
      A client can use the presence of this flag to help it make a
      decision whether to delegate credentials (either grant a proxy or
      a forwarded ticket-granting ticket) to this server.  It is
      acceptable to ignore the value of this flag. When setting this
      flag, an administrator should consider the security and placement
      of the server on which the service will run, as well as whether
      the service requires the use of delegated credentials.



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2.9. Other KDC options

      There are three additional options which MAY be set in a client's
      request of the KDC.

2.9.1. Renewable-OK

      The RENEWABLE-OK option indicates that the client will accept a
      renewable ticket if a ticket with the requested life cannot
      otherwise be provided. If a ticket with the requested life cannot
      be provided, then the KDC MAY issue a renewable ticket with a
      renew-till equal to the requested endtime. The value of the renew-
      till field MAY still be adjusted by site-determined limits or
      limits imposed by the individual principal or server.

2.9.2. ENC-TKT-IN-SKEY

      In its basic form the Kerberos protocol supports authentication in
      a client-server
       setting and is not well suited to authentication in a peer-to-
      peer environment because the long term key of the user does not
      remain on the workstation after initial login. Authentication of
      such peers may be supported by Kerberos in its user-to-user
      variant. The ENC-TKT-IN-SKEY option supports user-to-user
      authentication by allowing the KDC to issue a service ticket
      encrypted using the session key from another ticket-granting
      ticket issued to another user. The ENC-TKT-IN-SKEY option is
      honored only by the ticket-granting service. It indicates that the
      ticket to be issued for the end server is to be encrypted in the
      session key from the additional second ticket-granting ticket
      provided with the request. See section 3.3.3 for specific details.

2.9.3. Passwordless Hardware Authentication

      The OPT-HARDWARE-AUTH option indicates that the client wishes to
      use some form of hardware authentication instead of or in addition
      to the client's password or other long-lived encryption key. OPT-
      HARDWARE-AUTH is honored only by the authentication service. If
      supported and allowed by policy, the KDC will return an errorcode
      KDC_ERR_PREAUTH_REQUIRED and include the required METHOD-DATA to
      perform such authentication.

3. Message Exchanges

      The following sections describe the interactions between network
      clients and servers and the messages involved in those exchanges.

3.1. The Authentication Service Exchange



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                                Summary

            Message direction       Message type    Section
            1. Client to Kerberos   KRB_AS_REQ      5.4.1
            2. Kerberos to client   KRB_AS_REP or   5.4.2
                                    KRB_ERROR       5.9.1

      The Authentication Service (AS) Exchange between the client and
      the Kerberos Authentication Server is initiated by a client when
      it wishes to obtain authentication credentials for a given server
      but currently holds no credentials. In its basic form, the
      client's secret key is used for encryption and decryption. This
      exchange is typically used at the initiation of a login session to
      obtain credentials for a Ticket-Granting Server which will
      subsequently be used to obtain credentials for other servers (see
      section 3.3) without requiring further use of the client's secret
      key. This exchange is also used to request credentials for
      services which must not be mediated through the Ticket-Granting
      Service, but rather require a principal's secret key, such as the
      password-changing service[5]. This exchange does not by itself
      provide any assurance of the identity of the user[6].

      The exchange consists of two messages: KRB_AS_REQ from the client
      to Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for
      these messages are described in sections 5.4.1, 5.4.2, and 5.9.1.

      In the request, the client sends (in cleartext) its own identity
      and the identity of the server for which it is requesting
      credentials, other information about the credentials it is
      requesting, and a randomly generated nonce which can be used to
      detect replays, and to associate replies with the matching
      requests. This nonce MUST be generated randomly by the client and
      remembered for checking against the nonce in the expected reply.
      The response, KRB_AS_REP, contains a ticket for the client to
      present to the server, and a session key that will be shared by
      the client and the server.  The session key and additional
      information are encrypted in the client's secret key. The
      encrypted part of the KRB_AS_REP message also contains the nonce
      which MUST be matched with the nonce from the KRB_AS_REQ message.

      Without pre-authentication, the authentication server does not
      know whether the client is actually the principal named in the
      request. It simply sends a reply without knowing or caring whether
      they are the same. This is acceptable because nobody but the
      principal whose identity was given in the request will be able to
      use the reply. Its critical information is encrypted in that
      principal's key. However, an attacker can send a KRB_AS_REQ
      message to get known plaintext in order to attack the principal's



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      key. Especially if the key is based on a password, this may create
      a security exposure. So, the initial request supports an optional
      field that can be used to pass additional information that might
      be needed for the initial exchange. This field SHOULD be used for
      pre-authentication as described in sections 3.1.1 and 5.2.7.

      Various errors can occur; these are indicated by an error response
      (KRB_ERROR) instead of the KRB_AS_REP response. The error message
      is not encrypted. The KRB_ERROR message contains information which
      can be used to associate it with the message to which it replies.
      The contents of the KRB_ERROR message are not integrity-protected.
      As such, the client cannot detect replays, fabrications or
      modifications. A solution to this problem will be included in a
      future version of the protocol.

3.1.1. Generation of KRB_AS_REQ message

      The client may specify a number of options in the initial request.
      Among these options are whether pre-authentication is to be
      performed; whether the requested ticket is to be renewable,
      proxiable, or forwardable; whether it should be postdated or allow
      postdating of derivative tickets; and whether a renewable ticket
      will be accepted in lieu of a non-renewable ticket if the
      requested ticket expiration date cannot be satisfied by a non-
      renewable ticket (due to configuration constraints).

      The client prepares the KRB_AS_REQ message and sends it to the
      KDC.

3.1.2. Receipt of KRB_AS_REQ message

      If all goes well, processing the KRB_AS_REQ message will result in
      the creation of a ticket for the client to present to the server.
      The format for the ticket is described in section 5.3.

      Because Kerberos can run over unreliable transports such as UDP,
      the KDC MUST be prepared to retransmit responses in case they are
      lost. If a KDC receives a request identical to one it has recently
      successfully processed, the KDC MUST respond with a KRB_AS_REP
      message rather than a replay error.  In order to reduce ciphertext
      given to a potential attacker, KDCs MAY send the same response
      generated when the request was first handled. KDCs MUST obey this
      replay behavior even if the actual transport in use is reliable.

3.1.3. Generation of KRB_AS_REP message

      The authentication server looks up the client and server
      principals named in the KRB_AS_REQ in its database, extracting



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      their respective keys. If the requested client principal named in
      the request is not known because it doesn't exist in the KDC's
      principal database, then an error message with a
      KDC_ERR_C_PRINCIPAL_UNKNOWN is returned.

      If required, the server pre-authenticates the request, and if the
      pre-authentication check fails, an error message with the code
      KDC_ERR_PREAUTH_FAILED is returned. If pre-authentication is
      required, but was not present in the request, an error message
      with the code KDC_ERR_PREAUTH_REQUIRED is returned and a METHOD-
      DATA object will be stored in the e-data field of the KRB-ERROR
      message to specify which pre-authentication mechanisms are
      acceptable.  Usually this will include PA-ETYPE-INFO and/or PA-
      ETYPE-INFO2 elements as described below. If the server cannot
      accommodate any encryption type requested by the client, an error
      message with code KDC_ERR_ETYPE_NOSUPP is returned. Otherwise the
      KDC generates a 'random' session key[7].

      When responding to an AS request, if there are multiple encryption
      keys registered for a client in the Kerberos database, then the
      etype field from the AS request is used by the KDC to select the
      encryption method to be used to protect the encrypted part of the
      KRB_AS_REP message which is sent to the client. If there is more
      than one supported strong encryption type in the etype list, the
      KDC SHOULD use the first valid strong etype for which an
      encryption key is available.

      When the user's key is generated from a password or pass phrase,
      the string-to-key function for the particular encryption key type
      is used, as specified in [@KCRYPTO]. The salt value and additional
      parameters for the string-to-key function have default values
      (specified by section 4 and by the encryption mechanism
      specification, respectively) that may be overridden by pre-
      authentication data (PA-PW-SALT, PA-AFS3-SALT, PA-ETYPE-INFO, PA-
      ETYPE-INFO2, etc). Since the KDC is presumed to store a copy of
      the resulting key only, these values should not be changed for
      password-based keys except when changing the principal's key.

      When the AS server is to include pre-authentication data in a KRB-
      ERROR or in an AS-REP, it MUST use PA-ETYPE-INFO2, not PA-ETYPE-
      INFO, if the etype field of the client's AS-REQ lists at least one
      "newer" encryption type.  Otherwise (when the etype field of the
      client's AS-REQ does not list any "newer" encryption types) it
      MUST send both, PA-ETYPE-INFO2 and PA-ETYPE-INFO (both with an
      entry for each enctype).  A "newer" enctype is any enctype first
      officially specified concurrently with or subsequent to the issue
      of this RFC.  The enctypes DES, 3DES or RC4 and any defined in
      [RFC1510] are not "newer" enctypes.



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      It is not possible to reliably generate a user's key given a pass
      phrase without contacting the KDC, since it will not be known
      whether alternate salt or parameter values are required.

      The KDC will attempt to assign the type of the random session key
      from the list of methods in the etype field. The KDC will select
      the appropriate type using the list of methods provided together
      with information from the Kerberos database indicating acceptable
      encryption methods for the application server. The KDC will not
      issue tickets with a weak session key encryption type.

      If the requested start time is absent, indicates a time in the
      past, or is within the window of acceptable clock skew for the KDC
      and the POSTDATE option has not been specified, then the start
      time of the ticket is set to the authentication server's current
      time. If it indicates a time in the future beyond the acceptable
      clock skew, but the POSTDATED option has not been specified then
      the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise the
      requested start time is checked against the policy of the local
      realm (the administrator might decide to prohibit certain types or
      ranges of postdated tickets), and if acceptable, the ticket's
      start time is set as requested and the INVALID flag is set in the
      new ticket. The postdated ticket MUST be validated before use by
      presenting it to the KDC after the start time has been reached.

      The expiration time of the ticket will be set to the earlier of
      the requested endtime and a time determined by local policy,
      possibly determined using realm or principal specific factors. For
      example, the expiration time MAY be set to the earliest of the
      following:

      *  The expiration time (endtime) requested in the KRB_AS_REQ
         message.

      *  The ticket's start time plus the maximum allowable lifetime
         associated with the client principal from the authentication
         server's database.

      *  The ticket's start time plus the maximum allowable lifetime
         associated with the server principal.

      *  The ticket's start time plus the maximum lifetime set by the
         policy of the local realm.

   If the requested expiration time minus the start time (as determined
   above) is less than a site-determined minimum lifetime, an error
   message with code KDC_ERR_NEVER_VALID is returned. If the requested
   expiration time for the ticket exceeds what was determined as above,



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   and if the 'RENEWABLE-OK' option was requested, then the 'RENEWABLE'
   flag is set in the new ticket, and the renew-till value is set as if
   the 'RENEWABLE' option were requested (the field and option names are
   described fully in section 5.4.1).

   If the RENEWABLE option has been requested or if the RENEWABLE-OK
   option has been set and a renewable ticket is to be issued, then the
   renew-till field MAY be set to the earliest of:

      *  Its requested value.

      *  The start time of the ticket plus the minimum of the two
         maximum renewable lifetimes associated with the principals'
         database entries.

      *  The start time of the ticket plus the maximum renewable
         lifetime set by the policy of the local realm.

   The flags field of the new ticket will have the following options set
   if they have been requested and if the policy of the local realm
   allows: FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE.
   If the new ticket is postdated (the start time is in the future), its
   INVALID flag will also be set.

   If all of the above succeed, the server will encrypt the ciphertext
   part of the ticket using the encryption key extracted from the server
   principal's record in the Kerberos database using the encryption type
   associated with the server principal's key (this choice is NOT
   affected by the etype field in the request). It then formats a
   KRB_AS_REP message (see section 5.4.2), copying the addresses in the
   request into the caddr of the response, placing any required pre-
   authentication data into the padata of the response, and encrypts the
   ciphertext part in the client's key using an acceptable encryption
   method requested in the etype field of the request, or in some key
   specified by pre-authentication mechanisms being used.

3.1.4. Generation of KRB_ERROR message

      Several errors can occur, and the Authentication Server responds
      by returning an error message, KRB_ERROR, to the client, with the
      error-code and e-text fields set to appropriate values. The error
      message contents and details are described in Section 5.9.1.

3.1.5. Receipt of KRB_AS_REP message

      If the reply message type is KRB_AS_REP, then the client verifies
      that the cname and crealm fields in the cleartext portion of the
      reply match what it requested. If any padata fields are present,



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      they may be used to derive the proper secret key to decrypt the
      message. The client decrypts the encrypted part of the response
      using its secret key, verifies that the nonce in the encrypted
      part matches the nonce it supplied in its request (to detect
      replays). It also verifies that the sname and srealm in the
      response match those in the request (or are otherwise expected
      values), and that the host address field is also correct. It then
      stores the ticket, session key, start and expiration times, and
      other information for later use. The last-req field (and the
      deprecated key-expiration field) from the encrypted part of the
      response MAY be checked to notify the user of impending key
      expiration. This enables the client program to suggest remedial
      action, such as a password change.

      Upon validation of the KRB_AS_REP message (by checking the
      returned nonce against that sent in the KRB_AS_REQ message) the
      client knows that the current time on the KDC is that read from
      the authtime field of the encrypted part of the reply. The client
      can optionally use this value for clock synchronization in
      subsequent messages by recording with the ticket the difference
      (offset) between the authtime value and the local clock. This
      offset can then be used by the same user to adjust the time read
      from the system clock when generating messages [DGT96].

      This technique MUST be used when adjusting for clock skew instead
      of directly changing the system clock because the KDC reply is
      only authenticated to the user whose secret key was used, but not
      to the system or workstation. If the clock were adjusted, an
      attacker colluding with a user logging into a workstation could
      agree on a password, resulting in a KDC reply that would be
      correctly validated even though it did not originate from a KDC
      trusted by the workstation.

      Proper decryption of the KRB_AS_REP message is not sufficient for
      the host to verify the identity of the user; the user and an
      attacker could cooperate to generate a KRB_AS_REP format message
      which decrypts properly but is not from the proper KDC. If the
      host wishes to verify the identity of the user, it MUST require
      the user to present application credentials which can be verified
      using a securely-stored secret key for the host. If those
      credentials can be verified, then the identity of the user can be
      assured.

3.1.6. Receipt of KRB_ERROR message

      If the reply message type is KRB_ERROR, then the client interprets
      it as an error and performs whatever application-specific tasks
      are necessary to recover.



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3.2. The Client/Server Authentication Exchange

                                   Summary
      Message direction                         Message type    Section
      Client to Application server              KRB_AP_REQ      5.5.1
      [optional] Application server to client   KRB_AP_REP or   5.5.2
                                                KRB_ERROR       5.9.1

      The client/server authentication (CS) exchange is used by network
      applications to authenticate the client to the server and vice
      versa. The client MUST have already acquired credentials for the
      server using the AS or TGS exchange.

3.2.1. The KRB_AP_REQ message

      The KRB_AP_REQ contains authentication information which SHOULD be
      part of the first message in an authenticated transaction. It
      contains a ticket, an authenticator, and some additional
      bookkeeping information (see section 5.5.1 for the exact format).
      The ticket by itself is insufficient to authenticate a client,
      since tickets are passed across the network in cleartext[8], so
      the authenticator is used to prevent invalid replay of tickets by
      proving to the server that the client knows the session key of the
      ticket and thus is entitled to use the ticket. The KRB_AP_REQ
      message is referred to elsewhere as the 'authentication header.'

3.2.2. Generation of a KRB_AP_REQ message

      When a client wishes to initiate authentication to a server, it
      obtains (either through a credentials cache, the AS exchange, or
      the TGS exchange) a ticket and session key for the desired
      service. The client MAY re-use any tickets it holds until they
      expire. To use a ticket the client constructs a new Authenticator
      from the system time, its name, and optionally an application
      specific checksum, an initial sequence number to be used in
      KRB_SAFE or KRB_PRIV messages, and/or a session subkey to be used
      in negotiations for a session key unique to this particular
      session.  Authenticators MAY NOT be re-used and SHOULD be rejected
      if replayed to a server[9]. If a sequence number is to be
      included, it SHOULD be randomly chosen so that even after many
      messages have been exchanged it is not likely to collide with
      other sequence numbers in use.

      The client MAY indicate a requirement of mutual authentication or
      the use of a session-key based ticket (for user-to-user
      authentication - see section 3.7) by setting the appropriate
      flag(s) in the ap-options field of the message.




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      The Authenticator is encrypted in the session key and combined
      with the ticket to form the KRB_AP_REQ message which is then sent
      to the end server along with any additional application-specific
      information.

3.2.3. Receipt of KRB_AP_REQ message

      Authentication is based on the server's current time of day
      (clocks MUST be loosely synchronized), the authenticator, and the
      ticket. Several errors are possible. If an error occurs, the
      server is expected to reply to the client with a KRB_ERROR
      message. This message MAY be encapsulated in the application
      protocol if its raw form is not acceptable to the protocol.  The
      format of error messages is described in section 5.9.1.

      The algorithm for verifying authentication information is as
      follows. If the message type is not KRB_AP_REQ, the server returns
      the KRB_AP_ERR_MSG_TYPE error. If the key version indicated by the
      Ticket in the KRB_AP_REQ is not one the server can use (e.g., it
      indicates an old key, and the server no longer possesses a copy of
      the old key), the KRB_AP_ERR_BADKEYVER error is returned. If the
      USE-SESSION-KEY flag is set in the ap-options field, it indicates
      to the server that user-to-user authentication is in use, and that
      the ticket is encrypted in the session key from the server's
      ticket-granting ticket rather than in the server's secret key. See
      section 3.7 for a more complete description of the effect of user-
      to-user authentication on all messages in the Kerberos protocol.

      Since it is possible for the server to be registered in multiple
      realms, with different keys in each, the srealm field in the
      unencrypted portion of the ticket in the KRB_AP_REQ is used to
      specify which secret key the server should use to decrypt that
      ticket. The KRB_AP_ERR_NOKEY error code is returned if the server
      doesn't have the proper key to decipher the ticket.

      The ticket is decrypted using the version of the server's key
      specified by the ticket. If the decryption routines detect a
      modification of the ticket (each encryption system MUST provide
      safeguards to detect modified ciphertext), the
      KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that
      different keys were used to encrypt and decrypt).

      The authenticator is decrypted using the session key extracted
      from the decrypted ticket. If decryption shows it to have been
      modified, the KRB_AP_ERR_BAD_INTEGRITY error is returned. The name
      and realm of the client from the ticket are compared against the
      same fields in the authenticator.  If they don't match, the
      KRB_AP_ERR_BADMATCH error is returned; this normally is caused by



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      a client error or attempted attack. The addresses in the ticket
      (if any) are then searched for an address matching the operating-
      system reported address of the client. If no match is found or the
      server insists on ticket addresses but none are present in the
      ticket, the KRB_AP_ERR_BADADDR error is returned. If the local
      (server) time and the client time in the authenticator differ by
      more than the allowable clock skew (e.g., 5 minutes), the
      KRB_AP_ERR_SKEW error is returned.

      Unless the application server provides its own suitable means to
      protect against replay (for example, a challenge-response sequence
      initiated by the server after authentication, or use of a server-
      generated encryption subkey), the server MUST utilize a replay
      cache to remember any authenticator presented within the allowable
      clock skew. Careful analysis of the application protocol and
      implementation is recommended before eliminating this cache. The
      replay cache will store at least the server name, along with the
      client name, time and microsecond fields from the recently-seen
      authenticators and if a matching tuple is found, the
      KRB_AP_ERR_REPEAT error is returned [10]. If a server loses track
      of authenticators presented within the allowable clock skew, it
      MUST reject all requests until the clock skew interval has passed,
      providing assurance that any lost or replayed authenticators will
      fall outside the allowable clock skew and can no longer be
      successfully replayed [11].

      Implementation note: If a client generates multiple requests to
      the KDC with the same timestamp, including the microsecond field,
      all but the first of the requests received will be rejected as
      replays. This might happen, for example, if the resolution of the
      client's clock is too coarse.  Client implementations SHOULD
      ensure that the timestamps are not reused, possibly by
      incrementing the microseconds field in the time stamp when the
      clock returns the same time for multiple requests.

      If multiple servers (for example, different services on one
      machine, or a single service implemented on multiple machines)
      share a service principal (a practice we do not recommend in
      general, but acknowledge will be used in some cases), they MUST
      either share this replay cache, or the application protocol MUST
      be designed so as to eliminate the need for it. Note that this
      applies to all of the services, if any of the application
      protocols does not have replay protection built in; an
      authenticator used with such a service could later be replayed to
      a different service with the same service principal but no replay
      protection, if the former doesn't record the authenticator
      information in the common replay cache.




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      If a sequence number is provided in the authenticator, the server
      saves it for later use in processing KRB_SAFE and/or KRB_PRIV
      messages. If a subkey is present, the server either saves it for
      later use or uses it to help generate its own choice for a subkey
      to be returned in a KRB_AP_REP message.

      The server computes the age of the ticket: local (server) time
      minus the start time inside the Ticket. If the start time is later
      than the current time by more than the allowable clock skew or if
      the INVALID flag is set in the ticket, the KRB_AP_ERR_TKT_NYV
      error is returned. Otherwise, if the current time is later than
      end time by more than the allowable clock skew, the
      KRB_AP_ERR_TKT_EXPIRED error is returned.

      If all these checks succeed without an error, the server is
      assured that the client possesses the credentials of the principal
      named in the ticket and thus, the client has been authenticated to
      the server.

      Passing these checks provides only authentication of the named
      principal; it does not imply authorization to use the named
      service. Applications MUST make a separate authorization decision
      based upon the authenticated name of the user, the requested
      operation, local access control information such as that contained
      in a .k5login or .k5users file, and possibly a separate
      distributed authorization service.

3.2.4. Generation of a KRB_AP_REP message

      Typically, a client's request will include both the authentication
      information and its initial request in the same message, and the
      server need not explicitly reply to the KRB_AP_REQ. However, if
      mutual authentication (not only authenticating the client to the
      server, but also the server to the client) is being performed, the
      KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options
      field, and a KRB_AP_REP message is required in response. As with
      the error message, this message MAY be encapsulated in the
      application protocol if its "raw" form is not acceptable to the
      application's protocol. The timestamp and microsecond field used
      in the reply MUST be the client's timestamp and microsecond field
      (as provided in the authenticator) [12]. If a sequence number is
      to be included, it SHOULD be randomly chosen as described above
      for the authenticator. A subkey MAY be included if the server
      desires to negotiate a different subkey. The KRB_AP_REP message is
      encrypted in the session key extracted from the ticket.

3.2.5. Receipt of KRB_AP_REP message




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      If a KRB_AP_REP message is returned, the client uses the session
      key from the credentials obtained for the server [13] to decrypt
      the message, and verifies that the timestamp and microsecond
      fields match those in the Authenticator it sent to the server. If
      they match, then the client is assured that the server is genuine.
      The sequence number and subkey (if present) are retained for later
      use.

3.2.6. Using the encryption key

      After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client
      and server share an encryption key which can be used by the
      application. In some cases, the use of this session key will be
      implicit in the protocol; in others the method of use must be
      chosen from several alternatives. The actual encryption key to be
      used for KRB_PRIV, KRB_SAFE, or other application-specific uses
      MAY be chosen by the application based on the session key from the
      ticket and subkeys in the KRB_AP_REP message and the authenticator
      [14]. To mitigate the effect of failures in random number
      generation on the client it is strongly encouraged that any key
      derived by an application for subsequent use include the full key
      entropy derived from the KDC generated session key carried in the
      ticket. We leave the protocol negotiations of how to use the key
      (e.g. selecting an encryption or checksum type) to the application
      programmer; the Kerberos protocol does not constrain the
      implementation options, but an example of how this might be done
      follows.

      One way that an application may choose to negotiate a key to be
      used for subsequent integrity and privacy protection is for the
      client to propose a key in the subkey field of the authenticator.
      The server can then choose a key using the proposed key from the
      client as input, returning the new subkey in the subkey field of
      the application reply. This key could then be used for subsequent
      communication.

      With both the one-way and mutual authentication exchanges, the
      peers should take care not to send sensitive information to each
      other without proper assurances. In particular, applications that
      require privacy or integrity SHOULD use the KRB_AP_REP response
      from the server to client to assure both client and server of
      their peer's identity. If an application protocol requires privacy
      of its messages, it can use the KRB_PRIV message (section 3.5).
      The KRB_SAFE message (section 3.4) can be used to assure
      integrity.

3.3. The Ticket-Granting Service (TGS) Exchange




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                                Summary
            Message direction       Message type     Section
            1. Client to Kerberos   KRB_TGS_REQ      5.4.1
            2. Kerberos to client   KRB_TGS_REP or   5.4.2
                                    KRB_ERROR        5.9.1

      The TGS exchange between a client and the Kerberos Ticket-Granting
      Server is initiated by a client when it wishes to obtain
      authentication credentials for a given server (which might be
      registered in a remote realm), when it wishes to renew or validate
      an existing ticket, or when it wishes to obtain a proxy ticket. In
      the first case, the client must already have acquired a ticket for
      the Ticket-Granting Service using the AS exchange (the ticket-
      granting ticket is usually obtained when a client initially
      authenticates to the system, such as when a user logs in). The
      message format for the TGS exchange is almost identical to that
      for the AS exchange.  The primary difference is that encryption
      and decryption in the TGS exchange does not take place under the
      client's key. Instead, the session key from the ticket-granting
      ticket or renewable ticket, or sub-session key from an
      Authenticator is used. As is the case for all application servers,
      expired tickets are not accepted by the TGS, so once a renewable
      or ticket-granting ticket expires, the client must use a separate
      exchange to obtain valid tickets.

      The TGS exchange consists of two messages: A request (KRB_TGS_REQ)
      from the client to the Kerberos Ticket-Granting Server, and a
      reply (KRB_TGS_REP or KRB_ERROR). The KRB_TGS_REQ message includes
      information authenticating the client plus a request for
      credentials. The authentication information consists of the
      authentication header (KRB_AP_REQ) which includes the client's
      previously obtained ticket-granting, renewable, or invalid ticket.
      In the ticket-granting ticket and proxy cases, the request MAY
      include one or more of: a list of network addresses, a collection
      of typed authorization data to be sealed in the ticket for
      authorization use by the application server, or additional tickets
      (the use of which are described later). The TGS reply
      (KRB_TGS_REP) contains the requested credentials, encrypted in the
      session key from the ticket-granting ticket or renewable ticket,
      or if present, in the sub-session key from the Authenticator (part
      of the authentication header). The KRB_ERROR message contains an
      error code and text explaining what went wrong. The KRB_ERROR
      message is not encrypted. The KRB_TGS_REP message contains
      information which can be used to detect replays, and to associate
      it with the message to which it replies. The KRB_ERROR message
      also contains information which can be used to associate it with
      the message to which it replies. The same comments about integrity
      protection of KRB_ERROR messages mentioned in section 3.1 apply to



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      the TGS exchange.

3.3.1. Generation of KRB_TGS_REQ message

      Before sending a request to the ticket-granting service, the
      client MUST determine in which realm the application server is
      believed to be registered [15]. If the client knows the service
      principal name and realm and it does not already possess a ticket-
      granting ticket for the appropriate realm, then one must be
      obtained. This is first attempted by requesting a ticket-granting
      ticket for the destination realm from a Kerberos server for which
      the client possesses a ticket-granting ticket (using the
      KRB_TGS_REQ message recursively). The Kerberos server MAY return a
      TGT for the desired realm in which case one can proceed.
      Alternatively, the Kerberos server MAY return a TGT for a realm
      which is 'closer' to the desired realm (further along the standard
      hierarchical path between the client's realm and the requested
      realm server's realm). It should be noted in this case that
      misconfiguration of the Kerberos servers may cause loops in the
      resulting authentication path, which the client should be careful
      to detect and avoid.

      If the Kerberos server returns a TGT for a 'closer' realm other
      than the desired realm, the client MAY use local policy
      configuration to verify that the authentication path used is an
      acceptable one.  Alternatively, a client MAY choose its own
      authentication path, rather than relying on the Kerberos server to
      select one. In either case, any policy or configuration
      information used to choose or validate authentication paths,
      whether by the Kerberos server or client, MUST be obtained from a
      trusted source.

      When a client obtains a ticket-granting ticket that is 'closer' to
      the destination realm, the client MAY cache this ticket and reuse
      it in future KRB-TGS exchanges with services in the 'closer'
      realm. However, if the client were to obtain a ticket-granting
      ticket for the 'closer' realm by starting at the initial KDC
      rather than as part of obtaining another ticket, then a shorter
      path to the 'closer' realm might be used. This shorter path may be
      desirable because fewer intermediate KDCs would know the session
      key of the ticket involved. For this reason, clients SHOULD
      evaluate whether they trust the realms transited in obtaining the
      'closer' ticket when making a decision to use the ticket in
      future.

      Once the client obtains a ticket-granting ticket for the
      appropriate realm, it determines which Kerberos servers serve that
      realm, and contacts one. The list might be obtained through a



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      configuration file or network service or it MAY be generated from
      the name of the realm; as long as the secret keys exchanged by
      realms are kept secret, only denial of service results from using
      a false Kerberos server.

       As in the AS exchange, the client MAY specify a number of options
      in the KRB_TGS_REQ message. One of these options is the ENC-TKT-
      IN-SKEY option used for user-to-user authentication. An overview
      of user-to-user authentication can be found in section 3.7. When
      generating the KRB_TGS_REQ message, this option indicates that the
      client is including a ticket-granting ticket obtained from the
      application server in the additional tickets field of the request
      and that the KDC SHOULD encrypt the ticket for the application
      server using the session key from this additional ticket, instead
      of using a server key from the principal database.

      The client prepares the KRB_TGS_REQ message, providing an
      authentication header as an element of the padata field, and
      including the same fields as used in the KRB_AS_REQ message along
      with several optional fields: the enc-authorizatfion-data field
      for application server use and additional tickets required by some
      options.

      In preparing the authentication header, the client can select a
      sub-session key under which the response from the Kerberos server
      will be encrypted [16]. If the sub-session key is not specified,
      the session key from the ticket-granting ticket will be used. If
      the enc-authorization-data is present, it MUST be encrypted in the
      sub-session key, if present, from the authenticator portion of the
      authentication header, or if not present, using the session key
      from the ticket-granting ticket.

      Once prepared, the message is sent to a Kerberos server for the
      destination realm.

3.3.2. Receipt of KRB_TGS_REQ message

      The KRB_TGS_REQ message is processed in a manner similar to the
      KRB_AS_REQ message, but there are many additional checks to be
      performed. First, the Kerberos server MUST determine which server
      the accompanying ticket is for and it MUST select the appropriate
      key to decrypt it. For a normal KRB_TGS_REQ message, it will be
      for the ticket granting service, and the TGS's key will be used.
      If the TGT was issued by another realm, then the appropriate
      inter-realm key MUST be used. If the accompanying ticket is not a
      ticket-granting ticket for the current realm, but is for an
      application server in the current realm, the RENEW, VALIDATE, or
      PROXY options are specified in the request, and the server for



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      which a ticket is requested is the server named in the
      accompanying ticket, then the KDC will decrypt the ticket in the
      authentication header using the key of the server for which it was
      issued. If no ticket can be found in the padata field, the
      KDC_ERR_PADATA_TYPE_NOSUPP error is returned.

      Once the accompanying ticket has been decrypted, the user-supplied
      checksum in the Authenticator MUST be verified against the
      contents of the request, and the message rejected if the checksums
      do not match (with an error code of KRB_AP_ERR_MODIFIED) or if the
      checksum is not collision-proof (with an error code of
      KRB_AP_ERR_INAPP_CKSUM). If the checksum type is not supported,
      the KDC_ERR_SUMTYPE_NOSUPP error is returned. If the
      authorization-data are present, they are decrypted using the sub-
      session key from the Authenticator.

      If any of the decryptions indicate failed integrity checks, the
      KRB_AP_ERR_BAD_INTEGRITY error is returned.

      As discussed in section 3.1.2, the KDC MUST send a valid
      KRB_TGS_REP message if it receives a KRB_TGS_REQ message identical
      to one it has recently processed. However, if the authenticator is
      a replay, but the rest of the request is not identical, then the
      KDC SHOULD return KRB_AP_ERR_REPEAT.

3.3.3. Generation of KRB_TGS_REP message

      The KRB_TGS_REP message shares its format with the KRB_AS_REP
      (KRB_KDC_REP), but with its type field set to KRB_TGS_REP. The
      detailed specification is in section 5.4.2.

      The response will include a ticket for the requested server or for
      a ticket granting server of an intermediate KDC to be contacted to
      obtain the requested ticket. The Kerberos database is queried to
      retrieve the record for the appropriate server (including the key
      with which the ticket will be encrypted). If the request is for a
      ticket-granting ticket for a remote realm, and if no key is shared
      with the requested realm, then the Kerberos server will select the
      realm 'closest' to the requested realm with which it does share a
      key, and use that realm instead.  This is the only case where the
      response for the KDC will be for a different server than that
      requested by the client.

      By default, the address field, the client's name and realm, the
      list of transited realms, the time of initial authentication, the
      expiration time, and the authorization data of the newly-issued
      ticket will be copied from the ticket-granting ticket (TGT) or
      renewable ticket. If the transited field needs to be updated, but



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      the transited type is not supported, the KDC_ERR_TRTYPE_NOSUPP
      error is returned.

      If the request specifies an endtime, then the endtime of the new
      ticket is set to the minimum of (a) that request, (b) the endtime
      from the TGT, and (c) the starttime of the TGT plus the minimum of
      the maximum life for the application server and the maximum life
      for the local realm (the maximum life for the requesting principal
      was already applied when the TGT was issued). If the new ticket is
      to be a renewal, then the endtime above is replaced by the minimum
      of (a) the value of the renew_till field of the ticket and (b) the
      starttime for the new ticket plus the life (endtime-starttime) of
      the old ticket.

      If the FORWARDED option has been requested, then the resulting
      ticket will contain the addresses specified by the client. This
      option will only be honored if the FORWARDABLE flag is set in the
      TGT. The PROXY option is similar; the resulting ticket will
      contain the addresses specified by the client. It will be honored
      only if the PROXIABLE flag in the TGT is set. The PROXY option
      will not be honored on requests for additional ticket-granting
      tickets.

      If the requested start time is absent, indicates a time in the
      past, or is within the window of acceptable clock skew for the KDC
      and the POSTDATE option has not been specified, then the start
      time of the ticket is set to the authentication server's current
      time. If it indicates a time in the future beyond the acceptable
      clock skew, but the POSTDATED option has not been specified or the
      MAY-POSTDATE flag is not set in the TGT, then the error
      KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the ticket-
      granting ticket has the MAY-POSTDATE flag set, then the resulting
      ticket will be postdated and the requested starttime is checked
      against the policy of the local realm. If acceptable, the ticket's
      start time is set as requested, and the INVALID flag is set. The
      postdated ticket MUST be validated before use by presenting it to
      the KDC after the starttime has been reached. However, in no case
      may the starttime, endtime, or renew-till time of a newly-issued
      postdated ticket extend beyond the renew-till time of the ticket-
      granting ticket.

      If the ENC-TKT-IN-SKEY option has been specified and an additional
      ticket has been included in the request, it indicates that the
      client is using user- to-user authentication to prove its identity
      to a server that does not have access to a persistent key. Section
      3.7 describes the affect of this option on the entire Kerberos
      protocol. When generating the KRB_TGS_REP message, this option in
      the KRB_TGS_REQ message tells the KDC to decrypt the additional



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      ticket using the key for the server to which the additional ticket
      was issued and verify that it is a ticket-granting ticket. If the
      name of the requested server is missing from the request, the name
      of the client in the additional ticket will be used. Otherwise the
      name of the requested server will be compared to the name of the
      client in the additional ticket and if different, the request will
      be rejected. If the request succeeds, the session key from the
      additional ticket will be used to encrypt the new ticket that is
      issued instead of using the key of the server for which the new
      ticket will be used.

      If the name of the server in the ticket that is presented to the
      KDC as part of the authentication header is not that of the
      ticket-granting server itself, the server is registered in the
      realm of the KDC, and the RENEW option is requested, then the KDC
      will verify that the RENEWABLE flag is set in the ticket, that the
      INVALID flag is not set in the ticket, and that the renew_till
      time is still in the future. If the VALIDATE option is requested,
      the KDC will check that the starttime has passed and the INVALID
      flag is set. If the PROXY option is requested, then the KDC will
      check that the PROXIABLE flag is set in the ticket. If the tests
      succeed, and the ticket passes the hotlist check described in the
      next section, the KDC will issue the appropriate new ticket.

      The ciphertext part of the response in the KRB_TGS_REP message is
      encrypted in the sub-session key from the Authenticator, if
      present, or the session key from the ticket-granting ticket. It is
      not encrypted using the client's secret key. Furthermore, the
      client's key's expiration date and the key version number fields
      are left out since these values are stored along with the client's
      database record, and that record is not needed to satisfy a
      request based on a ticket-granting ticket.

3.3.3.1. Checking for revoked tickets

      Whenever a request is made to the ticket-granting server, the
      presented ticket(s) is(are) checked against a hot-list of tickets
      which have been canceled. This hot-list might be implemented by
      storing a range of issue timestamps for 'suspect tickets'; if a
      presented ticket had an authtime in that range, it would be
      rejected. In this way, a stolen ticket-granting ticket or
      renewable ticket cannot be used to gain additional tickets
      (renewals or otherwise) once the theft has been reported to the
      KDC for the realm in which the server resides. Any normal ticket
      obtained before it was reported stolen will still be valid
      (because they require no interaction with the KDC), but only until
      their normal expiration time. If TGT's have been issued for cross-
      realm authentication, use of the cross-realm TGT will not be



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      affected unless the hot-list is propagated to the KDCs for the
      realms for which such cross-realm tickets were issued.

3.3.3.2. Encoding the transited field

      If the identity of the server in the TGT that is presented to the
      KDC as part of the authentication header is that of the ticket-
      granting service, but the TGT was issued from another realm, the
      KDC will look up the inter-realm key shared with that realm and
      use that key to decrypt the ticket. If the ticket is valid, then
      the KDC will honor the request, subject to the constraints
      outlined above in the section describing the AS exchange.  The
      realm part of the client's identity will be taken from the ticket-
      granting ticket. The name of the realm that issued the ticket-
      granting ticket, if it is not the realm of the client principal,
      will be added to the transited field of the ticket to be issued.
      This is accomplished by reading the transited field from the
      ticket-granting ticket (which is treated as an unordered set of
      realm names), adding the new realm to the set, then constructing
      and writing out its encoded (shorthand) form (this may involve a
      rearrangement of the existing encoding).

      Note that the ticket-granting service does not add the name of its
      own realm. Instead, its responsibility is to add the name of the
      previous realm.  This prevents a malicious Kerberos server from
      intentionally leaving out its own name (it could, however, omit
      other realms' names).

      The names of neither the local realm nor the principal's realm are
      to be included in the transited field. They appear elsewhere in
      the ticket and both are known to have taken part in authenticating
      the principal. Since the endpoints are not included, both local
      and single-hop inter-realm authentication result in a transited
      field that is empty.

      Because the name of each realm transited is added to this field,
      it might potentially be very long. To decrease the length of this
      field, its contents are encoded. The initially supported encoding
      is optimized for the normal case of inter-realm communication: a
      hierarchical arrangement of realms using either domain or X.500
      style realm names. This encoding (called DOMAIN-X500-COMPRESS) is
      now described.

      Realm names in the transited field are separated by a ",". The
      ",", "\", trailing "."s, and leading spaces (" ") are special
      characters, and if they are part of a realm name, they MUST be
      quoted in the transited field by preceding them with a "\".




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      A realm name ending with a "." is interpreted as being prepended
      to the previous realm. For example, we can encode traversal of
      EDU, MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU, and
      CS.WASHINGTON.EDU as:

         "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".

      Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points,
      that they would not be included in this field, and we would have:

         "EDU,MIT.,WASHINGTON.EDU"

      A realm name beginning with a "/" is interpreted as being appended
      to the previous realm.  For the purpose of appending, the realm
      preceding the first listed realm is considered to be the null
      realm ("").  If a realm name beginning with a "/" is to stand by
      itself, then it SHOULD be preceded by a space (" "). For example,
      we can encode traversal of /COM/HP/APOLLO, /COM/HP, /COM, and
      /COM/DEC as:

         "/COM,/HP,/APOLLO, /COM/DEC".

      Like the example above, if /COM/HP/APOLLO and /COM/DEC are
      endpoints, they would not be included in this field, and we would
      have:

         "/COM,/HP"

      A null subfield preceding or following a "," indicates that all
      realms between the previous realm and the next realm have been
      traversed.  For the purpose of interpreting null subfields, the
      client's realm is considered to precede those in the transited
      field, and the server's realm is considered to follow them.  Thus,
      "," means that all realms along the path between the client and
      the server have been traversed. ",EDU, /COM," means that all
      realms from the client's realm up to EDU (in a domain style
      hierarchy) have been traversed, and that everything from /COM down
      to the server's realm in an X.500 style has also been traversed.
      This could occur if the EDU realm in one hierarchy shares an
      inter-realm key directly with the /COM realm in another hierarchy.

3.3.4. Receipt of KRB_TGS_REP message

      When the KRB_TGS_REP is received by the client, it is processed in
      the same manner as the KRB_AS_REP processing described above. The
      primary difference is that the ciphertext part of the response
      must be decrypted using the sub-session key from the
      Authenticator, if it was specified in the request, or the session



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      key from the ticket-granting ticket, rather than the client's
      secret key. The server name returned in the reply is the true
      principal name of the service.

3.4. The KRB_SAFE Exchange

      The KRB_SAFE message MAY be used by clients requiring the ability
      to detect modifications of messages they exchange. It achieves
      this by including a keyed collision-proof checksum of the user
      data and some control information. The checksum is keyed with an
      encryption key (usually the last key negotiated via subkeys, or
      the session key if no negotiation has occurred).

3.4.1. Generation of a KRB_SAFE message

      When an application wishes to send a KRB_SAFE message, it collects
      its data and the appropriate control information and computes a
      checksum over them.  The checksum algorithm should be the keyed
      checksum mandated to be implemented along with the crypto system
      used for the sub-session or session key. The checksum is generated
      using the sub-session key if present or the session key. Some
      implementations use a different checksum algorithm for the
      KRB_SAFE messages but doing so in a interoperable manner is not
      always possible.

      The control information for the KRB_SAFE message includes both a
      timestamp and a sequence number. The designer of an application
      using the KRB_SAFE message MUST choose at least one of the two
      mechanisms. This choice SHOULD be based on the needs of the
      application protocol.

      Sequence numbers are useful when all messages sent will be
      received by one's peer. Connection state is presently required to
      maintain the session key, so maintaining the next sequence number
      should not present an additional problem.

      If the application protocol is expected to tolerate lost messages
      without them being resent, the use of the timestamp is the
      appropriate replay detection mechanism. Using timestamps is also
      the appropriate mechanism for multi-cast protocols where all of
      one's peers share a common sub-session key, but some messages will
      be sent to a subset of one's peers.

      After computing the checksum, the client then transmits the
      information and checksum to the recipient in the message format
      specified in section 5.6.1.

3.4.2. Receipt of KRB_SAFE message



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      When an application receives a KRB_SAFE message, it verifies it as
      follows.  If any error occurs, an error code is reported for use
      by the application.

      The message is first checked by verifying that the protocol
      version and type fields match the current version and KRB_SAFE,
      respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or
      KRB_AP_ERR_MSG_TYPE error. The application verifies that the
      checksum used is a collision-proof keyed checksum that uses keys
      compatible with the sub-session or session key as appropriate (or
      with the application key derived from the session or sub-session
      keys), and if it is not, a KRB_AP_ERR_INAPP_CKSUM error is
      generated.  The sender's address MUST be included in the control
      information; the recipient verifies that the operating system's
      report of the sender's address matches the sender's address in the
      message, and (if a recipient address is specified or the recipient
      requires an address) that one of the recipient's addresses appears
      as the recipient's address in the message. To work with network
      address translation, senders MAY use the directional address type
      specified in section 8.1 for the sender address and not include
      recipient addresses. A failed match for either case generates a
      KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the
      sequence number fields are checked. If timestamp and usec are
      expected and not present, or they are present but not current, the
      KRB_AP_ERR_SKEW error is generated. Timestamps are not required to
      be strictly ordered; they are only required to be in the skew
      window.  If the server name, along with the client name, time and
      microsecond fields from the Authenticator match any recently-seen
      (sent or received) such tuples, the KRB_AP_ERR_REPEAT error is
      generated. If an incorrect sequence number is included, or a
      sequence number is expected but not present, the
      KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp
      and usec or a sequence number is present, a KRB_AP_ERR_MODIFIED
      error is generated. Finally, the checksum is computed over the
      data and control information, and if it doesn't match the received
      checksum, a KRB_AP_ERR_MODIFIED error is generated.

      If all the checks succeed, the application is assured that the
      message was generated by its peer and was not modified in transit.

      Implementations SHOULD accept any checksum algorithm they
      implement that both have adequate security and that have keys
      compatible with the sub-session or session key. Unkeyed or non-
      collision-proof checksums are not suitable for this use.

3.5. The KRB_PRIV Exchange

      The KRB_PRIV message MAY be used by clients requiring



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      confidentiality and the ability to detect modifications of
      exchanged messages. It achieves this by encrypting the messages
      and adding control information.

3.5.1. Generation of a KRB_PRIV message

      When an application wishes to send a KRB_PRIV message, it collects
      its data and the appropriate control information (specified in
      section 5.7.1) and encrypts them under an encryption key (usually
      the last key negotiated via subkeys, or the session key if no
      negotiation has occurred). As part of the control information, the
      client MUST choose to use either a timestamp or a sequence number
      (or both); see the discussion in section 3.4.1 for guidelines on
      which to use. After the user data and control information are
      encrypted, the client transmits the ciphertext and some 'envelope'
      information to the recipient.

3.5.2. Receipt of KRB_PRIV message

      When an application receives a KRB_PRIV message, it verifies it as
      follows.  If any error occurs, an error code is reported for use
      by the application.

      The message is first checked by verifying that the protocol
      version and type fields match the current version and KRB_PRIV,
      respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or
      KRB_AP_ERR_MSG_TYPE error. The application then decrypts the
      ciphertext and processes the resultant plaintext. If decryption
      shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY
      error is generated.

      The sender's address MUST be included in the control information;
      the recipient verifies that the operating system's report of the
      sender's address matches the sender's address in the message.  If
      a recipient address is specified or the recipient requires an
      address then one of the recipient's addresses MUST also appear as
      the recipient's address in the message.  Where a sender's or
      receiver's address might not otherwise match the address in a
      message because of network address translation, an application MAY
      be written to use addresses of the directional address type in
      place of the actual network address.

      A failed match for either case generates a KRB_AP_ERR_BADADDR
      error. To work with network address translation, implementations
      MAY use the directional address type defined in section 7.1 for
      the sender address and include no recipient address.

      Then the timestamp and usec and/or the sequence number fields are



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      checked. If timestamp and usec are expected and not present, or
      they are present but not current, the KRB_AP_ERR_SKEW error is
      generated. If the server name, along with the client name, time
      and microsecond fields from the Authenticator match any recently-
      seen such tuples, the KRB_AP_ERR_REPEAT error is generated. If an
      incorrect sequence number is included, or a sequence number is
      expected but not present, the KRB_AP_ERR_BADORDER error is
      generated. If neither a time-stamp and usec or a sequence number
      is present, a KRB_AP_ERR_MODIFIED error is generated.

      If all the checks succeed, the application can assume the message
      was generated by its peer, and was securely transmitted (without
      intruders able to see the unencrypted contents).

3.6. The KRB_CRED Exchange

      The KRB_CRED message MAY be used by clients requiring the ability
      to send Kerberos credentials from one host to another. It achieves
      this by sending the tickets together with encrypted data
      containing the session keys and other information associated with
      the tickets.

3.6.1. Generation of a KRB_CRED message

      When an application wishes to send a KRB_CRED message it first
      (using the KRB_TGS exchange) obtains credentials to be sent to the
      remote host. It then constructs a KRB_CRED message using the
      ticket or tickets so obtained, placing the session key needed to
      use each ticket in the key field of the corresponding KrbCredInfo
      sequence of the encrypted part of the KRB_CRED message.

      Other information associated with each ticket and obtained during
      the KRB_TGS exchange is also placed in the corresponding
      KrbCredInfo sequence in the encrypted part of the KRB_CRED
      message. The current time and, if specifically required by the
      application the nonce, s-address, and r-address fields, are placed
      in the encrypted part of the KRB_CRED message which is then
      encrypted under an encryption key previously exchanged in the
      KRB_AP exchange (usually the last key negotiated via subkeys, or
      the session key if no negotiation has occurred).

      Implementation note: When constructing a KRB_CRED message for
      inclusion in a GSSAPI initial context token, the MIT
      implementation of Kerberos will not encrypt the KRB_CRED message
      if the session key is a DES or triple DES key.  For
      interoperability with MIT, the Microsoft implementation will not
      encrypt the KRB_CRED in a GSSAPI token if it is using a DES
      session key. Starting at version 1.2.5, MIT Kerberos can receive



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      and decode either encrypted or unencrypted KRB_CRED tokens in the
      GSSAPI exchange. The Heimdal implementation of Kerberos can also
      accept either encrypted or unencrypted KRB_CRED messages. Since
      the KRB_CRED message in a GSSAPI token is encrypted in the
      authenticator, the MIT behavior does not present a security
      problem, although it is a violation of the Kerberos specification.

3.6.2. Receipt of KRB_CRED message

      When an application receives a KRB_CRED message, it verifies it.
      If any error occurs, an error code is reported for use by the
      application. The message is verified by checking that the protocol
      version and type fields match the current version and KRB_CRED,
      respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or
      KRB_AP_ERR_MSG_TYPE error. The application then decrypts the
      ciphertext and processes the resultant plaintext. If decryption
      shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY
      error is generated.

      If present or required, the recipient MAY verify that the
      operating system's report of the sender's address matches the
      sender's address in the message, and that one of the recipient's
      addresses appears as the recipient's address in the message. The
      address check does not provide any added security, since the
      address if present has already been checked in the KRB_AP_REQ
      message and there is not any benefit to be gained by an attacker
      in reflecting a KRB_CRED message back to its originator. Thus, the
      recipient MAY ignore the address even if present in order to work
      better in NAT environments. A failed match for either case
      generates a KRB_AP_ERR_BADADDR error. Recipients MAY skip the
      address check as the KRB_CRED message cannot generally be
      reflected back to the originator.  The timestamp and usec fields
      (and the nonce field if required) are checked next. If the
      timestamp and usec are not present, or they are present but not
      current, the KRB_AP_ERR_SKEW error is generated.

      If all the checks succeed, the application stores each of the new
      tickets in its credentials cache together with the session key and
      other information in the corresponding KrbCredInfo sequence from
      the encrypted part of the KRB_CRED message.

3.7. User-to-User Authentication Exchanges

      User-to-User authentication provides a method to perform
      authentication when the verifier does not have a access to long
      term service key. This might be the case when running a server
      (for example a window server) as a user on a workstation. In such
      cases, the server may have access to the ticket-granting ticket



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      obtained when the user logged in to the workstation, but because
      the server is running as an unprivileged user it might not have
      access to system keys. Similar situations may arise when running
      peer-to-peer applications.

                                Summary
          Message direction                    Message type     Sections
          0. Message from application server   Not Specified
          1. Client to Kerberos                KRB_TGS_REQ      3.3 + 5.4.1
          2. Kerberos to client                KRB_TGS_REP or   3.3 + 5.4.2
                                               KRB_ERROR        5.9.1
          3. Client to Application server      KRB_AP_REQ       3.2 + 5.5.1

      To address this problem, the Kerberos protocol allows the client
      to request that the ticket issued by the KDC be encrypted using a
      session key from a ticket-granting ticket issued to the party that
      will verify the authentication.  This ticket-granting ticket must
      be obtained from the verifier by means of an exchange external to
      the Kerberos protocol, usually as part of the application
      protocol. This message is shown in the summary above as message 0.
      Note that because the ticket-granting ticket is encrypted in the
      KDC's secret key, it can not be used for authentication without
      possession of the corresponding secret key.  Furthermore, because
      the verifier does not reveal the corresponding secret key,
      providing a copy of the verifier's ticket-granting ticket does not
      allow impersonation of the verifier.

      Message 0 in the table above represents an application specific
      negotiation between the client and server, at the end of which
      both have determined that they will use user-to-user
      authentication and the client has obtained the server's TGT.

      Next, the client includes the server's TGT as an additional ticket
      in its KRB_TGS_REQ request to the KDC (message 1 in the table
      above) and specifies the ENC-TKT-IN-SKEY option in its request.

      If validated according to the instructions in 3.3.3, the
      application ticket returned to the client (message 2 in the table
      above) will be encrypted using the session key from the additional
      ticket and the client will note this when it uses or stores the
      application ticket.

      When contacting the server using a ticket obtained for user-to-
      user authentication (message 3 in the table above), the client
      MUST specify the USE-SESSION-KEY flag in the ap-options field.
      This tells the application server to use the session key
      associated with its ticket-granting ticket to decrypt the server
      ticket provided in the application request.



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4. Encryption and Checksum Specifications

      The Kerberos protocols described in this document are designed to
      encrypt messages of arbitrary sizes, using stream or block
      encryption ciphers.  Encryption is used to prove the identities of
      the network entities participating in message exchanges. The Key
      Distribution Center for each realm is trusted by all principals
      registered in that realm to store a secret key in confidence.
      Proof of knowledge of this secret key is used to verify the
      authenticity of a principal.

      The KDC uses the principal's secret key (in the AS exchange) or a
      shared session key (in the TGS exchange) to encrypt responses to
      ticket requests; the ability to obtain the secret key or session
      key implies the knowledge of the appropriate keys and the identity
      of the KDC. The ability of a principal to decrypt the KDC response
      and present a Ticket and a properly formed Authenticator
      (generated with the session key from the KDC response) to a
      service verifies the identity of the principal; likewise the
      ability of the service to extract the session key from the Ticket
      and prove its knowledge thereof in a response verifies the
      identity of the service.

      [@KCRYPTO] defines a framework for defining encryption and
      checksum mechanisms for use with Kerberos. It also defines several
      such mechanisms, and more may be added in future updates to that
      document.

      The string-to-key operation provided by [@KCRYPTO] is used to
      produce a long-term key for a principal (generally for a user).
      The default salt string, if none is provided via pre-
      authentication data, is the concatenation of the principal's realm
      and name components, in order, with no separators.  Unless
      otherwise indicated, the default string-to-key opaque parameter
      set as defined in [@KCRYPTO] is used.

      Encrypted data, keys and checksums are transmitted using the
      EncryptedData, EncryptionKey and Checksum data objects defined in
      section 5.2.9. The encryption, decryption, and checksum operations
      described in this document use the corresponding encryption,
      decryption, and get_mic operations described in [@KCRYPTO], with
      implicit "specific key" generation using the "key usage" values
      specified in the description of each EncryptedData or Checksum
      object to vary the key for each operation. Note that in some
      cases, the value to be used is dependent on the method of choosing
      the key or the context of the message.

      Key usages are unsigned 32 bit integers; zero is not permitted.



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      The key usage values for encrypting or checksumming Kerberos
      messages are indicated in section 5 along with the message
      definitions. Key usage values 512-1023 are reserved for uses
      internal to a Kerberos implementation. (For example, seeding a
      pseudo-random number generator with a value produced by encrypting
      something with a session key and a key usage value not used for
      any other purpose.) Key usage values between 1024 and 2047
      (inclusive) are reserved for application use; applications SHOULD
      use even values for encryption and odd values for checksums within
      this range. Key usage values are also summarized in a table in
      section 7.5.1.

      There might exist other documents which define protocols in terms
      of the RFC1510 encryption types or checksum types. Such documents
      would not know about key usages. In order that these
      specifications continue to be meaningful until they are updated,
      if no key usage values are specified then key usages 1024 and 1025
      must be used to derive keys for encryption and checksums,
      respectively (this does not apply to protocols that do their own
      encryption independent of this framework, directly using the key
      resulting from the Kerberos authentication exchange.) New
      protocols defined in terms of the Kerberos encryption and checksum
      types SHOULD use their own key usage values.

      Unless otherwise indicated, no cipher state chaining is done from
      one encryption operation to another.

      Implementation note: While not recommended, some application
      protocols will continue to use the key data directly, even if only
      in currently existing protocol specifications. An implementation
      intended to support general Kerberos applications may therefore
      need to make key data available, as well as the attributes and
      operations described in [@KCRYPTO].  One of the more common
      reasons for directly performing encryption is direct control over
      negotiation and selection of a "sufficiently strong" encryption
      algorithm (in the context of a given application). While Kerberos
      does not directly provide a facility for negotiating encryption
      types between the application client and server, there are
      approaches for using Kerberos to facilitate this negotiation - for
      example, a client may request only "sufficiently strong" session
      key types from the KDC and expect that any type returned by the
      KDC will be understood and supported by the application server.

5. Message Specifications

      NOTE: The ASN.1 collected here should be identical to the contents
      of Appendix A. In case of conflict, the contents of Appendix A
      shall take precedence.



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      The Kerberos protocol is defined here in terms of Abstract Syntax
      Notation One (ASN.1) [X680], which provides a syntax for
      specifying both the abstract layout of protocol messages as well
      as their encodings. Implementors not utilizing an existing ASN.1
      compiler or support library are cautioned to thoroughly understand
      the actual ASN.1 specification to ensure correct implementation
      behavior, as there is more complexity in the notation than is
      immediately obvious, and some tutorials and guides to ASN.1 are
      misleading or erroneous.

      Note that in several places, there have been changes here from RFC
      1510 that change the abstract types. This is in part to address
      widespread assumptions that various implementors have made, in
      some cases resulting in unintentional violations of the ASN.1
      standard. These are clearly flagged where they occur. The
      differences between the abstract types in RFC 1510 and abstract
      types in this document can cause incompatible encodings to be
      emitted when certain encoding rules, e.g. the Packed Encoding
      Rules (PER), are used. This theoretical incompatibility should not
      be relevant for Kerberos, since Kerberos explicitly specifies the
      use of the Distinguished Encoding Rules (DER). It might be an
      issue for protocols wishing to use Kerberos types with other
      encoding rules. (This practice is not recommended.) With very few
      exceptions (most notably the usages of BIT STRING), the encodings
      resulting from using the DER remain identical between the types
      defined in RFC 1510 and the types defined in this document.

      The type definitions in this section assume an ASN.1 module
      definition of the following form:

      KerberosV5Spec2 {
              iso(1) identified-organization(3) dod(6) internet(1)
              security(5) kerberosV5(2) modules(4) krb5spec2(2)
      } DEFINITIONS EXPLICIT TAGS ::= BEGIN

      -- rest of definitions here

      END

      This specifies that the tagging context for the module will be
      explicit and non-automatic.

      Note that in some other publications [RFC1510] [RFC1964], the
      "dod" portion of the object identifier is erroneously specified as
      having the value "5".  In the case of RFC 1964, use of the
      "correct" OID value would result in a change in the wire protocol;
      therefore, it remains unchanged for now.




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      Note that elsewhere in this document, nomenclature for various
      message types is inconsistent, but largely follows C language
      conventions, including use of underscore (_) characters and all-
      caps spelling of names intended to be numeric constants. Also, in
      some places, identifiers (especially ones referring to constants)
      are written in all-caps in order to distinguish them from
      surrounding explanatory text.

      The ASN.1 notation does not permit underscores in identifiers, so
      in actual ASN.1 definitions, underscores are replaced with hyphens
      (-). Additionally, structure member names and defined values in
      ASN.1 MUST begin with a lowercase letter, while type names MUST
      begin with an uppercase letter.

5.1. Specific Compatibility Notes on ASN.1

      For compatibility purposes, implementors should heed the following
      specific notes regarding the use of ASN.1 in Kerberos. These notes
      do not describe deviations from standard usage of ASN.1. The
      purpose of these notes is to instead describe some historical
      quirks and non-compliance of various implementations, as well as
      historical ambiguities, which, while being valid ASN.1, can lead
      to confusion during implementation.

5.1.1. ASN.1 Distinguished Encoding Rules

      The encoding of Kerberos protocol messages shall obey the
      Distinguished Encoding Rules (DER) of ASN.1 as described in
      [X690]. Some implementations (believed to be primarily ones
      derived from DCE 1.1 and earlier) are known to use the more
      general Basic Encoding Rules (BER); in particular, these
      implementations send indefinite encodings of lengths.
      Implementations MAY accept such encodings in the interests of
      backwards compatibility, though implementors are warned that
      decoding fully-general BER is fraught with peril.

5.1.2. Optional Integer Fields

      Some implementations do not internally distinguish between an
      omitted optional integer value and a transmitted value of zero.
      The places in the protocol where this is relevant include various
      microseconds fields, nonces, and sequence numbers. Implementations
      SHOULD treat omitted optional integer values as having been
      transmitted with a value of zero, if the application is expecting
      this.

5.1.3. Empty SEQUENCE OF Types




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      There are places in the protocol where a message contains a
      SEQUENCE OF type as an optional member. This can result in an
      encoding that contains an empty SEQUENCE OF encoding. The Kerberos
      protocol does not semantically distinguish between an absent
      optional SEQUENCE OF type and a present optional but empty
      SEQUENCE OF type. Implementations SHOULD NOT send empty SEQUENCE
      OF encodings that are marked OPTIONAL, but SHOULD accept them as
      being equivalent to an omitted OPTIONAL type. In the ASN.1 syntax
      describing Kerberos messages, instances of these problematic
      optional SEQUENCE OF types are indicated with a comment.

5.1.4. Unrecognized Tag Numbers

      Future revisions to this protocol may include new message types
      with different APPLICATION class tag numbers. Such revisions
      should protect older implementations by only sending the message
      types to parties that are known to understand them, e.g. by means
      of a flag bit set by the receiver in a preceding request. In the
      interest of robust error handling, implementations SHOULD
      gracefully handle receiving a message with an unrecognized tag
      anyway, and return an error message if appropriate.

      In particular, KDCs SHOULD return KRB_AP_ERR_MSG_TYPE if the
      incorrect tag is sent over a TCP transport.  The KDCs SHOULD NOT
      respond to messages received with an unknown tag over UDP
      transport in order to avoid denial of service attacks.  For non-
      KDC applications, the Kerberos implementation typically indicates
      an error to the application which takes appropriate steps based on
      the application protocol.

5.1.5. Tag Numbers Greater Than 30

      A naive implementation of a DER ASN.1 decoder may experience
      problems with ASN.1 tag numbers greater than 30, due to such tag
      numbers being encoded using more than one byte. Future revisions
      of this protocol may utilize tag numbers greater than 30, and
      implementations SHOULD be prepared to gracefully return an error,
      if appropriate, if they do not recognize the tag.

5.2. Basic Kerberos Types

      This section defines a number of basic types that are potentially
      used in multiple Kerberos protocol messages.

5.2.1. KerberosString

      The original specification of the Kerberos protocol in RFC 1510
      uses GeneralString in numerous places for human-readable string



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      data. Historical implementations of Kerberos cannot utilize the
      full power of GeneralString.  This ASN.1 type requires the use of
      designation and invocation escape sequences as specified in
      ISO-2022/ECMA-35 [ISO-2022/ECMA-35] to switch character sets, and
      the default character set that is designated as G0 is the
      ISO-646/ECMA-6 [ISO-646,ECMA-6] International Reference Version
      (IRV) (aka U.S. ASCII), which mostly works.

      ISO-2022/ECMA-35 defines four character-set code elements (G0..G3)
      and two Control-function code elements (C0..C1). DER prohibits the
      designation of character sets as any but the G0 and C0 sets.
      Unfortunately, this seems to have the side effect of prohibiting
      the use of ISO-8859 (ISO Latin) [ISO-8859] character-sets or any
      other character-sets that utilize a 96-character set, since it is
      prohibited by ISO-2022/ECMA-35 to designate them as the G0 code
      element. This side effect is being investigated in the ASN.1
      standards community.

      In practice, many implementations treat GeneralStrings as if they
      were 8-bit strings of whichever character set the implementation
      defaults to, without regard for correct usage of character-set
      designation escape sequences. The default character set is often
      determined by the current user's operating system dependent
      locale. At least one major implementation places unescaped UTF-8
      encoded Unicode characters in the GeneralString. This failure to
      adhere to the GeneralString specifications results in
      interoperability issues when conflicting character encodings are
      utilized by the Kerberos clients, services, and KDC.

      This unfortunate situation is the result of improper documentation
      of the restrictions of the ASN.1 GeneralString type in prior
      Kerberos specifications.

      The new (post-RFC 1510) type KerberosString, defined below, is a
      GeneralString that is constrained to only contain characters in
      IA5String

         KerberosString  ::= GeneralString (IA5String)

      In general, US-ASCII control characters should not be used in
      KerberosString. Control characters SHOULD NOT be used in principal
      names or realm names.

      For compatibility, implementations MAY choose to accept
      GeneralString values that contain characters other than those
      permitted by IA5String, but they should be aware that character
      set designation codes will likely be absent, and that the encoding
      should probably be treated as locale-specific in almost every way.



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      Implementations MAY also choose to emit GeneralString values that
      are beyond those permitted by IA5String, but should be aware that
      doing so is extraordinarily risky from an interoperability
      perspective.

      Some existing implementations use GeneralString to encode
      unescaped locale-specific characters. This is a violation of the
      ASN.1 standard. Most of these implementations encode US-ASCII in
      the left-hand half, so as long the implementation transmits only
      US-ASCII, the ASN.1 standard is not violated in this regard. As
      soon as such an implementation encodes unescaped locale-specific
      characters with the high bit set, it violates the ASN.1 standard.

      Other implementations have been known to use GeneralString to
      contain a UTF-8 encoding. This also violates the ASN.1 standard,
      since UTF-8 is a different encoding, not a 94 or 96 character "G"
      set as defined by ISO 2022.  It is believed that these
      implementations do not even use the ISO 2022 escape sequence to
      change the character encoding. Even if implementations were to
      announce the change of encoding by using that escape sequence, the
      ASN.1 standard prohibits the use of any escape sequences other
      than those used to designate/invoke "G" or "C" sets allowed by
      GeneralString.

      Future revisions to this protocol will almost certainly allow for
      a more interoperable representation of principal names, probably
      including UTF8String.

      Note that applying a new constraint to a previously unconstrained
      type constitutes creation of a new ASN.1 type. In this particular
      case, the change does not result in a changed encoding under DER.

5.2.2. Realm and PrincipalName

      Realm           ::= KerberosString

      PrincipalName   ::= SEQUENCE {
              name-type       [0] Int32,
              name-string     [1] SEQUENCE OF KerberosString
      }

      Kerberos realm names are encoded as KerberosStrings. Realms shall
      not contain a character with the code 0 (the US-ASCII NUL). Most
      realms will usually consist of several components separated by
      periods (.), in the style of Internet Domain Names, or separated
      by slashes (/) in the style of X.500 names. Acceptable forms for
      realm names are specified in section 6.1.. A PrincipalName is a
      typed sequence of components consisting of the following sub-



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      fields:

   name-type
      This field specifies the type of name that follows. Pre-defined
      values for this field are specified in section 6.2. The name-type
      SHOULD be treated as a hint. Ignoring the name type, no two names
      can be the same (i.e. at least one of the components, or the
      realm, must be different).

   name-string
      This field encodes a sequence of components that form a name, each
      component encoded as a KerberosString. Taken together, a
      PrincipalName and a Realm form a principal identifier. Most
      PrincipalNames will have only a few components (typically one or
      two).

5.2.3. KerberosTime

      KerberosTime    ::= GeneralizedTime -- with no fractional seconds

      The timestamps used in Kerberos are encoded as GeneralizedTimes. A
      KerberosTime value shall not include any fractional portions of
      the seconds.  As required by the DER, it further shall not include
      any separators, and it shall specify the UTC time zone (Z).
      Example: The only valid format for UTC time 6 minutes, 27 seconds
      after 9 pm on 6 November 1985 is 19851106210627Z.

5.2.4. Constrained Integer types

      Some integer members of types SHOULD be constrained to values
      representable in 32 bits, for compatibility with reasonable
      implementation limits.

      Int32           ::= INTEGER (-2147483648..2147483647)
                          -- signed values representable in 32 bits

      UInt32          ::= INTEGER (0..4294967295)
                          -- unsigned 32 bit values

      Microseconds    ::= INTEGER (0..999999)
                          -- microseconds

      While this results in changes to the abstract types from the RFC
      1510 version, the encoding in DER should be unaltered. Historical
      implementations were typically limited to 32-bit integer values
      anyway, and assigned numbers SHOULD fall in the space of integer
      values representable in 32 bits in order to promote
      interoperability anyway.



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      There are several integer fields in messages that are constrained
      to fixed values.

   pvno
      also TKT-VNO or AUTHENTICATOR-VNO, this recurring field is always
      the constant integer 5. There is no easy way to make this field
      into a useful protocol version number, so its value is fixed.

   msg-type
      this integer field is usually identical to the application tag
      number of the containing message type.

5.2.5. HostAddress and HostAddresses

      HostAddress     ::= SEQUENCE  {
              addr-type       [0] Int32,
              address         [1] OCTET STRING
      }

      -- NOTE: HostAddresses is always used as an OPTIONAL field and
      -- should not be empty.
      HostAddresses   -- NOTE: subtly different from rfc1510,
                      -- but has a value mapping and encodes the same
              ::= SEQUENCE OF HostAddress

      The host address encodings consists of two fields:

   addr-type
      This field specifies the type of address that follows. Pre-defined
      values for this field are specified in section 7.5.3.

   address
      This field encodes a single address of type addr-type.

5.2.6. AuthorizationData

      -- NOTE: AuthorizationData is always used as an OPTIONAL field and
      -- should not be empty.
      AuthorizationData       ::= SEQUENCE OF SEQUENCE {
              ad-type         [0] Int32,
              ad-data         [1] OCTET STRING
      }

   ad-data
      This field contains authorization data to be interpreted according
      to the value of the corresponding ad-type field.

   ad-type



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      This field specifies the format for the ad-data subfield. All
      negative values are reserved for local use. Non-negative values
      are reserved for registered use.

   Each sequence of type and data is referred to as an authorization
   element.  Elements MAY be application specific, however, there is a
   common set of recursive elements that should be understood by all
   implementations. These elements contain other elements embedded
   within them, and the interpretation of the encapsulating element
   determines which of the embedded elements must be interpreted, and
   which may be ignored.

   These common authorization data elements are recursively defined,
   meaning the ad-data for these types will itself contain a sequence of
   authorization data whose interpretation is affected by the
   encapsulating element. Depending on the meaning of the encapsulating
   element, the encapsulated elements may be ignored, might be
   interpreted as issued directly by the KDC, or they might be stored in
   a separate plaintext part of the ticket. The types of the
   encapsulating elements are specified as part of the Kerberos
   specification because the behavior based on these values should be
   understood across implementations whereas other elements need only be
   understood by the applications which they affect.

   Authorization data elements are considered critical if present in a
   ticket or authenticator. Unless encapsulated in a known authorization
   data element amending the criticality of the elements it contains, if
   an unknown authorization data element type is received by a server
   either in an AP-REQ or in a ticket contained in an AP-REQ, then
   authentication MUST fail.  Authorization data is intended to restrict
   the use of a ticket. If the service cannot determine whether the
   restriction applies to that service then a security weakness may
   result if the ticket can be used for that service. Authorization
   elements that are optional can be enclosed in AD-IF-RELEVANT element.

   In the definitions that follow, the value of the ad-type for the
   element will be specified as the least significant part of the
   subsection number, and the value of the ad-data will be as shown in
   the ASN.1 structure that follows the subsection heading.

             contents of ad-data          ad-type

    DER encoding of AD-IF-RELEVANT        1

    DER encoding of AD-KDCIssued          4

    DER encoding of AD-AND-OR             5




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    DER encoding of AD-MANDATORY-FOR-KDC  8

5.2.6.1. IF-RELEVANT

      AD-IF-RELEVANT          ::= AuthorizationData

      AD elements encapsulated within the if-relevant element are
      intended for interpretation only by application servers that
      understand the particular ad-type of the embedded element.
      Application servers that do not understand the type of an element
      embedded within the if-relevant element MAY ignore the
      uninterpretable element. This element promotes interoperability
      across implementations which may have local extensions for
      authorization.  The ad-type for AD-IF-RELEVANT is (1).

5.2.6.2. KDCIssued

      AD-KDCIssued            ::= SEQUENCE {
              ad-checksum     [0] Checksum,
              i-realm         [1] Realm OPTIONAL,
              i-sname         [2] PrincipalName OPTIONAL,
              elements        [3] AuthorizationData
      }

   ad-checksum
      A cryptographic checksum computed over the DER encoding of the
      AuthorizationData in the "elements" field, keyed with the session
      key.  Its checksumtype is the mandatory checksum type for the
      encryption type of the session key, and its key usage value is 19.

   i-realm, i-sname
      The name of the issuing principal if different from the KDC
      itself.  This field would be used when the KDC can verify the
      authenticity of elements signed by the issuing principal and it
      allows this KDC to notify the application server of the validity
      of those elements.

   elements
      A sequence of authorization data elements issued by the KDC.

   The KDC-issued ad-data field is intended to provide a means for
   Kerberos principal credentials to embed within themselves privilege
   attributes and other mechanisms for positive authorization,
   amplifying the privileges of the principal beyond what can be done
   using a credentials without such an a-data element.

   This can not be provided without this element because the definition
   of the authorization-data field allows elements to be added at will



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   by the bearer of a TGT at the time that they request service tickets
   and elements may also be added to a delegated ticket by inclusion in
   the authenticator.

   For KDC-issued elements this is prevented because the elements are
   signed by the KDC by including a checksum encrypted using the
   server's key (the same key used to encrypt the ticket - or a key
   derived from that key). Elements encapsulated with in the KDC-issued
   element MUST  be ignored by the application server if this
   "signature" is not present. Further, elements encapsulated within
   this element from a ticket-granting ticket MAY be interpreted by the
   KDC, and used as a basis according to policy for including new signed
   elements within derivative tickets, but they will not be copied to a
   derivative ticket directly. If they are copied directly to a
   derivative ticket by a KDC that is not aware of this element, the
   signature will not be correct for the application ticket elements,
   and the field will be ignored by the application server.

   This element and the elements it encapsulates MAY be safely ignored
   by applications, application servers, and KDCs that do not implement
   this element.

   The ad-type for AD-KDC-ISSUED is (4).

5.2.6.3. AND-OR

      AD-AND-OR               ::= SEQUENCE {
              condition-count [0] INTEGER,
              elements        [1] AuthorizationData
      }


      When restrictive AD elements are encapsulated within the and-or
      element, the and-or element is considered satisfied if and only if
      at least the number of encapsulated elements specified in
      condition-count are satisfied.  Therefore, this element MAY be
      used to implement an "or" operation by setting the condition-count
      field to 1, and it MAY specify an "and" operation by setting the
      condition count to the number of embedded elements. Application
      servers that do not implement this element MUST reject tickets
      that contain authorization data elements of this type.

      The ad-type for AD-AND-OR is (5).

5.2.6.4. MANDATORY-FOR-KDC

      AD-MANDATORY-FOR-KDC    ::= AuthorizationData




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      AD elements encapsulated within the mandatory-for-kdc element are
      to be interpreted by the KDC. KDCs that do not understand the type
      of an element embedded within the mandatory-for-kdc element MUST
      reject the request.

      The ad-type for AD-MANDATORY-FOR-KDC is (8).

5.2.7. PA-DATA

      Historically, PA-DATA have been known as "pre-authentication
      data", meaning that they were used to augment the initial
      authentication with the KDC.  Since that time, they have also been
      used as a typed hole with which to extend protocol exchanges with
      the KDC.

      PA-DATA         ::= SEQUENCE {
              -- NOTE: first tag is [1], not [0]
              padata-type     [1] Int32,
              padata-value    [2] OCTET STRING -- might be encoded AP-REQ
      }

   padata-type
      indicates the way that the padata-value element is to be
      interpreted.  Negative values of padata-type are reserved for
      unregistered use; non-negative values are used for a registered
      interpretation of the element type.

   padata-value
      Usually contains the DER encoding of another type; the padata-type
      field identifies which type is encoded here.

       padata-type        name           contents of padata-value

       1            pa-tgs-req       DER encoding of AP-REQ

       2            pa-enc-timestamp DER encoding of PA-ENC-TIMESTAMP

       3            pa-pw-salt       salt (not ASN.1 encoded)

       11           pa-etype-info    DER encoding of ETYPE-INFO

       19           pa-etype-info2   DER encoding of ETYPE-INFO2

      This field MAY also contain information needed by certain
      extensions to the Kerberos protocol. For example, it might be used
      to initially verify the identity of a client before any response
      is returned.




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      The padata field can also contain information needed to help the
      KDC or the client select the key needed for generating or
      decrypting the response. This form of the padata is useful for
      supporting the use of certain token cards with Kerberos. The
      details of such extensions are specified in separate documents.
      See [Pat92] for additional uses of this field.

5.2.7.1. PA-TGS-REQ

      In the case of requests for additional tickets (KRB_TGS_REQ),
      padata-value will contain an encoded AP-REQ. The checksum in the
      authenticator (which MUST be collision-proof) is to be computed
      over the KDC-REQ-BODY encoding.

5.2.7.2. Encrypted Timestamp Pre-authentication

      There are pre-authentication types that may be used to pre-
      authenticate a client by means of an encrypted timestamp.

      PA-ENC-TIMESTAMP        ::= EncryptedData -- PA-ENC-TS-ENC

      PA-ENC-TS-ENC           ::= SEQUENCE {
              patimestamp     [0] KerberosTime -- client's time --,
              pausec          [1] Microseconds OPTIONAL
      }

      Patimestamp contains the client's time, and pausec contains the
      microseconds, which MAY be omitted if a client will not generate
      more than one request per second. The ciphertext (padata-value)
      consists of the PA-ENC-TS-ENC encoding, encrypted using the
      client's secret key and a key usage value of 1.

      This pre-authentication type was not present in RFC 1510, but many
      implementations support it.

5.2.7.3. PA-PW-SALT

      The padata-value for this pre-authentication type contains the
      salt for the string-to-key to be used by the client to obtain the
      key for decrypting the encrypted part of an AS-REP message.
      Unfortunately, for historical reasons, the character set to be
      used is unspecified and probably locale-specific.

      This pre-authentication type was not present in RFC 1510, but many
      implementations support it. It is necessary in any case where the
      salt for the string-to-key algorithm is not the default.

      In the trivial example, a zero-length salt string is very



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      commonplace for realms that have converted their principal
      databases from Kerberos 4.

      A KDC SHOULD NOT send PA-PW-SALT when issuing a KRB-ERROR message
      that requests additional pre-authentication. Implementation note:
      some KDC implementations issue an erroneous PA-PW-SALT when
      issuing a KRB-ERROR message that requests additional pre-
      authentication. Therefore, clients SHOULD ignore a PA-PW-SALT
      accompanying a KRB-ERROR message that requests additional pre-
      authentication.  As noted in section 3.1.3, a KDC MUST NOT send
      PA-PW-SALT when the client's AS-REQ includes at least one "newer"
      etype.

5.2.7.4. PA-ETYPE-INFO

      The ETYPE-INFO pre-authentication type is sent by the KDC in a
      KRB-ERROR indicating a requirement for additional pre-
      authentication. It is usually used to notify a client of which key
      to use for the encryption of an encrypted timestamp for the
      purposes of sending a PA-ENC-TIMESTAMP pre-authentication value.
      It MAY also be sent in an AS-REP to provide information to the
      client about which key salt to use for the string-to-key to be
      used by the client to obtain the key for decrypting the encrypted
      part the AS-REP.

      ETYPE-INFO-ENTRY        ::= SEQUENCE {
              etype           [0] Int32,
              salt            [1] OCTET STRING OPTIONAL
      }

      ETYPE-INFO              ::= SEQUENCE OF ETYPE-INFO-ENTRY

      The salt, like that of PA-PW-SALT, is also completely unspecified
      with respect to character set and is probably locale-specific.

      If ETYPE-INFO is sent in an AS-REP, there shall be exactly one
      ETYPE-INFO-ENTRY, and its etype shall match that of the enc-part
      in the AS-REP.

      This pre-authentication type was not present in RFC 1510, but many
      implementations that support encrypted timestamps for pre-
      authentication need to support ETYPE-INFO as well.  As noted in
      section 3.1.3, a KDC MUST NOT send PA-ETYPE-INFO when the client's
      AS-REQ includes at least one "newer" etype.

5.2.7.5. PA-ETYPE-INFO2

      The ETYPE-INFO2 pre-authentication type is sent by the KDC in a



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      KRB-ERROR indicating a requirement for additional pre-
      authentication. It is usually used to notify a client of which key
      to use for the encryption of an encrypted timestamp for the
      purposes of sending a PA-ENC-TIMESTAMP pre-authentication value.
      It MAY also be sent in an AS-REP to provide information to the
      client about which key salt to use for the string-to-key to be
      used by the client to obtain the key for decrypting the encrypted
      part the AS-REP.

      ETYPE-INFO2-ENTRY       ::= SEQUENCE {
              etype           [0] Int32,
              salt            [1] KerberosString OPTIONAL,
              s2kparams       [2] OCTET STRING OPTIONAL
      }

      ETYPE-INFO2              ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY

      The type of the salt is KerberosString, but existing installations
      might have locale-specific characters stored in salt strings, and
      implementors MAY choose to handle them.

      The interpretation of s2kparams is specified in the cryptosystem
      description associated with the etype. Each cryptosystem has a
      default interpretation of s2kparams that will hold if that element
      is omitted from the encoding of ETYPE-INFO2-ENTRY.

      If ETYPE-INFO2 is sent in an AS-REP, there shall be exactly one
      ETYPE-INFO2-ENTRY, and its etype shall match that of the enc-part
      in the AS-REP.

      The preferred ordering of the "hint" pre-authentication data that
      affect client key selection is: ETYPE-INFO2, followed by ETYPE-
      INFO, followed by PW-SALT.  As noted in section 3.1.3, a KDC MUST
      NOT send ETYPE-INFO or PW-SALT when the client's AS-REQ includes
      at least one "newer" etype.

      The ETYPE-INFO2 pre-authentication type was not present in RFC
      1510.

5.2.8. KerberosFlags

      For several message types, a specific constrained bit string type,
      KerberosFlags, is used.

      KerberosFlags   ::= BIT STRING (SIZE (32..MAX)) -- minimum number of bits
                          -- shall be sent, but no fewer than 32

      Compatibility note: the following paragraphs describe a change



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      from the RFC1510 description of bit strings that would result in
      incompatility in the case of an implementation that strictly
      conformed to ASN.1 DER and RFC1510.

      ASN.1 bit strings have multiple uses. The simplest use of a bit
      string is to contain a vector of bits, with no particular meaning
      attached to individual bits. This vector of bits is not
      necessarily a multiple of eight bits long.  The use in Kerberos of
      a bit string as a compact boolean vector wherein each element has
      a distinct meaning poses some problems. The natural notation for a
      compact boolean vector is the ASN.1 "NamedBit" notation, and the
      DER require that encodings of a bit string using "NamedBit"
      notation exclude any trailing zero bits. This truncation is easy
      to neglect, especially given C language implementations that
      naturally choose to store boolean vectors as 32 bit integers.

      For example, if the notation for KDCOptions were to include the
      "NamedBit" notation, as in RFC 1510, and a KDCOptions value to be
      encoded had only the "forwardable" (bit number one) bit set, the
      DER encoding MUST include only two bits: the first reserved bit
      ("reserved", bit number zero, value zero) and the one-valued bit
      (bit number one) for "forwardable".

      Most existing implementations of Kerberos unconditionally send 32
      bits on the wire when encoding bit strings used as boolean
      vectors. This behavior violates the ASN.1 syntax used for flag
      values in RFC 1510, but occurs on such a widely installed base
      that the protocol description is being modified to accommodate it.

      Consequently, this document removes the "NamedBit" notations for
      individual bits, relegating them to comments. The size constraint
      on the KerberosFlags type requires that at least 32 bits be
      encoded at all times, though a lenient implementation MAY choose
      to accept fewer than 32 bits and to treat the missing bits as set
      to zero.

      Currently, no uses of KerberosFlags specify more than 32 bits
      worth of flags, although future revisions of this document may do
      so. When more than 32 bits are to be transmitted in a
      KerberosFlags value, future revisions to this document will likely
      specify that the smallest number of bits needed to encode the
      highest-numbered one-valued bit should be sent. This is somewhat
      similar to the DER encoding of a bit string that is declared with
      the "NamedBit" notation.

5.2.9. Cryptosystem-related Types

      Many Kerberos protocol messages contain an EncryptedData as a



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      container for arbitrary encrypted data, which is often the
      encrypted encoding of another data type. Fields within
      EncryptedData assist the recipient in selecting a key with which
      to decrypt the enclosed data.

      EncryptedData   ::= SEQUENCE {
              etype   [0] Int32 -- EncryptionType --,
              kvno    [1] UInt32 OPTIONAL,
              cipher  [2] OCTET STRING -- ciphertext
      }

   etype
      This field identifies which encryption algorithm was used to
      encipher the cipher.

   kvno
      This field contains the version number of the key under which data
      is encrypted. It is only present in messages encrypted under long
      lasting keys, such as principals' secret keys.

   cipher
      This field contains the enciphered text, encoded as an OCTET
      STRING.  (Note that the encryption mechanisms defined in
      [@KCRYPTO] MUST incorporate integrity protection as well, so no
      additional checksum is required.)

   The EncryptionKey type is the means by which cryptographic keys used
   for encryption are transferred.

   EncryptionKey   ::= SEQUENCE {
           keytype         [0] Int32 -- actually encryption type --,
           keyvalue        [1] OCTET STRING
   }

   keytype
      This field specifies the encryption type of the encryption key
      that follows in the keyvalue field. While its name is "keytype",
      it actually specifies an encryption type. Previously, multiple
      cryptosystems that performed encryption differently but were
      capable of using keys with the same characteristics were permitted
      to share an assigned number to designate the type of key; this
      usage is now deprecated.

   keyvalue
      This field contains the key itself, encoded as an octet string.

   Messages containing cleartext data to be authenticated will usually
   do so by using a member of type Checksum. Most instances of Checksum



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   use a keyed hash, though exceptions will be noted.

   Checksum        ::= SEQUENCE {
           cksumtype       [0] Int32,
           checksum        [1] OCTET STRING
   }

   cksumtype
      This field indicates the algorithm used to generate the
      accompanying checksum.

   checksum
      This field contains the checksum itself, encoded as an octet
      string.

   See section 4 for a brief description of the use of encryption and
   checksums in Kerberos.

5.3. Tickets

      This section describes the format and encryption parameters for
      tickets and authenticators. When a ticket or authenticator is
      included in a protocol message it is treated as an opaque object.
      A ticket is a record that helps a client authenticate to a
      service. A Ticket contains the following information:

      Ticket          ::= [APPLICATION 1] SEQUENCE {
              tkt-vno         [0] INTEGER (5),
              realm           [1] Realm,
              sname           [2] PrincipalName,
              enc-part        [3] EncryptedData -- EncTicketPart
      }

      -- Encrypted part of ticket
      EncTicketPart   ::= [APPLICATION 3] SEQUENCE {
              flags                   [0] TicketFlags,
              key                     [1] EncryptionKey,
              crealm                  [2] Realm,
              cname                   [3] PrincipalName,
              transited               [4] TransitedEncoding,
              authtime                [5] KerberosTime,
              starttime               [6] KerberosTime OPTIONAL,
              endtime                 [7] KerberosTime,
              renew-till              [8] KerberosTime OPTIONAL,
              caddr                   [9] HostAddresses OPTIONAL,
              authorization-data      [10] AuthorizationData OPTIONAL
      }




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      -- encoded Transited field
      TransitedEncoding       ::= SEQUENCE {
              tr-type         [0] Int32 -- must be registered --,
              contents        [1] OCTET STRING
      }

      TicketFlags     ::= KerberosFlags
              -- reserved(0),
              -- forwardable(1),
              -- forwarded(2),
              -- proxiable(3),
              -- proxy(4),
              -- may-postdate(5),
              -- postdated(6),
              -- invalid(7),
              -- renewable(8),
              -- initial(9),
              -- pre-authent(10),
              -- hw-authent(11),
      -- the following are new since 1510
              -- transited-policy-checked(12),
              -- ok-as-delegate(13)

   tkt-vno
      This field specifies the version number for the ticket format.
      This document describes version number 5.

   realm
      This field specifies the realm that issued a ticket. It also
      serves to identify the realm part of the server's principal
      identifier. Since a Kerberos server can only issue tickets for
      servers within its realm, the two will always be identical.

   sname
      This field specifies all components of the name part of the
      server's identity, including those parts that identify a specific
      instance of a service.

   enc-part
      This field holds the encrypted encoding of the EncTicketPart
      sequence.  It is encrypted in the key shared by Kerberos and the
      end server (the server's secret key), using a key usage value of
      2.

   flags
      This field indicates which of various options were used or
      requested when the ticket was issued. The meanings of the flags
      are:



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         Bit(s)  Name                   Description

         0       reserved               Reserved for future expansion of this
                                        field.

                                        The FORWARDABLE flag is normally only
                                        interpreted by the TGS, and can be
                                        ignored by end servers. When set, this
         1       forwardable            flag tells the ticket-granting server
                                        that it is OK to issue a new
                                        ticket-granting ticket with a
                                        different network address based on the
                                        presented ticket.

                                        When set, this flag indicates that the
                                        ticket has either been forwarded or
         2       forwarded              was issued based on authentication
                                        involving a forwarded ticket-granting
                                        ticket.

                                        The PROXIABLE flag is normally only
                                        interpreted by the TGS, and can be
                                        ignored by end servers. The PROXIABLE
                                        flag has an interpretation identical
         3       proxiable              to that of the FORWARDABLE flag,
                                        except that the PROXIABLE flag tells
                                        the ticket-granting server that only
                                        non-ticket-granting tickets may be
                                        issued with different network
                                        addresses.

         4       proxy                  When set, this flag indicates that a
                                        ticket is a proxy.

                                        The MAY-POSTDATE flag is normally only
                                        interpreted by the TGS, and can be
         5       may-postdate           ignored by end servers. This flag
                                        tells the ticket-granting server that
                                        a post-dated ticket MAY be issued
                                        based on this ticket-granting ticket.

                                        This flag indicates that this ticket
                                        has been postdated. The end-service
         6       postdated              can check the authtime field to see
                                        when the original authentication
                                        occurred.

                                        This flag indicates that a ticket is



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                                        invalid, and it must be validated by
         7       invalid                the KDC before use. Application
                                        servers must reject tickets which have
                                        this flag set.

                                        The RENEWABLE flag is normally only
                                        interpreted by the TGS, and can
                                        usually be ignored by end servers
         8       renewable              (some particularly careful servers MAY
                                        disallow renewable tickets). A
                                        renewable ticket can be used to obtain
                                        a replacement ticket that expires at a
                                        later date.

                                        This flag indicates that this ticket
         9       initial                was issued using the AS protocol, and
                                        not issued based on a ticket-granting
                                        ticket.

                                        This flag indicates that during
                                        initial authentication, the client was
                                        authenticated by the KDC before a
         10      pre-authent            ticket was issued. The strength of the
                                        pre-authentication method is not
                                        indicated, but is acceptable to the
                                        KDC.

                                        This flag indicates that the protocol
                                        employed for initial authentication
                                        required the use of hardware expected
         11      hw-authent             to be possessed solely by the named
                                        client. The hardware authentication
                                        method is selected by the KDC and the
                                        strength of the method is not
                                        indicated.

                                        This flag indicates that the KDC for
                                        the realm has checked the transited
                                        field against a realm defined policy
                                        for trusted certifiers. If this flag
                                        is reset (0), then the application
                                        server must check the transited field
                                        itself, and if unable to do so it must
                                        reject the authentication. If the flag
         12      transited-             is set (1) then the application server
                 policy-checked         MAY skip its own validation of the
                                        transited field, relying on the
                                        validation performed by the KDC. At



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                                        its option the application server MAY
                                        still apply its own validation based
                                        on a separate policy for acceptance.

                                        This flag is new since RFC 1510.

                                        This flag indicates that the server
                                        (not the client) specified in the
                                        ticket has been determined by policy
                                        of the realm to be a suitable
                                        recipient of delegation. A client can
                                        use the presence of this flag to help
                                        it make a decision whether to delegate
                                        credentials (either grant a proxy or a
                                        forwarded ticket-granting ticket) to
         13      ok-as-delegate         this server. The client is free to
                                        ignore the value of this flag. When
                                        setting this flag, an administrator
                                        should consider the Security and
                                        placement of the server on which the
                                        service will run, as well as whether
                                        the service requires the use of
                                        delegated credentials.

                                        This flag is new since RFC 1510.

         14-31   reserved               Reserved for future use.

   key
      This field exists in the ticket and the KDC response and is used
      to pass the session key from Kerberos to the application server
      and the client.

   crealm
      This field contains the name of the realm in which the client is
      registered and in which initial authentication took place.

   cname
      This field contains the name part of the client's principal
      identifier.

   transited
      This field lists the names of the Kerberos realms that took part
      in authenticating the user to whom this ticket was issued. It does
      not specify the order in which the realms were transited. See
      section 3.3.3.2 for details on how this field encodes the
      traversed realms.  When the names of CA's are to be embedded in
      the transited field (as specified for some extensions to the



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      protocol), the X.500 names of the CA's SHOULD be mapped into items
      in the transited field using the mapping defined by RFC2253.

   authtime
      This field indicates the time of initial authentication for the
      named principal. It is the time of issue for the original ticket
      on which this ticket is based. It is included in the ticket to
      provide additional information to the end service, and to provide
      the necessary information for implementation of a `hot list'
      service at the KDC. An end service that is particularly paranoid
      could refuse to accept tickets for which the initial
      authentication occurred "too far" in the past. This field is also
      returned as part of the response from the KDC.  When returned as
      part of the response to initial authentication (KRB_AS_REP), this
      is the current time on the Kerberos server.  It is NOT recommended
      that this time value be used to adjust the workstation's clock
      since the workstation cannot reliably determine that such a
      KRB_AS_REP actually came from the proper KDC in a timely manner.


   starttime

      This field in the ticket specifies the time after which the ticket
      is valid. Together with endtime, this field specifies the life of
      the ticket. If the starttime field is absent from the ticket, then
      the authtime field SHOULD be used in its place to determine the
      life of the ticket.

   endtime
      This field contains the time after which the ticket will not be
      honored (its expiration time). Note that individual services MAY
      place their own limits on the life of a ticket and MAY reject
      tickets which have not yet expired. As such, this is really an
      upper bound on the expiration time for the ticket.

   renew-till
      This field is only present in tickets that have the RENEWABLE flag
      set in the flags field. It indicates the maximum endtime that may
      be included in a renewal. It can be thought of as the absolute
      expiration time for the ticket, including all renewals.

   caddr
      This field in a ticket contains zero (if omitted) or more (if
      present) host addresses. These are the addresses from which the
      ticket can be used. If there are no addresses, the ticket can be
      used from any location. The decision by the KDC to issue or by the
      end server to accept addressless tickets is a policy decision and
      is left to the Kerberos and end-service administrators; they MAY



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      refuse to issue or accept such tickets. Because of the wide
      deployment of network address translation, it is recommended that
      policy allow the issue and acceptance of such tickets.

      Network addresses are included in the ticket to make it harder for
      an attacker to use stolen credentials. Because the session key is
      not sent over the network in cleartext, credentials can't be
      stolen simply by listening to the network; an attacker has to gain
      access to the session key (perhaps through operating system
      security breaches or a careless user's unattended session) to make
      use of stolen tickets.

      It is important to note that the network address from which a
      connection is received cannot be reliably determined. Even if it
      could be, an attacker who has compromised the client's workstation
      could use the credentials from there. Including the network
      addresses only makes it more difficult, not impossible, for an
      attacker to walk off with stolen credentials and then use them
      from a "safe" location.

   authorization-data
      The authorization-data field is used to pass authorization data
      from the principal on whose behalf a ticket was issued to the
      application service. If no authorization data is included, this
      field will be left out. Experience has shown that the name of this
      field is confusing, and that a better name for this field would be
      restrictions. Unfortunately, it is not possible to change the name
      of this field at this time.

      This field contains restrictions on any authority obtained on the
      basis of authentication using the ticket. It is possible for any
      principal in possession of credentials to add entries to the
      authorization data field since these entries further restrict what
      can be done with the ticket.  Such additions can be made by
      specifying the additional entries when a new ticket is obtained
      during the TGS exchange, or they MAY be added during chained
      delegation using the authorization data field of the
      authenticator.

      Because entries may be added to this field by the holder of
      credentials, except when an entry is separately authenticated by
      encapsulation in the KDC-issued element, it is not allowable for
      the presence of an entry in the authorization data field of a
      ticket to amplify the privileges one would obtain from using a
      ticket.

      The data in this field may be specific to the end service; the
      field will contain the names of service specific objects, and the



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      rights to those objects. The format for this field is described in
      section 5.2.6.  Although Kerberos is not concerned with the format
      of the contents of the sub-fields, it does carry type information
      (ad-type).

      By using the authorization_data field, a principal is able to
      issue a proxy that is valid for a specific purpose. For example, a
      client wishing to print a file can obtain a file server proxy to
      be passed to the print server. By specifying the name of the file
      in the authorization_data field, the file server knows that the
      print server can only use the client's rights when accessing the
      particular file to be printed.

      A separate service providing authorization or certifying group
      membership may be built using the authorization-data field. In
      this case, the entity granting authorization (not the authorized
      entity), may obtain a ticket in its own name (e.g. the ticket is
      issued in the name of a privilege server), and this entity adds
      restrictions on its own authority and delegates the restricted
      authority through a proxy to the client. The client would then
      present this authorization credential to the application server
      separately from the authentication exchange.  Alternatively, such
      authorization credentials MAY be embedded in the ticket
      authenticating the authorized entity, when the authorization is
      separately authenticated using the KDC-issued authorization data
      element (see 5.2.6.2).

      Similarly, if one specifies the authorization-data field of a
      proxy and leaves the host addresses blank, the resulting ticket
      and session key can be treated as a capability. See [Neu93] for
      some suggested uses of this field.

      The authorization-data field is optional and does not have to be
      included in a ticket.

5.4. Specifications for the AS and TGS exchanges

      This section specifies the format of the messages used in the
      exchange between the client and the Kerberos server. The format of
      possible error messages appears in section 5.9.1.

5.4.1. KRB_KDC_REQ definition

      The KRB_KDC_REQ message has no application tag number of its own.
      Instead, it is incorporated into one of KRB_AS_REQ or KRB_TGS_REQ,
      which each have an application tag, depending on whether the
      request is for an initial ticket or an additional ticket. In
      either case, the message is sent from the client to the KDC to



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      request credentials for a service.

      The message fields are:

      AS-REQ          ::= [APPLICATION 10] KDC-REQ

      TGS-REQ         ::= [APPLICATION 12] KDC-REQ

      KDC-REQ         ::= SEQUENCE {
              -- NOTE: first tag is [1], not [0]
              pvno            [1] INTEGER (5) ,
              msg-type        [2] INTEGER (10 -- AS -- | 12 -- TGS --),
              padata          [3] SEQUENCE OF PA-DATA OPTIONAL
                                  -- NOTE: not empty --,
              req-body        [4] KDC-REQ-BODY
      }

      KDC-REQ-BODY    ::= SEQUENCE {
              kdc-options             [0] KDCOptions,
              cname                   [1] PrincipalName OPTIONAL
                                          -- Used only in AS-REQ --,
              realm                   [2] Realm
                                          -- Server's realm
                                          -- Also client's in AS-REQ --,
              sname                   [3] PrincipalName OPTIONAL,
              from                    [4] KerberosTime OPTIONAL,
              till                    [5] KerberosTime,
              rtime                   [6] KerberosTime OPTIONAL,
              nonce                   [7] UInt32,
              etype                   [8] SEQUENCE OF Int32 -- EncryptionType
                                          -- in preference order --,
              addresses               [9] HostAddresses OPTIONAL,
              enc-authorization-data  [10] EncryptedData -- AuthorizationData --,
              additional-tickets      [11] SEQUENCE OF Ticket OPTIONAL
                                              -- NOTE: not empty
      }

      KDCOptions      ::= KerberosFlags
              -- reserved(0),
              -- forwardable(1),
              -- forwarded(2),
              -- proxiable(3),
              -- proxy(4),
              -- allow-postdate(5),
              -- postdated(6),
              -- unused7(7),
              -- renewable(8),
              -- unused9(9),



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              -- unused10(10),
              -- opt-hardware-auth(11),
              -- unused12(12),
              -- unused13(13),
      -- 15 is reserved for canonicalize
              -- unused15(15),
      -- 26 was unused in 1510
              -- disable-transited-check(26),
      --
              -- renewable-ok(27),
              -- enc-tkt-in-skey(28),
              -- renew(30),
              -- validate(31)

   The fields in this message are:

   pvno
      This field is included in each message, and specifies the protocol
      version number. This document specifies protocol version 5.

   msg-type
      This field indicates the type of a protocol message. It will
      almost always be the same as the application identifier associated
      with a message. It is included to make the identifier more readily
      accessible to the application. For the KDC-REQ message, this type
      will be KRB_AS_REQ or KRB_TGS_REQ.

   padata
      Contains pre-authentication data. Requests for additional tickets
      (KRB_TGS_REQ) MUST contain a padata of PA-TGS-REQ.

      The padata (pre-authentication data) field contains a sequence of
      authentication information which may be needed before credentials
      can be issued or decrypted.

   req-body
      This field is a placeholder delimiting the extent of the remaining
      fields. If a checksum is to be calculated over the request, it is
      calculated over an encoding of the KDC-REQ-BODY sequence which is
      enclosed within the req-body field.

   kdc-options
      This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to
      the KDC and indicates the flags that the client wants set on the
      tickets as well as other information that is to modify the
      behavior of the KDC.  Where appropriate, the name of an option may
      be the same as the flag that is set by that option. Although in
      most case, the bit in the options field will be the same as that



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      in the flags field, this is not guaranteed, so it is not
      acceptable to simply copy the options field to the flags field.
      There are various checks that must be made before honoring an
      option anyway.

      The kdc_options field is a bit-field, where the selected options
      are indicated by the bit being set (1), and the unselected options
      and reserved fields being reset (0). The encoding of the bits is
      specified in section 5.2. The options are described in more detail
      above in section 2. The meanings of the options are:

         Bits    Name                     Description

         0       RESERVED                 Reserved for future expansion of
                                          this field.

                                          The FORWARDABLE option indicates
                                          that the ticket to be issued is to
                                          have its forwardable flag set. It
         1       FORWARDABLE              may only be set on the initial
                                          request, or in a subsequent request
                                          if the ticket-granting ticket on
                                          which it is based is also
                                          forwardable.

                                          The FORWARDED option is only
                                          specified in a request to the
                                          ticket-granting server and will only
                                          be honored if the ticket-granting
                                          ticket in the request has its
         2       FORWARDED                FORWARDABLE bit set. This option
                                          indicates that this is a request for
                                          forwarding. The address(es) of the
                                          host from which the resulting ticket
                                          is to be valid are included in the
                                          addresses field of the request.

                                          The PROXIABLE option indicates that
                                          the ticket to be issued is to have
                                          its proxiable flag set. It may only
         3       PROXIABLE                be set on the initial request, or in
                                          a subsequent request if the
                                          ticket-granting ticket on which it
                                          is based is also proxiable.

                                          The PROXY option indicates that this
                                          is a request for a proxy. This
                                          option will only be honored if the



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                                          ticket-granting ticket in the
         4       PROXY                    request has its PROXIABLE bit set.
                                          The address(es) of the host from
                                          which the resulting ticket is to be
                                          valid are included in the addresses
                                          field of the request.

                                          The ALLOW-POSTDATE option indicates
                                          that the ticket to be issued is to
                                          have its MAY-POSTDATE flag set. It
         5       ALLOW-POSTDATE           may only be set on the initial
                                          request, or in a subsequent request
                                          if the ticket-granting ticket on
                                          which it is based also has its
                                          MAY-POSTDATE flag set.

                                          The POSTDATED option indicates that
                                          this is a request for a postdated
                                          ticket. This option will only be
                                          honored if the ticket-granting
                                          ticket on which it is based has its
         6       POSTDATED                MAY-POSTDATE flag set. The resulting
                                          ticket will also have its INVALID
                                          flag set, and that flag may be reset
                                          by a subsequent request to the KDC
                                          after the starttime in the ticket
                                          has been reached.

         7       RESERVED                 This option is presently unused.

                                          The RENEWABLE option indicates that
                                          the ticket to be issued is to have
                                          its RENEWABLE flag set. It may only
                                          be set on the initial request, or
                                          when the ticket-granting ticket on
         8       RENEWABLE                which the request is based is also
                                          renewable. If this option is
                                          requested, then the rtime field in
                                          the request contains the desired
                                          absolute expiration time for the
                                          ticket.

         9       RESERVED                 Reserved for PK-Cross

         10      RESERVED                 Reserved for future use.

         11      RESERVED                 Reserved for opt-hardware-auth.




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         12-25   RESERVED                 Reserved for future use.

                                          By default the KDC will check the
                                          transited field of a
                                          ticket-granting-ticket against the
                                          policy of the local realm before it
                                          will issue derivative tickets based
                                          on the ticket-granting ticket. If
                                          this flag is set in the request,
                                          checking of the transited field is
                                          disabled. Tickets issued without the
         26      DISABLE-TRANSITED-CHECK  performance of this check will be
                                          noted by the reset (0) value of the
                                          TRANSITED-POLICY-CHECKED flag,
                                          indicating to the application server
                                          that the tranisted field must be
                                          checked locally. KDCs are
                                          encouraged but not required to honor
                                          the DISABLE-TRANSITED-CHECK option.

                                          This flag is new since RFC 1510

                                          The RENEWABLE-OK option indicates
                                          that a renewable ticket will be
                                          acceptable if a ticket with the
                                          requested life cannot otherwise be
                                          provided. If a ticket with the
                                          requested life cannot be provided,
         27      RENEWABLE-OK             then a renewable ticket may be
                                          issued with a renew-till equal to
                                          the requested endtime. The value
                                          of the renew-till field may still be
                                          limited by local limits, or limits
                                          selected by the individual principal
                                          or server.

                                          This option is used only by the
                                          ticket-granting service. The
                                          ENC-TKT-IN-SKEY option indicates
         28      ENC-TKT-IN-SKEY          that the ticket for the end server
                                          is to be encrypted in the session
                                          key from the additional
                                          ticket-granting ticket provided.

         29      RESERVED                 Reserved for future use.

                                          This option is used only by the
                                          ticket-granting service. The RENEW



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                                          option indicates that the present
                                          request is for a renewal. The ticket
                                          provided is encrypted in the secret
                                          key for the server on which it is
         30      RENEW                    valid. This option will only be
                                          honored if the ticket to be renewed
                                          has its RENEWABLE flag set and if
                                          the time in its renew-till field has
                                          not passed. The ticket to be renewed
                                          is passed in the padata field as
                                          part of the authentication header.

                                          This option is used only by the
                                          ticket-granting service. The
                                          VALIDATE option indicates that the
                                          request is to validate a postdated
                                          ticket. It will only be honored if
                                          the ticket presented is postdated,
                                          presently has its INVALID flag set,
         31      VALIDATE                 and would be otherwise usable at
                                          this time. A ticket cannot be
                                          validated before its starttime. The
                                          ticket presented for validation is
                                          encrypted in the key of the server
                                          for which it is valid and is passed
                                          in the padata field as part of the
                                          authentication header.
   cname and sname
      These fields are the same as those described for the ticket in
      section 5.3. The sname may only be absent when the ENC-TKT-IN-SKEY
      option is specified. If absent, the name of the server is taken
      from the name of the client in the ticket passed as additional-
      tickets.

   enc-authorization-data
      The enc-authorization-data, if present (and it can only be present
      in the TGS_REQ form), is an encoding of the desired authorization-
      data encrypted under the sub-session key if present in the
      Authenticator, or alternatively from the session key in the
      ticket-granting ticket (both the Authenticator and ticket-granting
      ticket come from the padata field in the KRB_TGS_REQ). The key
      usage value used when encrypting is 5 if a sub-session key is
      used, or 4 if the session key is used.

   realm
      This field specifies the realm part of the server's principal
      identifier. In the AS exchange, this is also the realm part of the
      client's principal identifier.



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   from
      This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket
      requests when the requested ticket is to be postdated. It
      specifies the desired start time for the requested ticket. If this
      field is omitted then the KDC SHOULD use the current time instead.

   till
      This field contains the expiration date requested by the client in
      a ticket request. It is not optional, but if the requested endtime
      is "19700101000000Z", the requested ticket is to have the maximum
      endtime permitted according to KDC policy. Implementation note:
      This special timestamp corresponds to a UNIX time_t value of zero
      on most systems.

   rtime
      This field is the requested renew-till time sent from a client to
      the KDC in a ticket request. It is optional.

   nonce
      This field is part of the KDC request and response. It is intended
      to hold a random number generated by the client. If the same
      number is included in the encrypted response from the KDC, it
      provides evidence that the response is fresh and has not been
      replayed by an attacker.  Nonces MUST NEVER be reused.

   etype
      This field specifies the desired encryption algorithm to be used
      in the response.

   addresses
      This field is included in the initial request for tickets, and
      optionally included in requests for additional tickets from the
      ticket-granting server. It specifies the addresses from which the
      requested ticket is to be valid. Normally it includes the
      addresses for the client's host. If a proxy is requested, this
      field will contain other addresses. The contents of this field are
      usually copied by the KDC into the caddr field of the resulting
      ticket.

   additional-tickets
      Additional tickets MAY be optionally included in a request to the
      ticket-granting server. If the ENC-TKT-IN-SKEY option has been
      specified, then the session key from the additional ticket will be
      used in place of the server's key to encrypt the new ticket. When
      the ENC-TKT-IN-SKEY option is used for user-to-user
      authentication, this additional ticket MAY be a TGT issued by the
      local realm or an inter-realm TGT issued for the current KDC's
      realm by a remote KDC. If more than one option which requires



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      additional tickets has been specified, then the additional tickets
      are used in the order specified by the ordering of the options
      bits (see kdc-options, above).

   The application tag number will be either ten (10) or twelve (12)
   depending on whether the request is for an initial ticket (AS-REQ) or
   for an additional ticket (TGS-REQ).

   The optional fields (addresses, authorization-data and additional-
   tickets) are only included if necessary to perform the operation
   specified in the kdc-options field.

   It should be noted that in KRB_TGS_REQ, the protocol version number
   appears twice and two different message types appear: the KRB_TGS_REQ
   message contains these fields as does the authentication header
   (KRB_AP_REQ) that is passed in the padata field.

5.4.2. KRB_KDC_REP definition

      The KRB_KDC_REP message format is used for the reply from the KDC
      for either an initial (AS) request or a subsequent (TGS) request.
      There is no message type for KRB_KDC_REP. Instead, the type will
      be either KRB_AS_REP or KRB_TGS_REP. The key used to encrypt the
      ciphertext part of the reply depends on the message type. For
      KRB_AS_REP, the ciphertext is encrypted in the client's secret
      key, and the client's key version number is included in the key
      version number for the encrypted data. For KRB_TGS_REP, the
      ciphertext is encrypted in the sub-session key from the
      Authenticator, or if absent, the session key from the ticket-
      granting ticket used in the request.  In that case, no version
      number will be present in the EncryptedData sequence.

      The KRB_KDC_REP message contains the following fields:

      AS-REP          ::= [APPLICATION 11] KDC-REP

      TGS-REP         ::= [APPLICATION 13] KDC-REP

      KDC-REP         ::= SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (11 -- AS -- | 13 -- TGS --),
              padata          [2] SEQUENCE OF PA-DATA OPTIONAL
                                      -- NOTE: not empty --,
              crealm          [3] Realm,
              cname           [4] PrincipalName,
              ticket          [5] Ticket,
              enc-part        [6] EncryptedData
                                      -- EncASRepPart or EncTGSRepPart,



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                                      -- as appropriate
      }

      EncASRepPart    ::= [APPLICATION 25] EncKDCRepPart

      EncTGSRepPart   ::= [APPLICATION 26] EncKDCRepPart

      EncKDCRepPart   ::= SEQUENCE {
              key             [0] EncryptionKey,
              last-req        [1] LastReq,
              nonce           [2] UInt32,
              key-expiration  [3] KerberosTime OPTIONAL,
              flags           [4] TicketFlags,
              authtime        [5] KerberosTime,
              starttime       [6] KerberosTime OPTIONAL,
              endtime         [7] KerberosTime,
              renew-till      [8] KerberosTime OPTIONAL,
              srealm          [9] Realm,
              sname           [10] PrincipalName,
              caddr           [11] HostAddresses OPTIONAL
      }

      LastReq         ::=     SEQUENCE OF SEQUENCE {
              lr-type         [0] Int32,
              lr-value        [1] KerberosTime
      }

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      either KRB_AS_REP or KRB_TGS_REP.

   padata
      This field is described in detail in section 5.4.1. One possible
      use for this field is to encode an alternate "salt" string to be
      used with a string-to-key algorithm. This ability is useful to
      ease transitions if a realm name needs to change (e.g. when a
      company is acquired); in such a case all existing password-derived
      entries in the KDC database would be flagged as needing a special
      salt string until the next password change.

   crealm, cname, srealm and sname
      These fields are the same as those described for the ticket in
      section 5.3.

   ticket
      The newly-issued ticket, from section 5.3.

   enc-part



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      This field is a place holder for the ciphertext and related
      information that forms the encrypted part of a message. The
      description of the encrypted part of the message follows each
      appearance of this field.

      The key usage value for encrypting this field is 3 in an AS-REP
      message, using the client's long-term key or another key selected
      via pre-authentication mechanisms. In a TGS-REP message, the key
      usage value is 8 if the TGS session key is used, or 9 if a TGS
      authenticator subkey is used.

      Compatibility note: Some implementations unconditionally send an
      encrypted EncTGSRepPart (application tag number 26) in this field
      regardless of whether the reply is a AS-REP or a TGS-REP. In the
      interests of compatibility, implementors MAY relax the check on
      the tag number of the decrypted ENC-PART.

   key
      This field is the same as described for the ticket in section 5.3.

   last-req
      This field is returned by the KDC and specifies the time(s) of the
      last request by a principal. Depending on what information is
      available, this might be the last time that a request for a
      ticket-granting ticket was made, or the last time that a request
      based on a ticket-granting ticket was successful. It also might
      cover all servers for a realm, or just the particular server. Some
      implementations MAY display this information to the user to aid in
      discovering unauthorized use of one's identity. It is similar in
      spirit to the last login time displayed when logging into
      timesharing systems.

      lr-type
         This field indicates how the following lr-value field is to be
         interpreted. Negative values indicate that the information
         pertains only to the responding server. Non-negative values
         pertain to all servers for the realm.

         If the lr-type field is zero (0), then no information is
         conveyed by the lr-value subfield. If the absolute value of the
         lr-type field is one (1), then the lr-value subfield is the
         time of last initial request for a TGT. If it is two (2), then
         the lr-value subfield is the time of last initial request. If
         it is three (3), then the lr-value subfield is the time of
         issue for the newest ticket-granting ticket used. If it is four
         (4), then the lr-value subfield is the time of the last
         renewal. If it is five (5), then the lr-value subfield is the
         time of last request (of any type).  If it is (6), then the lr-



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         value subfield is the time when the password will expire.  If
         it is (7), then the lr-value subfield is the time when the
         account will expire.

      lr-value
         This field contains the time of the last request. The time MUST
         be interpreted according to the contents of the accompanying
         lr-type subfield.

   nonce
      This field is described above in section 5.4.1.

   key-expiration
      The key-expiration field is part of the response from the KDC and
      specifies the time that the client's secret key is due to expire.
      The expiration might be the result of password aging or an account
      expiration. If present, it SHOULD be set to the earliest of the
      user's key expiration and account expiration.  The use of this
      field is deprecated and the last-req field SHOULD be used to
      convey this information instead.  This field will usually be left
      out of the TGS reply since the response to the TGS request is
      encrypted in a session key and no client information need be
      retrieved from the KDC database. It is up to the application
      client (usually the login program) to take appropriate action
      (such as notifying the user) if the expiration time is imminent.

   flags, authtime, starttime, endtime, renew-till and caddr
      These fields are duplicates of those found in the encrypted
      portion of the attached ticket (see section 5.3), provided so the
      client MAY verify they match the intended request and to assist in
      proper ticket caching. If the message is of type KRB_TGS_REP, the
      caddr field will only be filled in if the request was for a proxy
      or forwarded ticket, or if the user is substituting a subset of
      the addresses from the ticket-granting ticket. If the client-
      requested addresses are not present or not used, then the
      addresses contained in the ticket will be the same as those
      included in the ticket-granting ticket.

5.5. Client/Server (CS) message specifications

      This section specifies the format of the messages used for the
      authentication of the client to the application server.

5.5.1. KRB_AP_REQ definition

      The KRB_AP_REQ message contains the Kerberos protocol version
      number, the message type KRB_AP_REQ, an options field to indicate
      any options in use, and the ticket and authenticator themselves.



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      The KRB_AP_REQ message is often referred to as the 'authentication
      header'.

      AP-REQ          ::= [APPLICATION 14] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (14),
              ap-options      [2] APOptions,
              ticket          [3] Ticket,
              authenticator   [4] EncryptedData -- Authenticator
      }

      APOptions       ::= KerberosFlags
              -- reserved(0),
              -- use-session-key(1),
              -- mutual-required(2)

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_AP_REQ.

   ap-options
      This field appears in the application request (KRB_AP_REQ) and
      affects the way the request is processed. It is a bit-field, where
      the selected options are indicated by the bit being set (1), and
      the unselected options and reserved fields being reset (0). The
      encoding of the bits is specified in section 5.2. The meanings of
      the options are:

         Bit(s)  Name            Description

         0       reserved        Reserved for future expansion of this field.

                                 The USE-SESSION-KEY option indicates that the
                                 ticket the client is presenting to a server
         1       use-session-key is encrypted in the session key from the
                                 server's ticket-granting ticket. When this
                                 option is not specified, the ticket is
                                 encrypted in the server's secret key.

                                 The MUTUAL-REQUIRED option tells the server
         2       mutual-required that the client requires mutual
                                 authentication, and that it must respond with
                                 a KRB_AP_REP message.

         3-31    reserved        Reserved for future use.

   ticket
      This field is a ticket authenticating the client to the server.



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   authenticator
      This contains the encrypted authenticator, which includes the
      client's choice of a subkey.

   The encrypted authenticator is included in the AP-REQ; it certifies
   to a server that the sender has recent knowledge of the encryption
   key in the accompanying ticket, to help the server detect replays. It
   also assists in the selection of a "true session key" to use with the
   particular session.  The DER encoding of the following is encrypted
   in the ticket's session key, with a key usage value of 11 in normal
   application exchanges, or 7 when used as the PA-TGS-REQ PA-DATA field
   of a TGS-REQ exchange (see section 5.4.1):

   -- Unencrypted authenticator
   Authenticator   ::= [APPLICATION 2] SEQUENCE  {
           authenticator-vno       [0] INTEGER (5),
           crealm                  [1] Realm,
           cname                   [2] PrincipalName,
           cksum                   [3] Checksum OPTIONAL,
           cusec                   [4] Microseconds,
           ctime                   [5] KerberosTime,
           subkey                  [6] EncryptionKey OPTIONAL,
           seq-number              [7] UInt32 OPTIONAL,
           authorization-data      [8] AuthorizationData OPTIONAL
   }

   authenticator-vno
      This field specifies the version number for the format of the
      authenticator. This document specifies version 5.

   crealm and cname
      These fields are the same as those described for the ticket in
      section 5.3.

   cksum
      This field contains a checksum of the application data that
      accompanies the KRB_AP_REQ, computed using a key usage value of 10
      in normal application exchanges, or 6 when used in the TGS-REQ PA-
      TGS-REQ AP-DATA field.

   cusec
      This field contains the microsecond part of the client's
      timestamp. Its value (before encryption) ranges from 0 to 999999.
      It often appears along with ctime. The two fields are used
      together to specify a reasonably accurate timestamp.

   ctime
      This field contains the current time on the client's host.



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   subkey
      This field contains the client's choice for an encryption key
      which is to be used to protect this specific application session.
      Unless an application specifies otherwise, if this field is left
      out the session key from the ticket will be used.

   seq-number
      This optional field includes the initial sequence number to be
      used by the KRB_PRIV or KRB_SAFE messages when sequence numbers
      are used to detect replays (It may also be used by application
      specific messages).  When included in the authenticator this field
      specifies the initial sequence number for messages from the client
      to the server. When included in the AP-REP message, the initial
      sequence number is that for messages from the server to the
      client. When used in KRB_PRIV or KRB_SAFE messages, it is
      incremented by one after each message is sent.  Sequence numbers
      fall in the range of 0 through 2^32 - 1 and wrap to zero following
      the value 2^32 - 1.

      For sequence numbers to adequately support the detection of
      replays they SHOULD be non-repeating, even across connection
      boundaries. The initial sequence number SHOULD be random and
      uniformly distributed across the full space of possible sequence
      numbers, so that it cannot be guessed by an attacker and so that
      it and the successive sequence numbers do not repeat other
      sequences.  In the event that more than 2^32 messages are to be
      generated in a series of KRB_PRIV or KRB_SAFE messages, rekeying
      SHOULD be performed before sequence numbers are reused with the
      same encryption key.

      Implmentation note: historically, some implementations transmit
      signed twos-complement numbers for sequence numbers. In the
      interests of compatibility, implementations MAY accept the
      equivalent negative number where a positive number greater than
      2^31 - 1 is expected.

      Implementation note: as noted before, some implementations omit
      the optional sequence number when its value would be zero.
      Implementations MAY accept an omitted sequence number when
      expecting a value of zero, and SHOULD NOT transmit an
      Authenticator with a initial sequence number of zero.

   authorization-data
      This field is the same as described for the ticket in section 5.3.
      It is optional and will only appear when additional restrictions
      are to be placed on the use of a ticket, beyond those carried in
      the ticket itself.




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5.5.2. KRB_AP_REP definition

      The KRB_AP_REP message contains the Kerberos protocol version
      number, the message type, and an encrypted time-stamp. The message
      is sent in response to an application request (KRB_AP_REQ) where
      the mutual authentication option has been selected in the ap-
      options field.

      AP-REP          ::= [APPLICATION 15] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (15),
              enc-part        [2] EncryptedData -- EncAPRepPart
      }

      EncAPRepPart    ::= [APPLICATION 27] SEQUENCE {
              ctime           [0] KerberosTime,
              cusec           [1] Microseconds,
              subkey          [2] EncryptionKey OPTIONAL,
              seq-number      [3] UInt32 OPTIONAL
      }

      The encoded EncAPRepPart is encrypted in the shared session key of
      the ticket. The optional subkey field can be used in an
      application-arranged negotiation to choose a per association
      session key.

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_AP_REP.

   enc-part
      This field is described above in section 5.4.2. It is computed
      with a key usage value of 12.

   ctime
      This field contains the current time on the client's host.

   cusec
      This field contains the microsecond part of the client's
      timestamp.

   subkey
      This field contains an encryption key which is to be used to
      protect this specific application session. See section 3.2.6 for
      specifics on how this field is used to negotiate a key. Unless an
      application specifies otherwise, if this field is left out, the
      sub-session key from the authenticator, or if also left out, the
      session key from the ticket will be used.



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   seq-number
      This field is described above in section 5.3.2.

5.5.3. Error message reply

      If an error occurs while processing the application request, the
      KRB_ERROR message will be sent in response. See section 5.9.1 for
      the format of the error message. The cname and crealm fields MAY
      be left out if the server cannot determine their appropriate
      values from the corresponding KRB_AP_REQ message. If the
      authenticator was decipherable, the ctime and cusec fields will
      contain the values from it.

5.6. KRB_SAFE message specification

      This section specifies the format of a message that can be used by
      either side (client or server) of an application to send a tamper-
      proof message to its peer. It presumes that a session key has
      previously been exchanged (for example, by using the
      KRB_AP_REQ/KRB_AP_REP messages).

5.6.1. KRB_SAFE definition

      The KRB_SAFE message contains user data along with a collision-
      proof checksum keyed with the last encryption key negotiated via
      subkeys, or the session key if no negotiation has occurred. The
      message fields are:

      KRB-SAFE        ::= [APPLICATION 20] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (20),
              safe-body       [2] KRB-SAFE-BODY,
              cksum           [3] Checksum
      }

      KRB-SAFE-BODY   ::= SEQUENCE {
              user-data       [0] OCTET STRING,
              timestamp       [1] KerberosTime OPTIONAL,
              usec            [2] Microseconds OPTIONAL,
              seq-number      [3] UInt32 OPTIONAL,
              s-address       [4] HostAddress,
              r-address       [5] HostAddress OPTIONAL
      }

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_SAFE.




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   safe-body
      This field is a placeholder for the body of the KRB-SAFE message.

   cksum
      This field contains the checksum of the application data, computed
      with a key usage value of 15.

      The checksum is computed over the encoding of the KRB-SAFE
      sequence.  First, the cksum is set to a type zero, zero-length
      value and the checksum is computed over the encoding of the KRB-
      SAFE sequence, then the checksum is set to the result of that
      computation, and finally the KRB-SAFE sequence is encoded again.
      This method, while different than the one specified in RFC 1510,
      corresponds to existing practice.

   user-data
      This field is part of the KRB_SAFE and KRB_PRIV messages and
      contain the application specific data that is being passed from
      the sender to the recipient.

   timestamp
      This field is part of the KRB_SAFE and KRB_PRIV messages. Its
      contents are the current time as known by the sender of the
      message. By checking the timestamp, the recipient of the message
      is able to make sure that it was recently generated, and is not a
      replay.

   usec
      This field is part of the KRB_SAFE and KRB_PRIV headers. It
      contains the microsecond part of the timestamp.

   seq-number
      This field is described above in section 5.3.2.

   s-address
      Sender's address.

      This field specifies the address in use by the sender of the
      message.

   r-address
      This field specifies the address in use by the recipient of the
      message. It MAY be omitted for some uses (such as broadcast
      protocols), but the recipient MAY arbitrarily reject such
      messages. This field, along with s-address, can be used to help
      detect messages which have been incorrectly or maliciously
      delivered to the wrong recipient.




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5.7. KRB_PRIV message specification

      This section specifies the format of a message that can be used by
      either side (client or server) of an application to securely and
      privately send a message to its peer. It presumes that a session
      key has previously been exchanged (for example, by using the
      KRB_AP_REQ/KRB_AP_REP messages).

5.7.1. KRB_PRIV definition

      The KRB_PRIV message contains user data encrypted in the Session
      Key. The message fields are:

      KRB-PRIV        ::= [APPLICATION 21] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (21),
                              -- NOTE: there is no [2] tag
              enc-part        [3] EncryptedData -- EncKrbPrivPart
      }

      EncKrbPrivPart  ::= [APPLICATION 28] SEQUENCE {
              user-data       [0] OCTET STRING,
              timestamp       [1] KerberosTime OPTIONAL,
              usec            [2] Microseconds OPTIONAL,
              seq-number      [3] UInt32 OPTIONAL,
              s-address       [4] HostAddress -- sender's addr --,
              r-address       [5] HostAddress OPTIONAL -- recip's addr
      }

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_PRIV.

   enc-part
      This field holds an encoding of the EncKrbPrivPart sequence
      encrypted under the session key, with a key usage value of 13.
      This encrypted encoding is used for the enc-part field of the KRB-
      PRIV message.

   user-data, timestamp, usec, s-address and r-address
      These fields are described above in section 5.6.1.

   seq-number
      This field is described above in section 5.3.2.

5.8. KRB_CRED message specification

      This section specifies the format of a message that can be used to



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      send Kerberos credentials from one principal to another. It is
      presented here to encourage a common mechanism to be used by
      applications when forwarding tickets or providing proxies to
      subordinate servers. It presumes that a session key has already
      been exchanged perhaps by using the KRB_AP_REQ/KRB_AP_REP
      messages.

5.8.1. KRB_CRED definition

      The KRB_CRED message contains a sequence of tickets to be sent and
      information needed to use the tickets, including the session key
      from each.  The information needed to use the tickets is encrypted
      under an encryption key previously exchanged or transferred
      alongside the KRB_CRED message. The message fields are:

      KRB-CRED        ::= [APPLICATION 22] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (22),
              tickets         [2] SEQUENCE OF Ticket,
              enc-part        [3] EncryptedData -- EncKrbCredPart
      }

      EncKrbCredPart  ::= [APPLICATION 29] SEQUENCE {
              ticket-info     [0] SEQUENCE OF KrbCredInfo,
              nonce           [1] UInt32 OPTIONAL,
              timestamp       [2] KerberosTime OPTIONAL,
              usec            [3] Microseconds OPTIONAL,
              s-address       [4] HostAddress OPTIONAL,
              r-address       [5] HostAddress OPTIONAL
      }

      KrbCredInfo     ::= SEQUENCE {
              key             [0] EncryptionKey,
              prealm          [1] Realm OPTIONAL,
              pname           [2] PrincipalName OPTIONAL,
              flags           [3] TicketFlags OPTIONAL,
              authtime        [4] KerberosTime OPTIONAL,
              starttime       [5] KerberosTime OPTIONAL,
              endtime         [6] KerberosTime OPTIONAL,
              renew-till      [7] KerberosTime OPTIONAL,
              srealm          [8] Realm OPTIONAL,
              sname           [9] PrincipalName OPTIONAL,
              caddr           [10] HostAddresses OPTIONAL
      }

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_CRED.



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   tickets
      These are the tickets obtained from the KDC specifically for use
      by the intended recipient. Successive tickets are paired with the
      corresponding KrbCredInfo sequence from the enc-part of the KRB-
      CRED message.

   enc-part
      This field holds an encoding of the EncKrbCredPart sequence
      encrypted under the session key shared between the sender and the
      intended recipient, with a key usage value of 14. This encrypted
      encoding is used for the enc-part field of the KRB-CRED message.

      Implementation note: implementations of certain applications, most
      notably certain implementations of the Kerberos GSS-API mechanism,
      do not separately encrypt the contents of the EncKrbCredPart of
      the KRB-CRED message when sending it.  In the case of those GSS-
      API mechanisms, this is not a security vulnerability, as the
      entire KRB-CRED message is itself embedded in an encrypted
      message.

   nonce
      If practical, an application MAY require the inclusion of a nonce
      generated by the recipient of the message. If the same value is
      included as the nonce in the message, it provides evidence that
      the message is fresh and has not been replayed by an attacker. A
      nonce MUST NEVER be reused.

   timestamp and usec
      These fields specify the time that the KRB-CRED message was
      generated.  The time is used to provide assurance that the message
      is fresh.

   s-address and r-address
      These fields are described above in section 5.6.1. They are used
      optionally to provide additional assurance of the integrity of the
      KRB-CRED message.

   key
      This field exists in the corresponding ticket passed by the KRB-
      CRED message and is used to pass the session key from the sender
      to the intended recipient. The field's encoding is described in
      section 5.2.9.

   The following fields are optional. If present, they can be associated
   with the credentials in the remote ticket file. If left out, then it
   is assumed that the recipient of the credentials already knows their
   value.




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   prealm and pname
      The name and realm of the delegated principal identity.

   flags, authtime, starttime, endtime, renew-till, srealm, sname, and
      caddr
      These fields contain the values of the corresponding fields from
      the ticket found in the ticket field. Descriptions of the fields
      are identical to the descriptions in the KDC-REP message.

5.9. Error message specification

      This section specifies the format for the KRB_ERROR message. The
      fields included in the message are intended to return as much
      information as possible about an error. It is not expected that
      all the information required by the fields will be available for
      all types of errors. If the appropriate information is not
      available when the message is composed, the corresponding field
      will be left out of the message.

      Note that since the KRB_ERROR message is not integrity protected,
      it is quite possible for an intruder to synthesize or modify such
      a message. In particular, this means that the client SHOULD NOT
      use any fields in this message for security-critical purposes,
      such as setting a system clock or generating a fresh
      authenticator. The message can be useful, however, for advising a
      user on the reason for some failure.

5.9.1. KRB_ERROR definition

      The KRB_ERROR message consists of the following fields:

      KRB-ERROR       ::= [APPLICATION 30] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (30),
              ctime           [2] KerberosTime OPTIONAL,
              cusec           [3] Microseconds OPTIONAL,
              stime           [4] KerberosTime,
              susec           [5] Microseconds,
              error-code      [6] Int32,
              crealm          [7] Realm OPTIONAL,
              cname           [8] PrincipalName OPTIONAL,
              realm           [9] Realm -- service realm --,
              sname           [10] PrincipalName -- service name --,
              e-text          [11] KerberosString OPTIONAL,
              e-data          [12] OCTET STRING OPTIONAL
      }

   pvno and msg-type



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      These fields are described above in section 5.4.1. msg-type is
      KRB_ERROR.

   ctime
      This field is described above in section 5.5.2.

   cusec
      This field is described above in section 5.5.2.

   stime
      This field contains the current time on the server. It is of type
      KerberosTime.

   susec
      This field contains the microsecond part of the server's
      timestamp. Its value ranges from 0 to 999999. It appears along
      with stime. The two fields are used in conjunction to specify a
      reasonably accurate timestamp.

   error-code
      This field contains the error code returned by Kerberos or the
      server when a request fails. To interpret the value of this field
      see the list of error codes in section 7.5.9. Implementations are
      encouraged to provide for national language support in the display
      of error messages.

   crealm, cname, realm and sname
      These fields are described above in section 5.3.

   e-text
      This field contains additional text to help explain the error code
      associated with the failed request (for example, it might include
      a principal name which was unknown).

   e-data
      This field contains additional data about the error for use by the
      application to help it recover from or handle the error. If the
      errorcode is KDC_ERR_PREAUTH_REQUIRED, then the e-data field will
      contain an encoding of a sequence of padata fields, each
      corresponding to an acceptable pre-authentication method and
      optionally containing data for the method:

      METHOD-DATA     ::= SEQUENCE OF PA-DATA

   For error codes defined in this document other than
   KDC_ERR_PREAUTH_REQUIRED, the format and contents of the e-data field
   are implementation-defined. Similarly, for future error codes, the
   format and contents of the e-data field are implementation-defined



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   unless specified. Whether defined by the implementation or in a
   future document, the e-data field MAY take the form of TYPED-DATA:

   TYPED-DATA      ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
           data-type       [0] INTEGER,
           data-value      [1] OCTET STRING OPTIONAL
   }

5.10. Application Tag Numbers

      The following table lists the application class tag numbers used
      by various data types defined in this section.

       Tag Number(s)    Type Name    Comments

       0                             unused

       1              Ticket         PDU

       2              Authenticator  non-PDU

       3              EncTicketPart  non-PDU

       4-9                           unused

       10             AS-REQ         PDU

       11             AS-REP         PDU

       12             TGS-REQ        PDU

       13             TGS-REP        PDU

       14             AP-REQ         PDU

       15             AP-REP         PDU

       16             RESERVED16     TGT-REQ (for user-to-user)

       17             RESERVED17     TGT-REP (for user-to-user)

       18-19                         unused

       20             KRB-SAFE       PDU

       21             KRB-PRIV       PDU

       22             KRB-CRED       PDU



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       23-24                         unused

       25             EncASRepPart   non-PDU

       26             EncTGSRepPart  non-PDU

       27             EncApRepPart   non-PDU

       28             EncKrbPrivPart non-PDU

       29             EncKrbCredPart non-PDU

       30             KRB-ERROR      PDU

      The ASN.1 types marked as "PDU" (Protocol Data Unit) in the above
      are the only ASN.1 types intended as top-level types of the
      Kerberos protocol, and are the only types that may be used as
      elements in another protocol that makes use of Kerberos.

6. Naming Constraints

6.1. Realm Names

      Although realm names are encoded as GeneralStrings and although a
      realm can technically select any name it chooses, interoperability
      across realm boundaries requires agreement on how realm names are
      to be assigned, and what information they imply.

      To enforce these conventions, each realm MUST conform to the
      conventions itself, and it MUST require that any realms with which
      inter-realm keys are shared also conform to the conventions and
      require the same from its neighbors.

      Kerberos realm names are case sensitive. Realm names that differ
      only in the case of the characters are not equivalent. There are
      presently three styles of realm names: domain, X500, and other.
      Examples of each style follow:

           domain:   ATHENA.MIT.EDU
             X500:   C=US/O=OSF
            other:   NAMETYPE:rest/of.name=without-restrictions

      Domain style realm names MUST look like domain names: they consist
      of components separated by periods (.) and they contain neither
      colons (:) nor slashes (/). Though domain names themselves are
      case insensitive, in order for realms to match, the case must
      match as well. When establishing a new realm name based on an
      internet domain name it is recommended by convention that the



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      characters be converted to upper case.

      X.500 names contain an equal (=) and cannot contain a colon (:)
      before the equal. The realm names for X.500 names will be string
      representations of the names with components separated by slashes.
      Leading and trailing slashes will not be included. Note that the
      slash separator is consistent with Kerberos implementations based
      on RFC1510, but it is different from the separator recommended in
      RFC2253.

      Names that fall into the other category MUST begin with a prefix
      that contains no equal (=) or period (.) and the prefix MUST be
      followed by a colon (:) and the rest of the name. All prefixes
      expect those beginning with used. Presently none are assigned.

      The reserved category includes strings which do not fall into the
      first three categories. All names in this category are reserved.
      It is unlikely that names will be assigned to this category unless
      there is a very strong argument for not using the 'other'
      category.

      These rules guarantee that there will be no conflicts between the
      various name styles. The following additional constraints apply to
      the assignment of realm names in the domain and X.500 categories:
      the name of a realm for the domain or X.500 formats must either be
      used by the organization owning (to whom it was assigned) an
      Internet domain name or X.500 name, or in the case that no such
      names are registered, authority to use a realm name MAY be derived
      from the authority of the parent realm. For example, if there is
      no domain name for E40.MIT.EDU, then the administrator of the
      MIT.EDU realm can authorize the creation of a realm with that
      name.

      This is acceptable because the organization to which the parent is
      assigned is presumably the organization authorized to assign names
      to its children in the X.500 and domain name systems as well. If
      the parent assigns a realm name without also registering it in the
      domain name or X.500 hierarchy, it is the parent's responsibility
      to make sure that there will not in the future exist a name
      identical to the realm name of the child unless it is assigned to
      the same entity as the realm name.

6.2. Principal Names

      As was the case for realm names, conventions are needed to ensure
      that all agree on what information is implied by a principal name.
      The name-type field that is part of the principal name indicates
      the kind of information implied by the name. The name-type SHOULD



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      be treated only as a hint to interpreting the meaning of a name.
      It is not significant when checking for equivalence. Principal
      names that differ only in the name-type identify the same
      principal. The name type does not partition the name space.
      Ignoring the name type, no two names can be the same (i.e. at
      least one of the components, or the realm, MUST be different). The
      following name types are defined:

      name-type      value   meaning

      name types

      NT-UNKNOWN        0  Name type not known
      NT-PRINCIPAL      1  Just the name of the principal as in DCE, or for users
      NT-SRV-INST       2  Service and other unique instance (krbtgt)
      NT-SRV-HST        3  Service with host name as instance (telnet, rcommands)
      NT-SRV-XHST       4  Service with host as remaining components
      NT-UID            5  Unique ID
      NT-X500-PRINCIPAL 6  Encoded X.509 Distingished name [RFC 2253]
      NT-SMTP-NAME      7  Name in form of SMTP email name (e.g. user@foo.com)
      NT-ENTERPRISE    10   Enterprise name - may be mapped to principal name

      When a name implies no information other than its uniqueness at a
      particular time the name type PRINCIPAL SHOULD be used. The
      principal name type SHOULD be used for users, and it might also be
      used for a unique server. If the name is a unique machine
      generated ID that is guaranteed never to be reassigned then the
      name type of UID SHOULD be used (note that it is generally a bad
      idea to reassign names of any type since stale entries might
      remain in access control lists).

      If the first component of a name identifies a service and the
      remaining components identify an instance of the service in a
      server specified manner, then the name type of SRV-INST SHOULD be
      used. An example of this name type is the Kerberos ticket-granting
      service whose name has a first component of krbtgt and a second
      component identifying the realm for which the ticket is valid.

      If the first component of a name identifies a service and there is
      a single component following the service name identifying the
      instance as the host on which the server is running, then the name
      type SRV-HST SHOULD be used. This type is typically used for
      Internet services such as telnet and the Berkeley R commands. If
      the separate components of the host name appear as successive
      components following the name of the service, then the name type
      SRV-XHST SHOULD be used. This type might be used to identify
      servers on hosts with X.500 names where the slash (/) might
      otherwise be ambiguous.



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      A name type of NT-X500-PRINCIPAL SHOULD be used when a name from
      an X.509 certificate is translated into a Kerberos name. The
      encoding of the X.509 name as a Kerberos principal shall conform
      to the encoding rules specified in RFC 2253.

      A name type of SMTP allows a name to be of a form that resembles a
      SMTP email name. This name, including an "@" and a domain name, is
      used as the one component of the principal name.

      A name type of UNKNOWN SHOULD be used when the form of the name is
      not known. When comparing names, a name of type UNKNOWN will match
      principals authenticated with names of any type. A principal
      authenticated with a name of type UNKNOWN, however, will only
      match other names of type UNKNOWN.

      Names of any type with an initial component of 'krbtgt' are
      reserved for the Kerberos ticket granting service. See section 7.3
      for the form of such names.

6.2.1. Name of server principals

      The principal identifier for a server on a host will generally be
      composed of two parts: (1) the realm of the KDC with which the
      server is registered, and (2) a two-component name of type NT-SRV-
      HST if the host name is an Internet domain name or a multi-
      component name of type NT-SRV-XHST if the name of the host is of a
      form such as X.500 that allows slash (/) separators. The first
      component of the two- or multi-component name will identify the
      service and the latter components will identify the host. Where
      the name of the host is not case sensitive (for example, with
      Internet domain names) the name of the host MUST be lower case. If
      specified by the application protocol for services such as telnet
      and the Berkeley R commands which run with system privileges, the
      first component MAY be the string 'host' instead of a service
      specific identifier.

7. Constants and other defined values

7.1. Host address types

      All negative values for the host address type are reserved for
      local use.  All non-negative values are reserved for officially
      assigned type fields and interpretations.

   Internet (IPv4) Addresses

      Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded
      in MSB order. The IPv4 loopback address SHOULD NOT appear in a



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      Kerberos packet. The type of IPv4 addresses is two (2).

   Internet (IPv6) Addresses

      IPv6 addresses [RFC2373] are 128-bit (16-octet) quantities,
      encoded in MSB order. The type of IPv6 addresses is twenty-four
      (24). The following addresses MUST NOT appear in any Kerberos
      packet:

         *  the Unspecified Address
         *  the Loopback Address
         *  Link-Local addresses

      IPv4-mapped IPv6 addresses MUST be represented as addresses of
      type 2.

   DECnet Phase IV addresses

      DECnet Phase IV addresses are 16-bit addresses, encoded in LSB
      order. The type of DECnet Phase IV addresses is twelve (12).

   Netbios addresses

      Netbios addresses are 16-octet addresses typically composed of 1
      to 15 alphanumeric characters and padded with the US-ASCII SPC
      character (code 32).  The 16th octet MUST be the US-ASCII NUL
      character (code 0).  The type of Netbios addresses is twenty (20).

   Directional Addresses

      In many environments, including the sender address in KRB_SAFE and
      KRB_PRIV messages is undesirable because the addresses may be
      changed in transport by network address translators. However, if
      these addresses are removed, the messages may be subject to a
      reflection attack in which a message is reflected back to its
      originator. The directional address type provides a way to avoid
      transport addresses and reflection attacks. Directional addresses
      are encoded as four byte unsigned integers in network byte order.
      If the message is originated by the party sending the original
      KRB_AP_REQ message, then an address of 0 SHOULD be used. If the
      message is originated by the party to whom that KRB_AP_REQ was
      sent, then the address 1 SHOULD be used. Applications involving
      multiple parties can specify the use of other addresses.

      Directional addresses MUST only be used for the sender address
      field in the KRB_SAFE or KRB_PRIV messages. They MUST NOT be used
      as a ticket address or in a KRB_AP_REQ message. This address type
      SHOULD only be used in situations where the sending party knows



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      that the receiving party supports the address type. This generally
      means that directional addresses may only be used when the
      application protocol requires their support. Directional addresses
      are type (3).

7.2. KDC messaging - IP Transports

      Kerberos defines two IP transport mechanisms for communication
      between clients and servers: UDP/IP and TCP/IP.

7.2.1. UDP/IP transport

      Kerberos servers (KDCs) supporting IP transports MUST accept UDP
      requests and SHOULD listen for such requests on port 88 (decimal)
      unless specifically configured to listen on an alternative UDP
      port. Alternate ports MAY be used when running multiple KDCs for
      multiple realms on the same host.

      Kerberos clients supporting IP transports SHOULD support the
      sending of UDP requests. Clients SHOULD use KDC discovery [7.2.3]
      to identify the IP address and port to which they will send their
      request.

      When contacting a KDC for a KRB_KDC_REQ request using UDP/IP
      transport, the client shall send a UDP datagram containing only an
      encoding of the request to the KDC. The KDC will respond with a
      reply datagram containing only an encoding of the reply message
      (either a KRB_ERROR or a KRB_KDC_REP) to the sending port at the
      sender's IP address. The response to a request made through UDP/IP
      transport MUST also use UDP/IP transport. If the response can not
      be handled using UDP (for example because it is too large), the
      KDC MUST return KRB_ERR_RESPONSE_TOO_BIG, forcing the client to
      retry the request using the TCP transport.

7.2.2. TCP/IP transport

      Kerberos servers (KDCs) supporting IP transports MUST accept TCP
      requests and SHOULD listen for such requests on port 88 (decimal)
      unless specifically configured to listen on an alternate TCP port.
      Alternate ports MAY be used when running multiple KDCs for
      multiple realms on the same host.

      Clients MUST support the sending of TCP requests, but MAY choose
      to initially try a request using the UDP transport. Clients SHOULD
      use KDC discovery [7.2.3] to identify the IP address and port to
      which they will send their request.

      Implementation note: Some extensions to the Kerberos protocol will



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      not succeed if any client or KDC not supporting the TCP transport
      is involved.  Implementations of RFC 1510 were not required to
      support TCP/IP transports.

      When the KRB_KDC_REQ message is sent to the KDC over a TCP stream,
      the response (KRB_KDC_REP or KRB_ERROR message) MUST be returned
      to the client on the same TCP stream that was established for the
      request. The KDC MAY close the TCP stream after sending a
      response, but MAY leave the stream open for a reasonable period of
      time if it expects a followup. Care must be taken in managing
      TCP/IP connections on the KDC to prevent denial of service attacks
      based on the number of open TCP/IP connections.

      The client MUST be prepared to have the stream closed by the KDC
      at anytime after the receipt of a response. A stream closure
      SHOULD NOT be treated as a fatal error. Instead, if multiple
      exchanges are required (e.g., certain forms of pre-authentication)
      the client may need to establish a new connection when it is ready
      to send subsequent messages. A client MAY close the stream after
      receiving a response, and SHOULD close the stream if it does not
      expect to send followup messages.

      A client MAY send multiple requests before receiving responses,
      though it must be prepared to handle the connection being closed
      after the first response.

      Each request (KRB_KDC_REQ) and response (KRB_KDC_REP or KRB_ERROR)
      sent over the TCP stream is preceded by the length of the request
      as 4 octets in network byte order. The high bit of the length is
      reserved for future expansion and MUST currently be set to zero.
      If a KDC that does not understand how to interpret a set high bit
      of the length encoding receives a request with the high order bit
      of the length set, it MUST return a KRB-ERROR message with the
      error KRB_ERR_FIELD_TOOLONG and MUST close the TCP stream.

      If multiple requests are sent over a single TCP connection, and
      the KDC sends multiple responses, the KDC is not required to send
      the responses in the order of the corresponding requests. This may
      permit some implementations to send each response as soon as it is
      ready even if earlier requests are still being processed (for
      example, waiting for a response from an external device or
      database).

7.2.3. KDC Discovery on IP Networks

      Kerberos client implementations MUST provide a means for the
      client to determine the location of the Kerberos Key Distribution
      Centers (KDCs).  Traditionally, Kerberos implementations have



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      stored such configuration information in a file on each client
      machine. Experience has shown this method of storing configuration
      information presents problems with out-of-date information and
      scaling problems, especially when using cross-realm
      authentication. This section describes a method for using the
      Domain Name System [RFC 1035] for storing KDC location
      information.

7.2.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names

      In Kerberos, realm names are case sensitive. While it is strongly
      encouraged that all realm names be all upper case this
      recommendation has not been adopted by all sites. Some sites use
      all lower case names and other use mixed case. DNS on the other
      hand is case insensitive for queries. Since the realm names
      "MYREALM", "myrealm", and "MyRealm" are all different, but resolve
      the same in the domain name system, it is necessary that only one
      of the possible combinations of upper and lower case characters be
      used in realm names.

7.2.3.2. Specifying KDC Location information with DNS SRV records

      KDC location information is to be stored using the DNS SRV RR [RFC
      2782].  The format of this RR is as follows:

         _Service._Proto.Realm TTL Class SRV Priority Weight Port Target

      The Service name for Kerberos is always "kerberos".

      The Proto can be one of "udp", "tcp". If these SRV records are to
      be used, both "udp" and "tcp" records MUST be specified for all
      KDC deployments.

      The Realm is the Kerberos realm that this record corresponds to.
      The realm MUST be a domain style realm name.

      TTL, Class, SRV, Priority, Weight, and Target have the standard
      meaning as defined in RFC 2782.

      As per RFC 2782 the Port number used for "_udp" and "_tcp" SRV
      records SHOULD be the value assigned to "kerberos" by the Internet
      Assigned Number Authority: 88 (decimal) unless the KDC is
      configured to listen on an alternate TCP port.

      Implementation note: Many existing client implementations do not
      support KDC Discovery and are configured to send requests to the
      IANA assigned port (88 decimal), so it is strongly recommended
      that KDCs be configured to listen on that port.



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7.2.3.3. KDC Discovery for Domain Style Realm Names on IP Networks

      These are DNS records for a Kerberos realm EXAMPLE.COM. It has two
      Kerberos servers, kdc1.example.com and kdc2.example.com. Queries
      should be directed to kdc1.example.com first as per the specified
      priority. Weights are not used in these sample records.

        _kerberos._udp.EXAMPLE.COM.     IN   SRV   0 0 88 kdc1.example.com.
        _kerberos._udp.EXAMPLE.COM.     IN   SRV   1 0 88 kdc2.example.com.
        _kerberos._tcp.EXAMPLE.COM.     IN   SRV   0 0 88 kdc1.example.com.
        _kerberos._tcp.EXAMPLE.COM.     IN   SRV   1 0 88 kdc2.example.com.

7.3. Name of the TGS

      The principal identifier of the ticket-granting service shall be
      composed of three parts: (1) the realm of the KDC issuing the TGS
      ticket (2) a two-part name of type NT-SRV-INST, with the first
      part "krbtgt" and the second part the name of the realm which will
      accept the ticket-granting ticket. For example, a ticket-granting
      ticket issued by the ATHENA.MIT.EDU realm to be used to get
      tickets from the ATHENA.MIT.EDU KDC has a principal identifier of
      "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A
      ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be
      used to get tickets from the MIT.EDU realm has a principal
      identifier of "ATHENA.MIT.EDU" (realm), ("krbtgt", "MIT.EDU")
      (name).

7.4. OID arc for KerberosV5

      This OID MAY be used to identify Kerberos protocol messages
      encapsulated in other protocols. It also designates the OID arc
      for KerberosV5-related OIDs assigned by future IETF action.
      Implementation note:: RFC 1510 had an incorrect value (5) for
      "dod" in its OID.

      id-krb5         OBJECT IDENTIFIER ::= {
              iso(1) identified-organization(3) dod(6) internet(1)
              security(5) kerberosV5(2)
      }


      Assignment of OIDs beneath the id-krb5 arc must be obtained by
      contacting the registrar for the id-krb5 arc, or its designee.  At
      the time of the issuance of this RFC, such registrations can be
      obtained by contacting krb5-oid-registrar@mit.edu.

7.5. Protocol constants and associated values




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      The following tables list constants used in the protocol and
      define their meanings. Ranges are specified in the "specification"
      section that limit the values of constants for which values are
      defined here. This allows implementations to make assumptions
      about the maximum values that will be received for these
      constants. Implementation receiving values outside the range
      specified in the "specification" section MAY reject the request,
      but they MUST recover cleanly.

7.5.1. Key usage numbers

      The encryption and checksum specifications in [@KCRYPTO] require
      as input a "key usage number", to alter the encryption key used in
      any specific message, to make certain types of cryptographic
      attack more difficult. These are the key usage values assigned in
      this document:

              1.          AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted
                          with the client key (section 5.2.7.2)
              2.          AS-REP Ticket and TGS-REP Ticket (includes TGS session
                          key or application session key), encrypted with the
                          service key (section 5.3)
              3.          AS-REP encrypted part (includes TGS session key or
                          application session key), encrypted with the client key
                          (section 5.4.2)
              4.          TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
                          the TGS session key (section 5.4.1)
              5.          TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
                          the TGS authenticator subkey (section 5.4.1)
              6.          TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum,
                          keyed with the TGS session key (sections 5.5.1)
              7.          TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator
                          (includes TGS authenticator subkey), encrypted with the
                          TGS session key (section 5.5.1)
              8.          TGS-REP encrypted part (includes application session
                          key), encrypted with the TGS session key (section
                          5.4.2)
              9.          TGS-REP encrypted part (includes application session
                          key), encrypted with the TGS authenticator subkey
                          (section 5.4.2)
              10.         AP-REQ Authenticator cksum, keyed with the application
                          session key (section 5.5.1)
              11.         AP-REQ Authenticator (includes application
                          authenticator subkey), encrypted with the application
                          session key (section 5.5.1)
              12.         AP-REP encrypted part (includes application session
                          subkey), encrypted with the application session key
                          (section 5.5.2)



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              13.         KRB-PRIV encrypted part, encrypted with a key chosen by
                          the application (section 5.7.1)
              14.         KRB-CRED encrypted part, encrypted with a key chosen by
                          the application (section 5.8.1)
              15.         KRB-SAFE cksum, keyed with a key chosen by the
                          application (section 5.6.1)
              19.         AD-KDC-ISSUED checksum (ad-checksum in 5.2.6.4)
            22-25.        Reserved for use in GSSAPI mechanisms derived from RFC
                          1964. (raeburn/MIT)
       16-18,20-21,26-511. Reserved for future use in Kerberos and related
                          protocols.
           512-1023.      Reserved for uses internal to a Kerberos
                          implementation.
            1024.         Encryption for application use in protocols that
                          do not specify key usage values
            1025.         Checksums for application use in protocols that
                          do not specify key usage values
          1026-2047.      Reserved for application use.


7.5.2. PreAuthentication Data Types

      padata and data types           padata-type value  comment

      PA-TGS-REQ                      1
      PA-ENC-TIMESTAMP                2
      PA-PW-SALT                      3
      [reserved]                      4
      PA-ENC-UNIX-TIME                5        (deprecated)
      PA-SANDIA-SECUREID              6
      PA-SESAME                       7
      PA-OSF-DCE                      8
      PA-CYBERSAFE-SECUREID           9
      PA-AFS3-SALT                    10
      PA-ETYPE-INFO                   11
      PA-SAM-CHALLENGE                12       (sam/otp)
      PA-SAM-RESPONSE                 13       (sam/otp)
      PA-PK-AS-REQ                    14       (pkinit)
      PA-PK-AS-REP                    15       (pkinit)
      PA-ETYPE-INFO2                  19       (replaces pa-etype-info)
      PA-USE-SPECIFIED-KVNO           20
      PA-SAM-REDIRECT                 21       (sam/otp)
      PA-GET-FROM-TYPED-DATA          22       (embedded in typed data)
      TD-PADATA                       22       (embeds padata)
      PA-SAM-ETYPE-INFO               23       (sam/otp)
      PA-ALT-PRINC                    24       (crawdad@fnal.gov)
      PA-SAM-CHALLENGE2               30       (kenh@pobox.com)
      PA-SAM-RESPONSE2                31       (kenh@pobox.com)



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      PA-EXTRA-TGT                    41       Reserved extra TGT
      TD-PKINIT-CMS-CERTIFICATES      101      CertificateSet from CMS
      TD-KRB-PRINCIPAL                102      PrincipalName
      TD-KRB-REALM                    103      Realm
      TD-TRUSTED-CERTIFIERS           104      from PKINIT
      TD-CERTIFICATE-INDEX            105      from PKINIT
      TD-APP-DEFINED-ERROR            106      application specific
      TD-REQ-NONCE                    107      INTEGER
      TD-REQ-SEQ                      108      INTEGER
      PA-PAC-REQUEST                  128      (jbrezak@exchange.microsoft.com)

7.5.3. Address Types

      Address type                   value

      IPv4                             2
      Directional                      3
      ChaosNet                         5
      XNS                              6
      ISO                              7
      DECNET Phase IV                 12
      AppleTalk DDP                   16
      NetBios                         20
      IPv6                            24

7.5.4. Authorization Data Types

      authorization data type         ad-type value
      AD-IF-RELEVANT                     1
      AD-INTENDED-FOR-SERVER             2
      AD-INTENDED-FOR-APPLICATION-CLASS  3
      AD-KDC-ISSUED                      4
      AD-AND-OR                          5
      AD-MANDATORY-TICKET-EXTENSIONS     6
      AD-IN-TICKET-EXTENSIONS            7
      AD-MANDATORY-FOR-KDC               8
      reserved values                    9-63
      OSF-DCE                            64
      SESAME                             65
      AD-OSF-DCE-PKI-CERTID              66         (hemsath@us.ibm.com)
      AD-WIN2K-PAC                      128         (jbrezak@exchange.microsoft.com)

7.5.5. Transited Encoding Types

      transited encoding type         tr-type value
      DOMAIN-X500-COMPRESS            1
      reserved values                 all others




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7.5.6. Protocol Version Number

      Label               Value   Meaning or MIT code

      pvno                    5   current Kerberos protocol version number

7.5.7. Kerberos Message Types

      message types

      KRB_AS_REQ             10   Request for initial authentication
      KRB_AS_REP             11   Response to KRB_AS_REQ request
      KRB_TGS_REQ            12   Request for authentication based on TGT
      KRB_TGS_REP            13   Response to KRB_TGS_REQ request
      KRB_AP_REQ             14   application request to server
      KRB_AP_REP             15   Response to KRB_AP_REQ_MUTUAL
      KRB_RESERVED16         16   Reserved for user-to-user krb_tgt_request
      KRB_RESERVED17         17   Reserved for user-to-user krb_tgt_reply
      KRB_SAFE               20   Safe (checksummed) application message
      KRB_PRIV               21   Private (encrypted) application message
      KRB_CRED               22   Private (encrypted) message to forward credentials
      KRB_ERROR              30   Error response

7.5.8. Name Types

      name types

      KRB_NT_UNKNOWN        0  Name type not known
      KRB_NT_PRINCIPAL      1  Just the name of the principal as in DCE, or for users
      KRB_NT_SRV_INST       2  Service and other unique instance (krbtgt)
      KRB_NT_SRV_HST        3  Service with host name as instance (telnet, rcommands)
      KRB_NT_SRV_XHST       4  Service with host as remaining components
      KRB_NT_UID            5  Unique ID
      KRB_NT_X500_PRINCIPAL 6  Encoded X.509 Distingished name [RFC 2253]
      KRB_NT_SMTP_NAME      7  Name in form of SMTP email name (e.g. user@foo.com)
      KRB_NT_ENTERPRISE    10   Enterprise name - may be mapped to principal name

7.5.9. Error Codes

      error codes

      KDC_ERR_NONE                    0   No error
      KDC_ERR_NAME_EXP                1   Client's entry in database has expired
      KDC_ERR_SERVICE_EXP             2   Server's entry in database has expired
      KDC_ERR_BAD_PVNO                3   Requested protocol version number
                                             not supported
      KDC_ERR_C_OLD_MAST_KVNO         4   Client's key encrypted in old master key
      KDC_ERR_S_OLD_MAST_KVNO         5   Server's key encrypted in old master key



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      KDC_ERR_C_PRINCIPAL_UNKNOWN     6   Client not found in Kerberos database
      KDC_ERR_S_PRINCIPAL_UNKNOWN     7   Server not found in Kerberos database
      KDC_ERR_PRINCIPAL_NOT_UNIQUE    8   Multiple principal entries in database
      KDC_ERR_NULL_KEY                9   The client or server has a null key
      KDC_ERR_CANNOT_POSTDATE        10   Ticket not eligible for postdating
      KDC_ERR_NEVER_VALID            11   Requested start time is later than end time
      KDC_ERR_POLICY                 12   KDC policy rejects request
      KDC_ERR_BADOPTION              13   KDC cannot accommodate requested option
      KDC_ERR_ETYPE_NOSUPP           14   KDC has no support for encryption type
      KDC_ERR_SUMTYPE_NOSUPP         15   KDC has no support for checksum type
      KDC_ERR_PADATA_TYPE_NOSUPP     16   KDC has no support for padata type
      KDC_ERR_TRTYPE_NOSUPP          17   KDC has no support for transited type
      KDC_ERR_CLIENT_REVOKED         18   Clients credentials have been revoked
      KDC_ERR_SERVICE_REVOKED        19   Credentials for server have been revoked
      KDC_ERR_TGT_REVOKED            20   TGT has been revoked
      KDC_ERR_CLIENT_NOTYET          21   Client not yet valid - try again later
      KDC_ERR_SERVICE_NOTYET         22   Server not yet valid - try again later
      KDC_ERR_KEY_EXPIRED            23   Password has expired
                                                - change password to reset
      KDC_ERR_PREAUTH_FAILED         24   Pre-authentication information was invalid
      KDC_ERR_PREAUTH_REQUIRED       25   Additional pre-authenticationrequired
      KDC_ERR_SERVER_NOMATCH         26   Requested server and ticket don't match
      KDC_ERR_MUST_USE_USER2USER     27   Server principal valid for user2user only
      KDC_ERR_PATH_NOT_ACCPETED      28   KDC Policy rejects transited path
      KDC_ERR_SVC_UNAVAILABLE        29   A service is not available
      KRB_AP_ERR_BAD_INTEGRITY       31   Integrity check on decrypted field failed
      KRB_AP_ERR_TKT_EXPIRED         32   Ticket expired
      KRB_AP_ERR_TKT_NYV             33   Ticket not yet valid
      KRB_AP_ERR_REPEAT              34   Request is a replay
      KRB_AP_ERR_NOT_US              35   The ticket isn't for us
      KRB_AP_ERR_BADMATCH            36   Ticket and authenticator don't match
      KRB_AP_ERR_SKEW                37   Clock skew too great
      KRB_AP_ERR_BADADDR             38   Incorrect net address
      KRB_AP_ERR_BADVERSION          39   Protocol version mismatch
      KRB_AP_ERR_MSG_TYPE            40   Invalid msg type
      KRB_AP_ERR_MODIFIED            41   Message stream modified
      KRB_AP_ERR_BADORDER            42   Message out of order
      KRB_AP_ERR_BADKEYVER           44   Specified version of key is not available
      KRB_AP_ERR_NOKEY               45   Service key not available
      KRB_AP_ERR_MUT_FAIL            46   Mutual authentication failed
      KRB_AP_ERR_BADDIRECTION        47   Incorrect message direction
      KRB_AP_ERR_METHOD              48   Alternative authentication method required
      KRB_AP_ERR_BADSEQ              49   Incorrect sequence number in message
      KRB_AP_ERR_INAPP_CKSUM         50   Inappropriate type of checksum in message
      KRB_AP_PATH_NOT_ACCEPTED       51   Policy rejects transited path
      KRB_ERR_RESPONSE_TOO_BIG       52   Response too big for UDP, retry with TCP
      KRB_ERR_GENERIC                60   Generic error (description in e-text)
      KRB_ERR_FIELD_TOOLONG          61   Field is too long for this implementation



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      KDC_ERROR_CLIENT_NOT_TRUSTED      62 Reserved for PKINIT
      KDC_ERROR_KDC_NOT_TRUSTED         63 Reserved for PKINIT
      KDC_ERROR_INVALID_SIG             64 Reserved for PKINIT
      KDC_ERR_KEY_TOO_WEAK              65 Reserved for PKINIT
      KDC_ERR_CERTIFICATE_MISMATCH      66 Reserved for PKINIT
      KRB_AP_ERR_NO_TGT                 67 No TGT available to validate USER-TO-USER
      KDC_ERR_WRONG_REALM               68 USER-TO-USER TGT issued different KDC
      KRB_AP_ERR_USER_TO_USER_REQUIRED  69 Ticket must be for USER-TO-USER
      KDC_ERR_CANT_VERIFY_CERTIFICATE   70 Reserved for PKINIT
      KDC_ERR_INVALID_CERTIFICATE             71 Reserved for PKINIT
      KDC_ERR_REVOKED_CERTIFICATE             72 Reserved for PKINIT
      KDC_ERR_REVOCATION_STATUS_UNKNOWN       73 Reserved for PKINIT
      KDC_ERR_REVOCATION_STATUS_UNAVAILABLE   74 Reserved for PKINIT
      KDC_ERR_CLIENT_NAME_MISMATCH            75 Reserved for PKINIT
      KDC_ERR_KDC_NAME_MISMATCH               76 Reserved for PKINIT

8. Interoperability requirements

      Version 5 of the Kerberos protocol supports a myriad of options.
      Among these are multiple encryption and checksum types,
      alternative encoding schemes for the transited field, optional
      mechanisms for pre-authentication, the handling of tickets with no
      addresses, options for mutual authentication, user-to-user
      authentication, support for proxies, forwarding, postdating, and
      renewing tickets, the format of realm names, and the handling of
      authorization data.

      In order to ensure the interoperability of realms, it is necessary
      to define a minimal configuration which must be supported by all
      implementations. This minimal configuration is subject to change
      as technology does. For example, if at some later date it is
      discovered that one of the required encryption or checksum
      algorithms is not secure, it will be replaced.

8.1. Specification 2

      This section defines the second specification of these options.
      Implementations which are configured in this way can be said to
      support Kerberos Version 5 Specification 2 (5.2). Specification 1
      (deprecated) may be found in RFC1510.

   Transport

      TCP/IP and UDP/IP transport MUST be supported by clients and KDCs
      claiming conformance to specification 2.

   Encryption and checksum methods




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      The following encryption and checksum mechanisms MUST be
      supported.

      Encryption: AES256-CTS-HMAC-SHA1-96
      Checksums: HMAC-SHA1-96-AES256

      Implementations SHOULD support other mechanisms as well, but the
      additional mechanisms may only be used when communicating with
      principals known to also support them. The mechanisms that SHOULD
      be supported are:

      Encryption:  DES-CBC-MD5, DES3-CBC-SHA1-KD
      Checksums:   DES-MD5, HMAC-SHA1-DES3-KD

      Implementations MAY support other mechanisms as well, but the
      additional mechanisms may only be used when communicating with
      principals known to also support them.

      Implementation note: earlier implementations of Kerberos generate
      messages using the CRC-32, RSA-MD5 checksum methods. For
      interoperability with these earlier releases implementors MAY
      consider supporting these checksum methods but should carefully
      analyze the security impplications to limit the situations within
      which these methods are accepted.

   Realm Names

      All implementations MUST understand hierarchical realms in both
      the Internet Domain and the X.500 style. When a ticket-granting
      ticket for an unknown realm is requested, the KDC MUST be able to
      determine the names of the intermediate realms between the KDCs
      realm and the requested realm.

   Transited field encoding

      DOMAIN-X500-COMPRESS (described in section 3.3.3.2) MUST be
      supported.  Alternative encodings MAY be supported, but they may
      be used only when that encoding is supported by ALL intermediate
      realms.

   Pre-authentication methods

      The TGS-REQ method MUST be supported. The TGS-REQ method is not
      used on the initial request. The PA-ENC-TIMESTAMP method MUST be
      supported by clients but whether it is enabled by default MAY be
      determined on a realm by realm basis. If not used in the initial
      request and the error KDC_ERR_PREAUTH_REQUIRED is returned
      specifying PA-ENC-TIMESTAMP as an acceptable method, the client



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      SHOULD retry the initial request using the PA-ENC-TIMESTAMP pre-
      authentication method. Servers need not support the PA-ENC-
      TIMESTAMP method, but if not supported the server SHOULD ignore
      the presence of PA-ENC-TIMESTAMP pre-authentication in a request.

      The ETYPE-INFO2 method MUST be supported; this method is used to
      communicate the set of supported encryption types, and
      corresponding salt and string to key paramters. The ETYPE-INFO
      method SHOULD be supported for interoperability with older
      implementation.

   Mutual authentication

      Mutual authentication (via the KRB_AP_REP message) MUST be
      supported.

   Ticket addresses and flags

      All KDCs MUST pass through tickets that carry no addresses (i.e.
      if a TGT contains no addresses, the KDC will return derivative
      tickets).  Implementations SHOULD default to requesting
      addressless tickets as this significantly increases
      interoperability with network address translation.  In some cases
      realms or application servers MAY require that tickets have an
      address.

      Implementations SHOULD accept directional address type for the
      KRB_SAFE and KRB_PRIV message and SHOULD include directional
      addresses in these messages when other address types are not
      available.

      Proxies and forwarded tickets MUST be supported. Individual realms
      and application servers can set their own policy on when such
      tickets will be accepted.

      All implementations MUST recognize renewable and postdated
      tickets, but need not actually implement them. If these options
      are not supported, the starttime and endtime in the ticket shall
      specify a ticket's entire useful life. When a postdated ticket is
      decoded by a server, all implementations shall make the presence
      of the postdated flag visible to the calling server.

   User-to-user authentication

      Support for user-to-user authentication (via the ENC-TKT-IN-SKEY
      KDC option) MUST be provided by implementations, but individual
      realms MAY decide as a matter of policy to reject such requests on
      a per-principal or realm-wide basis.



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   Authorization data

      Implementations MUST pass all authorization data subfields from
      ticket-granting tickets to any derivative tickets unless directed
      to suppress a subfield as part of the definition of that
      registered subfield type (it is never incorrect to pass on a
      subfield, and no registered subfield types presently specify
      suppression at the KDC).

      Implementations MUST make the contents of any authorization data
      subfields available to the server when a ticket is used.
      Implementations are not required to allow clients to specify the
      contents of the authorization data fields.

   Constant ranges

      All protocol constants are constrained to 32 bit (signed) values
      unless further constrained by the protocol definition. This limit
      is provided to allow implementations to make assumptions about the
      maximum values that will be received for these constants.
      Implementation receiving values outside this range MAY reject the
      request, but they MUST recover cleanly.

8.2. Recommended KDC values

      Following is a list of recommended values for a KDC configuration.

      minimum lifetime              5 minutes
      maximum renewable lifetime    1 week
      maximum ticket lifetime       1 day
      acceptable clock skew         5 minutes
      empty addresses               Allowed.
      proxiable, etc.               Allowed.

9. IANA considerations

      Section 7 of this document specifies protocol constants and other
      defined values required for the interoperability of multiple
      implementations. Until otherwise specified in a subsequent RFC, or
      upon disbanding of the Kerberos working group, allocations of
      additional protocol constants and other defined values required
      for extensions to the Kerberos protocol will be administered by
      the kerberos working group.  Following the recomendations outlined
      in [RFC 2434], guidance is provided to the IANA as follows:

      "reserved" realm name types in section 6.1 and "other" realm types
      except those beginning with "X-" or "x-" will not be registered
      without IETF standards action, at which point guidlines for



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      further assignment will be specified.  Realm name types beginning
      with "X-" or "x-" are for private use.

      For host address types described in section 7.1, negative values
      are for private use.  Assignment of additional positive numbers is
      subject to review by the kerberos working group or other expert
      review.

      Additional key usage numbers as defined in section 7.5.1 will be
      assigned subject to review by the kerberos working group or other
      expert review.

      Additional preauthentciation data type values as defined in
      section 7.5.2 will be assigned subject to review by the kerberos
      working group or other expert review.

      Additional Authorization Data Types as defined in section 7.5.4
      will be assigned subject to review by the kerberos working group
      or other expert review.  Although it is anticipated that there may
      be significant demand for private use types, provision is
      intentionaly not made for a private use portion of the namespace
      because conficts between privately assigned values coule have
      detrimental security implications.

      Additional Transited Encoding Types as defined in section 7.5.5
      present special concerns for interoperability with existing
      implementations.  As such, such assignments will only be made by
      standards action, except that the Kerberos working group or
      another other working group with competent jurisdiction may make
      preliminary assignments for documents which are moving through the
      standards process.

      Additional Kerberos Message Types as described in section 7.5.7
      will be assigned subject to review by the kerberos working group
      or other expert review.

      Additional Name Types as described in section 7.5.8 will be
      assigned subject to review by the kerberos working group or other
      expert review.

      Additional error codes described in section 7.5.9 will be assigned
      subject to review by the kerberos working group or other expert
      review.

10. Security Considerations

      As an authentication service, Kerberos provides a means of
      verifying the identity of principals on a network. Kerberos does



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      not, by itself, provide authorization. Applications should not
      accept the issuance of a service ticket by the Kerberos server as
      granting authority to use the service, since such applications may
      become vulnerable to the bypass of this authorization check in an
      environment if they inter-operate with other KDCs or where other
      options for application authentication are provided.

      Denial of service attacks are not solved with Kerberos. There are
      places in the protocols where an intruder can prevent an
      application from participating in the proper authentication steps.
      Because authentication is a required step for the use of many
      services, successful denial of service attacks on a Kerberos
      server might result in the denial of other network services that
      rely on Kerberos for authentication. Kerberos is vulnerable to
      many kinds of denial of service attacks: denial of service attacks
      on the network which would prevent clients from contacting the
      KDC; denial of service attacks on the domain name system which
      could prevent a client from finding the IP address of the Kerberos
      server; and denial of service attack by overloading the Kerberos
      KDC itself with repeated requests.

      Interoperability conflicts caused by incompatible character-set
      usage (see 5.2.1) can result in denial of service for clients that
      utilize character-sets in Kerberos strings other than those stored
      in the KDC database.

      Authentication servers maintain a database of principals (i.e.,
      users and servers) and their secret keys. The security of the
      authentication server machines is critical. The breach of security
      of an authentication server will compromise the security of all
      servers that rely upon the compromised KDC, and will compromise
      the authentication of any principals registered in the realm of
      the compromised KDC.

      Principals must keep their secret keys secret. If an intruder
      somehow steals a principal's key, it will be able to masquerade as
      that principal or impersonate any server to the legitimate
      principal.

      Password guessing attacks are not solved by Kerberos. If a user
      chooses a poor password, it is possible for an attacker to
      successfully mount an off-line dictionary attack by repeatedly
      attempting to decrypt, with successive entries from a dictionary,
      messages obtained which are encrypted under a key derived from the
      user's password.

      Unless pre-authentication options are required by the policy of a
      realm, the KDC will not know whether a request for authentication



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      succeeds. An attacker can request a reply with credentials for any
      principal. These credentials will likely not be of much use to the
      attacker unless it knows the client's secret key, but the
      availability of the response encrypted in the client's secret key
      provides the attacker with ciphertext that may be used to mount
      brute force or dictionary attacks to decrypt the credentials, by
      guessing the user's password. For this reason it is strongly
      encouraged that Kerberos realms require the use of pre-
      authentication. Even with pre-authentication, attackers may try
      brute force or dictionary attacks against credentials that are
      observed by eavesdropping on the network.

      Because a client can request a ticket for any server principal and
      can attempt a brute force or dictionary attack against the server
      principal's key using that ticket, it is strongly encouraged that
      keys be randomly generated (rather than generated from passwords)
      for any principals that are usable as the target principal for a
      KRB_TGS_REQ or KRB_AS_REQ messages. [RFC1750]

      Although the DES-CBC-MD5 encryption method and DES-MD5 checksum
      methods are listed as SHOULD be implemented for backward
      compatibility, the single DES encryption algorithm on which these
      are based is weak and stronger algorithms should be used whenever
      possible.

      Each host on the network must have a clock which is loosely
      synchronized to the time of the other hosts; this synchronization
      is used to reduce the bookkeeping needs of application servers
      when they do replay detection. The degree of "looseness" can be
      configured on a per-server basis, but is typically on the order of
      5 minutes. If the clocks are synchronized over the network, the
      clock synchronization protocol MUST itself be secured from network
      attackers.

      Principal identifiers must not recycled on a short-term basis. A
      typical mode of access control will use access control lists
      (ACLs) to grant permissions to particular principals. If a stale
      ACL entry remains for a deleted principal and the principal
      identifier is reused, the new principal will inherit rights
      specified in the stale ACL entry. By not reusing principal
      identifiers, the danger of inadvertent access is removed.

      Proper decryption of an KRB_AS_REP message from the KDC is not
      sufficient for the host to verify the identity of the user; the
      user and an attacker could cooperate to generate a KRB_AS_REP
      format message which decrypts properly but is not from the proper
      KDC. To authenticate a user logging on to a local system, the
      credentials obtained in the AS exchange may first be used in a TGS



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      exchange to obtain credentials for a local server. Those
      credentials must then be verified by a local server through
      successful completion of the Client/Server exchange.

      Many RFC 1510 compliant implementations ignore unknown
      authorization data elements. Depending on these implementations to
      honor authorization data restrictions may create a security
      weakness.

      Kerberos credentials contain clear-text information identifying
      the principals to which they apply. If privacy of this information
      is needed, this exchange should itself be encapsulated in a
      protocol providing for confidentiality on the exchange of these
      credentials.

      Applications must take care to protect communications subsequent
      to authentication either by using the KRB_PRIV or KRB_SAFE
      messages as appropriate, or by applying their own confidentiality
      or integrity mechanisms on such communications. Completion of the
      KRB_AP_REQ and KRB_AP_REP exchange without subsequent use of
      confidentiality and integrity mechanisms provides only for
      authentication of the parties to the communication and not
      confidentiality and integrity of the subsequent communication.
      Application applying confidentiality and integrity protection
      mechanisms other than KRB_PRIV and KRB_SAFE must make sure that
      the authentication step is appropriately linked with the protected
      communication channel that is established by the application.

      Unless the application server provides its own suitable means to
      protect against replay (for example, a challenge-response sequence
      initiated by the server after authentication, or use of a server-
      generated encryption subkey), the server must utilize a replay
      cache to remember any authenticator presented within the allowable
      clock skew. All services sharing a key need to use the same replay
      cache. If separate replay caches are used, then and authenticator
      used with one such service could later be replayed to a different
      service with the same service principal.

      If a server loses track of authenticators presented within the
      allowable clock skew, it must reject all requests until the clock
      skew interval has passed, providing assurance that any lost or
      replayed authenticators will fall outside the allowable clock skew
      and can no longer be successfully replayed.

      Implementations of Kerberos should not use untrusted directory
      servers to determine the realm of a host. To allow such would
      allow the compromise of the directory server to enable an attacker
      to direct the client to accept authentication with the wrong



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      principal (i.e. one with a similar name, but in a realm with which
      the legitimate host was not registered).

      Implementations of Kerberos must not use DNS to map one name to
      another (canonicalize) to determine the host part of the principal
      name with which one is to communicate.  To allow such
      canonicalization would allow a compromise of the DNS to result in
      a client obtaining credentials and correctly authenticating to the
      wrong principal. Though the client will know who it is
      communicating with, it will not be the principal with which it
      intended to communicate.

      If the Kerberos server returns a TGT for a 'closer' realm other
      than the desired realm, the client may use local policy
      configuration to verify that the authentication path used is an
      acceptable one.  Alternatively, a client may choose its own
      authentication path, rather than relying on the Kerberos server to
      select one. In either case, any policy or configuration
      information used to choose or validate authentication paths,
      whether by the Kerberos server or client, must be obtained from a
      trusted source.

      The Kerberos protocol in its basic form does not provide perfect
      forward secrecy for communications. If traffic has been recorded
      by an eavesdropper, then messages encrypted using the KRB_PRIV
      message, or messages encrypted using application specific
      encryption under keys exchanged using Kerberos can be decrypted if
      any of the user's, application server's, or KDC's key is
      subsequently discovered. This is because the session key use to
      encrypt such messages is transmitted over the network encrypted in
      the key of the application server, and also encrypted under the
      session key from the user's ticket-granting ticket when returned
      to the user in the KRB_TGS_REP message. The session key from the
      ticket-granting ticket was sent to the user in the KRB_AS_REP
      message encrypted in the user's secret key, and embedded in the
      ticket-granting ticket, which was encrypted in the key of the KDC.
      Application requiring perfect forward secrecy must exchange keys
      through mechanisms that provide such assurance, but may use
      Kerberos for authentication of the encrypted channel established
      through such other means.

11. Author's Addresses


          Clifford Neuman
          Information Sciences Institute
          University of Southern California
          4676 Admiralty Way



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          Marina del Rey, CA 90292, USA
          Email: bcn@isi.edu

          Tom Yu
          Massachusetts Institute of Technology
          77 Massachusetts Avenue
          Cambridge, MA 02139, USA
          Email: tlyu@mit.edu

          Sam Hartman
          Massachusetts Institute of Technology
          77 Massachusetts Avenue
          Cambridge, MA 02139, USA
          Email: hartmans@mit.edu

          Kenneth Raeburn
          Massachusetts Institute of Technology
          77 Massachusetts Avenue
          Cambridge, MA 02139, USA
          Email: raeburn@MIT.EDU


12. Acknowledgements

      This document is a revision to RFC1510 which was co-authored with
      John Kohl.  The specification of the Kerberos protocol described
      in this document is the result of many years of effort.  Over this
      period many individuals have contributed to the definition of the
      protocol and to the writing of the specification. Unfortunately it
      is not possible to list all contributors as authors of this
      document, though there are many not listed who are authors in
      spirit, because they contributed text for parts of some sections,
      because they contributed to the design of parts of the protocol,
      or because they contributed significantly to the discussion of the
      protocol in the IETF common authentication technology (CAT) and
      Kerberos working groups.

      Among those contributing to the development and specification of
      Kerberos were Jeffrey Altman, John Brezak, Marc Colan, Johan
      Danielsson, Don Davis, Doug Engert, Dan Geer, Paul Hill, John
      Kohl, Marc Horowitz, Matt Hur, Jeffrey Hutzelman, Paul Leach, John
      Linn, Ari Medvinsky, Sasha Medvinsky, Steve Miller, Jon Rochlis,
      Jerome Saltzer, Jeffrey Schiller, Jennifer Steiner, Ralph Swick,
      Mike Swift, Jonathan Trostle, Theodore Ts'o, Brian Tung, Jacques
      Vidrine, Assar Westerlund, and Nicolas Williams. Many other
      members of MIT Project Athena, the MIT networking group, and the
      Kerberos and CAT working groups of the IETF contributed but are
      not listed.



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      Funding for the RFC Editor function is currently provided by the
      Internet Society.

13. REFERENCES

13.1 NORMATIVE REFERENCES

   [@KCRYPTO]
      RFC-Editor: To be replaced by RFC number for draft-ietf-krb-wg-
      crypto.

   [@AES]
      RFC-Editor: To be replaced by RFC number for draft-raeburn0krb-
      rijndael-krb.

   [ISO-646/ECMA-6]
      7-bit Coded Character Set

   [ISO-2022/ECMA-35]
      Character Code Structure and Extension Techniques

   [ISO-4873/ECMA-43]
      8-bit Coded Character Set Structure and Rules

   [RFC1035]
      P.V. Mockapetris, RFC1035: "Domain Names - Implementations and
      Specification," November 1, 1987, Obsoletes - RFC973, RFC882,
      RFC883. Updated by RFC1101, RFC1183, RFC1348, RFCRFC1876, RFC1982,
      RFC1995, RFC1996, RFC2065, RFC2136, RFC2137, RFC2181, RFC2308,
      RFC2535, RFC2845, and RFC3425. Status: Standard.

   [RFC2119]

      S. Bradner, RFC2119: "Key words for use in RFC's to Indicate
      Requirement Levels", March 1997.

   [RFC2434]
      T. Narten, H. Alvestrand, RFC2434: "Guidelines for writing IANA
      Consideration Secionts in RFCs" October, 1998.

   [RFC2782]
      A. Gulbrandsen,  P. Vixie and L. Esibov., RFC2782: "A DNS RR for
      Specifying the Location of Services (DNS SRV)," February 2000.

   [RFC2253]
      M. Wahl, S. Killie, and T. Howes, RFC2253: "Lightweight Directory
      Access Protocol (v3): UTF-8 String Representation or Distinguished
      Names," December 1997, Obsoletes - RFC1779, Updated by RFC3377,



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      Status: Proposed Standard.

   [RFC2373]
      R. Hinden, S. Deering, RFC2373: "IP Version 6 Addressing
      Architecture," July 1998, Status: Proposed Standard.

   [X680]
      Abstract Syntax Notation One (ASN.1): Specification of Basic
      Notation, ITU-T Recommendation X.680 (1997) | ISO/IEC
      International Standard 8824-1:1998.

   [X690]
      ASN.1 encoding rules: Specification of Basic Encoding Rules (BER),
      Canonical Encoding Rules (CER) and Distinguished Encoding Rules
      (DER), ITU-T Recommendation X.690 (1997)| ISO/IEC International
      Standard 8825-1:1998.

13.2 INFORMATIVE REFERENCES

   [DGT96]
      Don Davis, Daniel Geer, and Theodore Ts'o, "Kerberos With Clocks
      Adrift: History, Protocols, and Implementation", USENIX Computing
      Systems 9:1 (January 1996).

   [DS81]
      Dorothy E. Denning and Giovanni Maria Sacco, "Time-stamps in Key
      Distribution Protocols," Communications of the ACM, Vol. 24(8),
      pp.  533-536 (August 1981).

   [KNT94]

      John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o, "The
      Evolution of the Kerberos Authentication System". In Distributed
      Open Systems, pages 78-94. IEEE Computer Society Press, 1994.

   [MNSS87]
      S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. Saltzer,
      Section E.2.1: Kerberos Authentication and Authorization System,
      M.I.T. Project Athena, Cambridge, Massachusetts (December 21,
      1987).

   [NS78]
      Roger M. Needham and Michael D. Schroeder, "Using Encryption for
      Authentication in Large Networks of Computers," Communications of
      the ACM, Vol. 21(12), pp. 993-999 (December, 1978).

   [Neu93]
      B. Clifford Neuman, "Proxy-Based Authorization and Accounting for



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      Distributed Systems," in Proceedings of the 13th International
      Conference on Distributed Computing Systems, Pittsburgh, PA (May,
      1993).

   [NT94]
      B. Clifford Neuman and Theodore Y. Ts'o, "An Authentication
      Service for Computer Networks," IEEE Communications Magazine, Vol.
      32(9), pp.  33-38 (September 1994).

   [Pat92].
      J. Pato, Using Pre-Authentication to Avoid Password Guessing
      Attacks, Open Software Foundation DCE Request for Comments 26
      (December 1992).

   [RFC1510]
      J. Kohl and  B. C. Neuman, RFC1510: "The Kerberos Network
      Authentication Service (v5)," September 1993, Status: Proposed
      Standard.

   [RFC1750]
      D. Eastlake, S. Crocker, and J. Schiller "Randomness
      Recommendation for Security" December 1994, Status: Informational.

   [RFC2026]
      S. Bradner, RFC2026:  "The Internet Standard Process - Revision
      3," October 1996, Obsoletes - RFC 1602, Status: Best Current
      Practice.

   [SNS88]
      J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Kerberos: An
      Authentication Service for Open Network Systems," pp. 191-202 in
      Usenix Conference Proceedings, Dallas, Texas (February, 1988).


14. Copyright Statement

      Copyright (C) The Internet Society (2004).  This document is
      subject to the rights, licenses and restrictions contained in BCP
      78 and except as set forth therein, the authors retain all their
      rights.

      This document and the information contained herein are provided on
      an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
      REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
      THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
      EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
      THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
      ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A



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      PARTICULAR PURPOSE.

15. Intellectual Property

      The IETF takes no position regarding the validity or scope of any
      Intellectual Property Rights or other rights that might be claimed
      to pertain to the implementation or use of the technology
      described in this document or the extent to which any license
      under such rights might or might not be available; nor does it
      represent that it has made any independent effort to identify any
      such rights.  Information on the procedures with respect to rights
      in RFC documents can be found in BCP 78 and BCP 79.

      Copies of IPR disclosures made to the IETF Secretariat and any
      assurances of licenses to be made available, or the result of an
      attempt made to obtain a general license or permission for the use
      of such proprietary rights by implementers or users of this
      specification can be obtained from the IETF on-line IPR repository
      at http://www.ietf.org/ipr.

      The IETF invites any interested party to bring to its attention
      any copyrights, patents or patent applications, or other
      proprietary rights that may cover technology that may be required
      to implement this standard.  Please address the information to the
      IETF at ietf-ipr@ietf.org.

A. ASN.1 module

      KerberosV5Spec2 {
              iso(1) identified-organization(3) dod(6) internet(1)
              security(5) kerberosV5(2) modules(4) krb5spec2(2)
      } DEFINITIONS EXPLICIT TAGS ::= BEGIN

      -- OID arc for KerberosV5
      --
      -- This OID may be used to identify Kerberos protocol messages
      -- encapsulated in other protocols.
      --
      -- This OID also designates the OID arc for KerberosV5-related OIDs.
      --
      -- NOTE: RFC 1510 had an incorrect value (5) for "dod" in its OID.
      id-krb5         OBJECT IDENTIFIER ::= {
              iso(1) identified-organization(3) dod(6) internet(1)
              security(5) kerberosV5(2)
      }

      Int32           ::= INTEGER (-2147483648..2147483647)
                          -- signed values representable in 32 bits



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      UInt32          ::= INTEGER (0..4294967295)
                          -- unsigned 32 bit values

      Microseconds    ::= INTEGER (0..999999)
                          -- microseconds

      KerberosString  ::= GeneralString (IA5String)

      Realm           ::= KerberosString

      PrincipalName   ::= SEQUENCE {
              name-type       [0] Int32,
              name-string     [1] SEQUENCE OF KerberosString
      }

      KerberosTime    ::= GeneralizedTime -- with no fractional seconds

      HostAddress     ::= SEQUENCE  {
              addr-type       [0] Int32,
              address         [1] OCTET STRING
      }

      -- NOTE: HostAddresses is always used as an OPTIONAL field and
      -- should not be empty.
      HostAddresses   -- NOTE: subtly different from rfc1510,
                      -- but has a value mapping and encodes the same
              ::= SEQUENCE OF HostAddress

      -- NOTE: AuthorizationData is always used as an OPTIONAL field and
      -- should not be empty.
      AuthorizationData       ::= SEQUENCE OF SEQUENCE {
              ad-type         [0] Int32,
              ad-data         [1] OCTET STRING
      }

      PA-DATA         ::= SEQUENCE {
              -- NOTE: first tag is [1], not [0]
              padata-type     [1] Int32,
              padata-value    [2] OCTET STRING -- might be encoded AP-REQ
      }

      KerberosFlags   ::= BIT STRING (SIZE (32..MAX)) -- minimum number of bits
                          -- shall be sent, but no fewer than 32

      EncryptedData   ::= SEQUENCE {
              etype   [0] Int32 -- EncryptionType --,
              kvno    [1] UInt32 OPTIONAL,
              cipher  [2] OCTET STRING -- ciphertext



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      }

      EncryptionKey   ::= SEQUENCE {
              keytype         [0] Int32 -- actually encryption type --,
              keyvalue        [1] OCTET STRING
      }

      Checksum        ::= SEQUENCE {
              cksumtype       [0] Int32,
              checksum        [1] OCTET STRING
      }

      Ticket          ::= [APPLICATION 1] SEQUENCE {
              tkt-vno         [0] INTEGER (5),
              realm           [1] Realm,
              sname           [2] PrincipalName,
              enc-part        [3] EncryptedData -- EncTicketPart
      }

      -- Encrypted part of ticket
      EncTicketPart   ::= [APPLICATION 3] SEQUENCE {
              flags                   [0] TicketFlags,
              key                     [1] EncryptionKey,
              crealm                  [2] Realm,
              cname                   [3] PrincipalName,
              transited               [4] TransitedEncoding,
              authtime                [5] KerberosTime,
              starttime               [6] KerberosTime OPTIONAL,
              endtime                 [7] KerberosTime,
              renew-till              [8] KerberosTime OPTIONAL,
              caddr                   [9] HostAddresses OPTIONAL,
              authorization-data      [10] AuthorizationData OPTIONAL
      }

      -- encoded Transited field
      TransitedEncoding       ::= SEQUENCE {
              tr-type         [0] Int32 -- must be registered --,
              contents        [1] OCTET STRING
      }

      TicketFlags     ::= KerberosFlags
              -- reserved(0),
              -- forwardable(1),
              -- forwarded(2),
              -- proxiable(3),
              -- proxy(4),
              -- may-postdate(5),
              -- postdated(6),



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              -- invalid(7),
              -- renewable(8),
              -- initial(9),
              -- pre-authent(10),
              -- hw-authent(11),
      -- the following are new since 1510
              -- transited-policy-checked(12),
              -- ok-as-delegate(13)

      AS-REQ          ::= [APPLICATION 10] KDC-REQ

      TGS-REQ         ::= [APPLICATION 12] KDC-REQ

      KDC-REQ         ::= SEQUENCE {
              -- NOTE: first tag is [1], not [0]
              pvno            [1] INTEGER (5) ,
              msg-type        [2] INTEGER (10 -- AS -- | 12 -- TGS --),
              padata          [3] SEQUENCE OF PA-DATA OPTIONAL
                                  -- NOTE: not empty --,
              req-body        [4] KDC-REQ-BODY
      }

      KDC-REQ-BODY    ::= SEQUENCE {
              kdc-options             [0] KDCOptions,
              cname                   [1] PrincipalName OPTIONAL
                                          -- Used only in AS-REQ --,
              realm                   [2] Realm
                                          -- Server's realm
                                          -- Also client's in AS-REQ --,
              sname                   [3] PrincipalName OPTIONAL,
              from                    [4] KerberosTime OPTIONAL,
              till                    [5] KerberosTime,
              rtime                   [6] KerberosTime OPTIONAL,
              nonce                   [7] UInt32,
              etype                   [8] SEQUENCE OF Int32 -- EncryptionType
                                          -- in preference order --,
              addresses               [9] HostAddresses OPTIONAL,
              enc-authorization-data  [10] EncryptedData -- AuthorizationData --,
              additional-tickets      [11] SEQUENCE OF Ticket OPTIONAL
                                              -- NOTE: not empty
      }

      KDCOptions      ::= KerberosFlags
              -- reserved(0),
              -- forwardable(1),
              -- forwarded(2),
              -- proxiable(3),
              -- proxy(4),



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              -- allow-postdate(5),
              -- postdated(6),
              -- unused7(7),
              -- renewable(8),
              -- unused9(9),
              -- unused10(10),
              -- opt-hardware-auth(11),
              -- unused12(12),
              -- unused13(13),
      -- 15 is reserved for canonicalize
              -- unused15(15),
      -- 26 was unused in 1510
              -- disable-transited-check(26),
      --
              -- renewable-ok(27),
              -- enc-tkt-in-skey(28),
              -- renew(30),
              -- validate(31)

      AS-REP          ::= [APPLICATION 11] KDC-REP

      TGS-REP         ::= [APPLICATION 13] KDC-REP

      KDC-REP         ::= SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (11 -- AS -- | 13 -- TGS --),
              padata          [2] SEQUENCE OF PA-DATA OPTIONAL
                                      -- NOTE: not empty --,
              crealm          [3] Realm,
              cname           [4] PrincipalName,
              ticket          [5] Ticket,
              enc-part        [6] EncryptedData
                                      -- EncASRepPart or EncTGSRepPart,
                                      -- as appropriate
      }

      EncASRepPart    ::= [APPLICATION 25] EncKDCRepPart

      EncTGSRepPart   ::= [APPLICATION 26] EncKDCRepPart

      EncKDCRepPart   ::= SEQUENCE {
              key             [0] EncryptionKey,
              last-req        [1] LastReq,
              nonce           [2] UInt32,
              key-expiration  [3] KerberosTime OPTIONAL,
              flags           [4] TicketFlags,
              authtime        [5] KerberosTime,
              starttime       [6] KerberosTime OPTIONAL,



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              endtime         [7] KerberosTime,
              renew-till      [8] KerberosTime OPTIONAL,
              srealm          [9] Realm,
              sname           [10] PrincipalName,
              caddr           [11] HostAddresses OPTIONAL
      }

      LastReq         ::=     SEQUENCE OF SEQUENCE {
              lr-type         [0] Int32,
              lr-value        [1] KerberosTime
      }

      AP-REQ          ::= [APPLICATION 14] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (14),
              ap-options      [2] APOptions,
              ticket          [3] Ticket,
              authenticator   [4] EncryptedData -- Authenticator
      }

      APOptions       ::= KerberosFlags
              -- reserved(0),
              -- use-session-key(1),
              -- mutual-required(2)

      -- Unencrypted authenticator
      Authenticator   ::= [APPLICATION 2] SEQUENCE  {
              authenticator-vno       [0] INTEGER (5),
              crealm                  [1] Realm,
              cname                   [2] PrincipalName,
              cksum                   [3] Checksum OPTIONAL,
              cusec                   [4] Microseconds,
              ctime                   [5] KerberosTime,
              subkey                  [6] EncryptionKey OPTIONAL,
              seq-number              [7] UInt32 OPTIONAL,
              authorization-data      [8] AuthorizationData OPTIONAL
      }

      AP-REP          ::= [APPLICATION 15] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (15),
              enc-part        [2] EncryptedData -- EncAPRepPart
      }

      EncAPRepPart    ::= [APPLICATION 27] SEQUENCE {
              ctime           [0] KerberosTime,
              cusec           [1] Microseconds,
              subkey          [2] EncryptionKey OPTIONAL,



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              seq-number      [3] UInt32 OPTIONAL
      }

      KRB-SAFE        ::= [APPLICATION 20] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (20),
              safe-body       [2] KRB-SAFE-BODY,
              cksum           [3] Checksum
      }

      KRB-SAFE-BODY   ::= SEQUENCE {
              user-data       [0] OCTET STRING,
              timestamp       [1] KerberosTime OPTIONAL,
              usec            [2] Microseconds OPTIONAL,
              seq-number      [3] UInt32 OPTIONAL,
              s-address       [4] HostAddress,
              r-address       [5] HostAddress OPTIONAL
      }

      KRB-PRIV        ::= [APPLICATION 21] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (21),
                              -- NOTE: there is no [2] tag
              enc-part        [3] EncryptedData -- EncKrbPrivPart
      }

      EncKrbPrivPart  ::= [APPLICATION 28] SEQUENCE {
              user-data       [0] OCTET STRING,
              timestamp       [1] KerberosTime OPTIONAL,
              usec            [2] Microseconds OPTIONAL,
              seq-number      [3] UInt32 OPTIONAL,
              s-address       [4] HostAddress -- sender's addr --,
              r-address       [5] HostAddress OPTIONAL -- recip's addr
      }

      KRB-CRED        ::= [APPLICATION 22] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (22),
              tickets         [2] SEQUENCE OF Ticket,
              enc-part        [3] EncryptedData -- EncKrbCredPart
      }

      EncKrbCredPart  ::= [APPLICATION 29] SEQUENCE {
              ticket-info     [0] SEQUENCE OF KrbCredInfo,
              nonce           [1] UInt32 OPTIONAL,
              timestamp       [2] KerberosTime OPTIONAL,
              usec            [3] Microseconds OPTIONAL,
              s-address       [4] HostAddress OPTIONAL,



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              r-address       [5] HostAddress OPTIONAL
      }

      KrbCredInfo     ::= SEQUENCE {
              key             [0] EncryptionKey,
              prealm          [1] Realm OPTIONAL,
              pname           [2] PrincipalName OPTIONAL,
              flags           [3] TicketFlags OPTIONAL,
              authtime        [4] KerberosTime OPTIONAL,
              starttime       [5] KerberosTime OPTIONAL,
              endtime         [6] KerberosTime OPTIONAL,
              renew-till      [7] KerberosTime OPTIONAL,
              srealm          [8] Realm OPTIONAL,
              sname           [9] PrincipalName OPTIONAL,
              caddr           [10] HostAddresses OPTIONAL
      }

      KRB-ERROR       ::= [APPLICATION 30] SEQUENCE {
              pvno            [0] INTEGER (5),
              msg-type        [1] INTEGER (30),
              ctime           [2] KerberosTime OPTIONAL,
              cusec           [3] Microseconds OPTIONAL,
              stime           [4] KerberosTime,
              susec           [5] Microseconds,
              error-code      [6] Int32,
              crealm          [7] Realm OPTIONAL,
              cname           [8] PrincipalName OPTIONAL,
              realm           [9] Realm -- service realm --,
              sname           [10] PrincipalName -- service name --,
              e-text          [11] KerberosString OPTIONAL,
              e-data          [12] OCTET STRING OPTIONAL
      }

      METHOD-DATA     ::= SEQUENCE OF PA-DATA

      TYPED-DATA      ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
              data-type       [0] INTEGER,
              data-value      [1] OCTET STRING OPTIONAL
      }

      -- preauth stuff follows

      PA-ENC-TIMESTAMP        ::= EncryptedData -- PA-ENC-TS-ENC

      PA-ENC-TS-ENC           ::= SEQUENCE {
              patimestamp     [0] KerberosTime -- client's time --,
              pausec          [1] Microseconds OPTIONAL
      }



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      ETYPE-INFO-ENTRY        ::= SEQUENCE {
              etype           [0] Int32,
              salt            [1] OCTET STRING OPTIONAL
      }

      ETYPE-INFO              ::= SEQUENCE OF ETYPE-INFO-ENTRY

      ETYPE-INFO2-ENTRY       ::= SEQUENCE {
              etype           [0] Int32,
              salt            [1] KerberosString OPTIONAL,
              s2kparams       [2] OCTET STRING OPTIONAL
      }

      ETYPE-INFO2             ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY

      AD-IF-RELEVANT          ::= AuthorizationData

      AD-KDCIssued            ::= SEQUENCE {
              ad-checksum     [0] Checksum,
              i-realm         [1] Realm OPTIONAL,
              i-sname         [2] PrincipalName OPTIONAL,
              elements        [3] AuthorizationData
      }

      AD-AND-OR               ::= SEQUENCE {
              condition-count [0] INTEGER,
              elements        [1] AuthorizationData
      }

      AD-MANDATORY-FOR-KDC    ::= AuthorizationData

      END

B. Changes since RFC-1510

      This document replaces RFC-1510 and clarifies specification of
      items that were not completely specified. Where changes to
      recommended implementation choices were made, or where new options
      were added, those changes are described within the document and
      listed in this section. More significantly, "Specification 2" in
      section 8 changes the required encryption and checksum methods to
      bring them in line with the best current practices and to
      deprecate methods that are no longer considered sufficiently
      strong.

      Discussion was added to section 1 regarding the ability to rely on
      the KDC to check the transited field, and on the inclusion of a
      flag in a ticket indicating that this check has occurred. This is



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      a new capability not present in RFC1510. Pre-existing
      implementations may ignore or not set this flag without negative
      security implications.

      The definition of the secret key says that in the case of a user
      the key may be derived from a password. In 1510, it said that the
      key was derived from the password. This change was made to
      accommodate situations where the user key might be stored on a
      smart-card, or otherwise obtained independent of a password.

      The introduction mentions the use of public key cryptography for
      initial authentication in Kerberos by reference. RFC1510 did not
      include such a reference.

      Section 1.2 was added to explain that while Kerberos provides
      authentication of a named principal, it is still the
      responsibility of the application to ensure that the authenticated
      name is the entity with which the application wishes to
      communicate.

      Discussion of extensibility has been added to the introduction.

      Discussion of how extensibility affects ticket flags and KDC
      options was added to the introduction of section 2. No changes
      were made to existing options and flags specified in RFC1510,
      though some of the sections in the specification were renumbered,
      and text was revised to make the description and intent of
      existing options clearer, especially with respect to the ENC-TKT-
      IN-SKEY option (now section 2.9.2) which is used for user-to-user
      authentication.  The new option and ticket flag transited policy
      checking (section 2.7) was added.

      A warning regarding generation of session keys for application use
      was added to section 3, urging the inclusion of key entropy from
      the KDC generated session key in the ticket. An example regarding
      use of the sub-session key was added to section 3.2.6.
      Descriptions of the pa-etype-info, pa-etype-info2, and pa-pw-salt
      pre-authentication data items were added. The recommendation for
      use of pre-authentication was changed from "may" to "should" and a
      note was added regarding known plaintext attacks.

      In RFC 1510, section 4 described the database in the KDC. This
      discussion was not necessary for interoperability and
      unnecessarily constrained implementation. The old section 4 was
      removed.

      The current section 4 was formerly section 6 on encryption and
      checksum specifications. The major part of this section was



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      brought up to date to support new encryption methods, and move to
      a separate document. Those few remaining aspects of the encryption
      and checksum specification specific to Kerberos are now specified
      in section 4.

      Significant changes were made to the layout of section 5 to
      clarify the correct behavior for optional fields. Many of these
      changes were made necessary because of improper ASN.1 description
      in the original Kerberos specification which left the correct
      behavior underspecified. Additionally, the wording in this section
      was tightened wherever possible to ensure that implementations
      conforming to this specification will be extensible with the
      addition of new fields in future specifications.

      Text was added describing time_t=0 issues in the ASN.1. Text was
      also added, clarifying issues with implementations treating
      omitted optional integers as zero. Text was added clarifying
      behavior for optional SEQUENCE or SEQUENCE OF that may be empty.
      Discussion was added regarding sequence numbers and behavior of
      some implementations, including "zero" behavior and negative
      numbers. A compatibility note was added regarding the
      unconditional sending of EncTGSRepPart regardless of the enclosing
      reply type. Minor changes were made to the description of the
      HostAddresses type. Integer types were constrained. KerberosString
      was defined as a (significantly) constrained GeneralString.
      KerberosFlags was defined to reflect existing implementation
      behavior that departs from the definition in RFC 1510. The
      transited-policy-checked(12) and the ok-as-delegate(13) ticket
      flags were added. The disable-transited-check(26) KDC option was
      added.

      Descriptions of commonly implemented PA-DATA were added to section
      5. The description of KRB-SAFE has been updated to note the
      existing implementation behavior of double-encoding.

      There were two definitions of METHOD-DATA in RFC 1510. The second
      one, intended for use with KRB_AP_ERR_METHOD was removed leaving
      the SEQUENCE OF PA-DATA definition.

      Section 7, naming constraints, from RFC1510 was moved to section
      6.

      Words were added describing the convention that domain based realm
      names for newly created realms should be specified as upper case.
      This recommendation does not make lower case realm names illegal.
      Words were added highlighting that the slash separated components
      in the X500 style of realm names is consistent with existing
      RFC1510 based implementations, but that it conflicts with the



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      general recommendation of X.500 name representation specified in
      RFC2253.

      Section 8, network transport, constants and defined values, from
      RFC1510 was moved to section 7.  Since RFC1510, the definition of
      the TCP transport for Kerberos messages was added, and the
      encryption and checksum number assignments have been moved into a
      separate document.

      "Specification 2" in section 8 of the current document changes the
      required encryption and checksum methods to bring them in line
      with the best current practices and to deprecate methods that are
      no longer considered sufficiently strong.

      Two new sections, on IANA considerations and security
      considerations were added.

      The pseudo-code has been removed from the appendix. The pseudo-
      code was sometimes misinterpreted to limit implementation choices
      and in RFC 1510, it was not always consistent with the words in
      the specification. Effort was made to clear up any ambiguities in
      the specification, rather than to rely on the pseudo-code.

      An appendix was added containing the complete ASN.1 module drawn
      from the discussion in section 5 of the current document.

END NOTES

      [TM] Project Athena, Athena, and Kerberos are trademarks of the
      Massachusetts Institute of Technology (MIT). No commercial use of
      these trademarks may be made without prior written permission of
      MIT.

      [1] Note, however, that many applications use Kerberos' functions
      only upon the initiation of a stream-based network connection.
      Unless an application subsequently provides integrity protection
      for the data stream, the identity verification applies only to the
      initiation of the connection, and does not guarantee that
      subsequent messages on the connection originate from the same
      principal.

      [2] Secret and private are often used interchangeably in the
      literature.  In our usage, it takes two (or more) to share a
      secret, thus a shared DES key is a secret key. Something is only
      private when no one but its owner knows it. Thus, in public key
      cryptosystems, one has a public and a private key.

      [3] Of course, with appropriate permission the client could



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      arrange registration of a separately-named principal in a remote
      realm, and engage in normal exchanges with that realm's services.
      However, for even small numbers of clients this becomes
      cumbersome, and more automatic methods as described here are
      necessary.

      [4] Though it is permissible to request or issue tickets with no
      network addresses specified.

      [5] The password-changing request must not be honored unless the
      requester can provide the old password (the user's current secret
      key). Otherwise, it would be possible for someone to walk up to an
      unattended session and change another user's password.

      [6] To authenticate a user logging on to a local system, the
      credentials obtained in the AS exchange may first be used in a TGS
      exchange to obtain credentials for a local server. Those
      credentials must then be verified by a local server through
      successful completion of the Client/Server exchange.

      [7] "Random" means that, among other things, it should be
      impossible to guess the next session key based on knowledge of
      past session keys. This can only be achieved in a pseudo-random
      number generator if it is based on cryptographic principles. It is
      more desirable to use a truly random number generator, such as one
      based on measurements of random physical phenomena.  See [RFC1750]
      for an in depth discussion of randomness.

      [8] Tickets contain both an encrypted and unencrypted portion, so
      cleartext here refers to the entire unit, which can be copied from
      one message and replayed in another without any cryptographic
      skill.

      [9] Note that this can make applications based on unreliable
      transports difficult to code correctly. If the transport might
      deliver duplicated messages, either a new authenticator must be
      generated for each retry, or the application server must match
      requests and replies and replay the first reply in response to a
      detected duplicate.

      [10] Note also that the rejection here is restricted to
      authenticators from the same principal to the same server. Other
      client principals communicating with the same server principal
      should not be have their authenticators rejected if the time and
      microsecond fields happen to match some other client's
      authenticator.

      [11] If this is not done, an attacker could subvert the



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      authentication by recording the ticket and authenticator sent over
      the network to a server and replaying them following an event that
      caused the server to lose track of recently seen authenticators.

      [12] In the Kerberos version 4 protocol, the timestamp in the
      reply was the client's timestamp plus one. This is not necessary
      in version 5 because version 5 messages are formatted in such a
      way that it is not possible to create the reply by judicious
      message surgery (even in encrypted form) without knowledge of the
      appropriate encryption keys.

      [13] Note that for encrypting the KRB_AP_REP message, the sub-
      session key is not used, even if present in the Authenticator.

      [14] Implementations of the protocol may provide routines to
      choose subkeys based on session keys and random numbers and to
      generate a negotiated key to be returned in the KRB_AP_REP
      message.

      [15]This can be accomplished in several ways. It might be known
      beforehand (since the realm is part of the principal identifier),
      it might be stored in a nameserver, or it might be obtained from a
      configuration file. If the realm to be used is obtained from a
      nameserver, there is a danger of being spoofed if the nameservice
      providing the realm name is not authenticated.  This might result
      in the use of a realm which has been compromised, and would result
      in an attacker's ability to compromise the authentication of the
      application server to the client.

      [16] If the client selects a sub-session key, care must be taken
      to ensure the randomness of the selected sub-session key. One
      approach would be to generate a random number and XOR it with the
      session key from the ticket-granting ticket.


















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