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Network Working Group                                        A. Melnikov
Internet Draft                                                    Editor
Document: draft-ietf-sasl-rfc2222bis-10.txt                February 2005
Obsoletes: RFC 2222                                Expires in six months


            Simple Authentication and Security Layer (SASL)

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   Internet Drafts are working documents of the Internet Engineering
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   A revised version of this draft document will be submitted to the RFC
   editor as a Standards Track RFC for the Internet Community.
   Discussion and suggestions for improvement are requested.
   Distribution of this draft is unlimited.

   When published as an RFC this document will obsolete RFC 2222.















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Abstract

   The Simple Authentication and Security Layer (SASL) is a framework
   for providing authentication and data security services in
   connection-oriented protocols via replaceable mechanisms. It provides
   a structured interface between protocols and mechanisms.  The
   resulting framework allows new protocols to reuse existing mechanisms
   and allows old protocols to make use of new mechanisms.  The
   framework also provides a protocol for securing subsequent protocol
   exchanges within a data security layer.

   This document describes how a SASL mechanism is structured, describes
   how protocols add support for SASL, and defines the protocol for
   carrying a data security layer over a connection.  Additionally, this
   document defines one SASL mechanism, the EXTERNAL mechanism.


1.  Conventions used in this document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as defined in "Key words for
   use in RFCs to Indicate Requirement Levels" [KEYWORDS].

   Character names in this document use the notation for code points and
   names from the Unicode Standard [Unicode].  For example, the letter
   "a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.

   This document uses terms "integrity protection" and "confidentiality
   protection". The former refers to a security layer (see Section
   "Introduction" below for the definition) designed to provide "data
   integrity service" as defined in [Sec-Glossary]. Confidentiality
   protection is a security layer that provides "data confidentiality
   service" as defined in [Sec-Glossary]. The term "confidentiality
   protection" implies "integrity protection". Security layers may offer
   other kinds of security services, for example re-keying, truncation
   detection, as well as other services, e.g. compression.


2.    Introduction

   The Simple Authentication and Security Layer (SASL) is a framework
   for providing authentication and data security services in
   connection-oriented protocols via replaceable mechanisms.  SASL
   provides a structured interface between protocols and mechanisms.
   SASL also provides a protocol for securing subsequent protocol



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   exchanges within a data security layer.

   SASL's design is intended to allow new protocols to reuse existing
   mechanisms without requiring redesign of the mechanisms and allows
   existing protocols to make use of new mechanisms without redesign of
   protocols.

   The SASL is conceptually a framework which provides a layer between
   protocols and mechanisms, as illustrated in the following diagram.

                SMTP Protocol     LDAP Protocol          Other Protocols
                   Profile           Profile            . . .
                          \-----        |       -----/
                                \       |      /
                                 SASL framework
                                /       |      \
                          /-----        |       -----\
                  DIGEST-MD5         EXTERNAL            Other Mechanisms
                SASL mechanism    SASL mechanism        . . .


   It is through the interfaces of this layer that the framework allows
   any protocol to be utilized with any mechanism.  While the layer does
   generally hide the particulars of protocols from mechanisms and the
   particulars of mechanisms from protocols, the layer does not
   generally hide the particulars of mechanisms from protocol
   implementations.  For example, different mechanisms require different
   information to operate, some of them use password based
   authentication, some of then require realm information, others make
   use of Kerberos tickets, certificates, etc.  Also, in order to
   perform authorization, server implementations have to implement a
   mapping from a mechanism-specific authentication identity format to a
   protocol-specific format.

   It is possible to design and implement this framework in ways which
   do abstract away particulars of similar mechanisms.  Such
   implementation could also be designed to be shared by multiple
   implementations of various protocols.

   As illustrated above, the SASL framework interfaces with both
   protocols and mechanisms.

   To use SASL, a protocol includes a command for identifying and
   authenticating a user to a server and for optionally negotiating a
   security layer for subsequent protocol interactions.  If the use of a
   security layer is negotiated, that security layer is inserted between
   the protocol and the connection.  Section 4 ("Protocol profile
   requirements") profiles the requirements that a protocol



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   specification must fulfill to make use of SASL. A SASL protocol
   profile is a part of the protocol specification that satisfies the
   requirements of Section 4.

   A SASL mechanism is a series of server challenges and client
   responses specific to the mechanism.  Each SASL mechanism is
   identified by a registered name.  Section 5 ("Mechanism profile
   guidelines") profiles the requirements that a mechanism specification
   must fulfill to define a SASL mechanism.

   This document is written to serve several different audiences:

   - protocol designers using this specification to support
   authentication in their protocol,

   - mechanism designers that define new SASL mechanisms, and

   - implementors of clients or servers for those protocols using this
   specification.

   The sections "Authentication mechanisms", "Protocol profile
   requirements", "Specific issues", and "Security considerations" cover
   issues that protocol designers need to understand and address in
   profiling this specification for use in a specific protocol.

   The sections "Authentication mechanisms", "Mechanism profile
   guidelines", "Security considerations" and "Registration procedure"
   cover issues that mechanism designers need to understand and address
   in designing new SASL mechanisms.

   The sections "Authentication mechanisms", "Protocol profile
   requirements", "Specific issues" and "Security considerations" cover
   issues that implementors of a protocol that uses SASL framework need
   to understand.  The implementors will also need to understand a
   specification of a SASL profile specific to the protocol, as well as
   aspects of mechanism specifications they intend to use (regardless of
   whether they are implementing the mechanisms themselves or using an
   existing implementation) to understand, for instance, the mechanism-
   specific authentication identity forms, the offered services, and
   security and other considerations.

2.1.  Relationship to other documents

   This document obsoletes RFC 2222.  It replaces all portions of RFC
   2222 excepting sections 7.1 (Kerberos version 4 mechanism), 7.2
   (GSSAPI mechanism), 7.3 (S/Key mechanism).  The Kerberos version 4
   (KERBEROS_IV) and S/Key (SKEY) mechanisms are now viewed as obsolete.
   The GSSAPI mechanism is now separately specified [SASL-GSSAPI].



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3.    Authentication mechanisms

   SASL mechanisms are named by strings, from 1 to 20 characters in
   length, consisting of ASCII [ASCII] upper-case letters, digits,
   hyphens, and/or underscores.  Names of SASL mechanisms or families of
   mechanisms must be registered with the Internet Assigned Numbers
   Authority (IANA) as described in section 8.2.

   The "sasl-mech" ABNF production below defines the syntax of a SASL
   mechanism name.  This uses the Augmented Backus-Naur Form (ABNF)
   notation as specified in [ABNF].

   sasl-mech    = 1*20mech-char
   mech-char    = UPPER-ALPHA / DIGIT / HYPHEN / UNDERSCORE
                  ; mech-char is restricted to "A"-"Z", "0"-"9", "-",
                  ; and "_" from ASCII character set.

   UPPER-ALPHA  = %x41-5A
                  ; "A"-"Z"

   DIGIT        = %x30-39
                  ; "0"-"9"

   HYPHEN       = %x2D
                  ; "-"

   UNDERSCORE   = %x5F
                  ; "_"


3.1.  Authentication Exchange

   A SASL mechanism is responsible for conducting an authentication
   exchange.  This consists of a series of server challenges and client
   responses, the contents of which are specific to and defined by the
   mechanism.  To the application protocol, the challenges and responses
   are opaque binary tokens of arbitrary length (including 0-length).
   The protocol's profile then specifies how these binary tokens are
   encoded for transfer over the connection.

   After receiving an authentication command or any client response, a
   server mechanism may issue a challenge, indicate failure, or indicate
   completion.  The server mechanism may return additional data with a
   completion indication.  The protocol's profile specifies how each of
   these is then represented over the connection.

   After receiving a challenge, a client mechanism may issue a response
   or abort the exchange.  The protocol's profile specifies how each of



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   these are then represented over the connection.

   During the authentication exchange, the mechanism performs
   authentication, transmits an authorization identity (sometimes known
   as a username) from the client to server, and may negotiate the use
   of a mechanism-specific security layer.  If the use of a security
   layer is agreed upon, then the mechanism must also define or
   negotiate the maximum security layer buffer size that each side is
   able to receive.

3.2.  Identity Concepts


   Conceptually, SASL framework involves two identities:
     1) an identity associated with the authentication
     credentials (termed the authentication identity), and
     2) an identity to act as (termed the authorization
     identity).

   The client provides its credentials and, optionally, a string
   representing the requested authorization identity as part of the SASL
   exchange.  When this string is omitted or empty, the client is
   requesting to act as the identity associated with the credentials
   (e.g., the user is requesting to act as the authentication identity).

   The server is responsible for verifying the client's credentials and
   verifying that the client is allowed to act as the authorization
   identity.  A SASL exchange fails if either (or both) of these
   verifications fails.

   SASL mechanism specifications describe the form of credentials used
   to authenticate clients, and SASL application profiles describe the
   form of authorization identities transferred as part of
   authentication exchange.  However, the precise form(s) of the
   authentication identities (used within the server in its
   verifications, or otherwise) and the precise form(s) of the
   authorization identities (used in making authorization decisions, or
   otherwise) is beyond the scope of the SASL and this specification.
   In some circumstances, the precise identity forms used outside of the
   SASL exchange may be dictated by other specifications.  For instance,
   the authorization policy specification for an application protocol
   may dictate the precise form that an authorization identity is to be
   represented in the authorization policy.

   <<Need to address few issues in the two remaining paragraphs>> Any
   normalization of the authentication identity (for the purposes of
   conducting authentication exchange) is defined by a particular SASL
   mechanism, the protocol profile doesn't influence it.  Note, that the



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   mechanism specification doesn't control how authentication identity
   information is represented elsewhere <<need to add few examples>>.

   The mechanism MUST preserve Unicode codepoints when transferring
   authorization identity (e.g. the mechanism can't apply any form of
   normalization).


3.2.1.  Authorization identities and proxy authentication


   A mechanism which is incapable of transmitting an authorization
   identity must be treated as if it always transmits an authorization
   identity of an empty string.

   If the authorization identity transmitted during the authentication
   exchange is not the empty string, this is typically referred to as
   "proxy authentication".  This feature permits agents such as proxy
   servers to authenticate using their own credentials, yet request the
   access privileges of the identity for which they are proxying.

   The server makes an implementation-defined policy decision as to
   whether the authentication identity is permitted to have the access
   privileges of the authorization identity and whether the
   authorization identity is permitted to receive service.  If it is
   not, the server indicates failure of the authentication exchange.

   As a client might not have the same information as the server,
   clients SHOULD NOT derive authorization identities from
   authentication identities. Instead, clients SHOULD provide no (or
   empty) authorization identity when the user has not provided an
   authorization identity.

   The server SHOULD verify that a received authorization identity is in
   the correct form. Protocol profiles whose authorization identities
   are simple user names (e.g. IMAP [RFC 3501]) SHOULD use "SASLprep"
   profile [SASLprep] of the "stringprep" algorithm [StringPrep] to
   prepare these names for matching. The profiles MAY use a stringprep
   profile that is more strict than "SASLprep". If the preparation of
   the authorization identity fails or results in an empty string, the
   server MUST fail the authentication exchange. The only exception to
   this rule is when the received authorization identity is already the
   empty string.

3.2.2.  Authorization Identity Format

   An authorization identity is a string of zero or more Unicode
   [Unicode] coded characters.  The NUL <U+0000> character is prohibited



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   in authorization identities.

   The character encoding scheme used for transmitting an authorization
   identity over the protocol is specified in each authentication
   mechanism.  All IETF-defined mechanisms MUST, and all other
   mechanisms SHOULD, use UTF-8 [UTF-8]. (See [CHARSET-POLICY] for IETF
   policy regarding character sets and encoding schemes.)

   Mechanisms are expected to be capable of carrying the entire Unicode
   repertoire (with the exception of the NUL character). An
   authorization identity of the empty string and an absent
   authorization identity MUST be treated as equivalent. A mechanism
   which provides an optional field for an authorization identity,
   SHOULD NOT allow that field, when present, to be empty.  The meaning
   of the empty string as an authorization identity is described in
   Section 3.2.

3.3.  Security layers


   SASL mechanisms may offer a wide range of services in "security
   layers".  Typical examples of such services are data integrity, data
   confidentiality, re-keying, truncation detection and compression.

   If use of a security layer is negotiated by the authentication
   protocol exchange, the security layer is applied to all subsequent
   data sent over the connection (until another security layer is
   negotiated ( see also section 6.3) or underlying connection is
   closed). The security layer takes effect immediately following the
   last response of the authentication exchange for data sent by the
   client and the completion indication for data sent by the server. The
   exact position MUST be defined by the protocol profile (see section 4
   part 5).

   Once the security layer is in effect the protocol stream is processed
   by the security layer into buffers of protected data.  If the
   security layer is not able to produce a buffer, the connection MUST
   be closed. If the security layer is not able to decode a received
   buffer, the connection MUST be closed. In both cases the underlying
   connection SHOULD be closed gracefully.

   Each buffer of protected data is transferred over the connection as a
   stream of octets prepended with a four octet field in network byte
   order that represents the length of the buffer.  The length of the
   protected data buffer MUST be no larger than the maximum size that
   was either defined in the mechanism specification or negotiated by
   the other side during the authentication exchange.  Upon the receipt
   of a data buffer which is larger than the defined/negotiated maximal



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   buffer size the receiver SHOULD close the connection, as this might
   be a sign of an attack.

   SASL mechanisms which are unable to negotiate a security layer are
   treated as selecting no security layer.

4.    Protocol profile requirements

   In order to use this specification, a protocol definition MUST supply
   the following information:

     1) A service name, to be selected from the IANA registry of
     "service" elements for the GSSAPI host-based service name form
     [GSSAPI]. This service name is made available to the authentication
     mechanism.

     The registry is available at the URL
     <http://www.iana.org/assignments/gssapi-service-names>.

     2) A definition of the command to initiate the authentication
     protocol exchange.  This command must have as a parameter the name
     of the mechanism being selected by the client.

     The command SHOULD have an optional parameter giving an initial
     response.  If the protocol allows for the initial response, the
     protocol profile MUST also describe how an empty initial response
     is encoded.  This optional parameter allows the client to avoid a
     round trip when using a mechanism which is defined to have the
     client send data first.  When this initial response is sent by the
     client and the selected mechanism is defined to have the server
     start with an initial challenge, the command fails.  See section
     6.1 of this document for further information.

     3) A definition of the method by which the authentication protocol
     exchange is carried out, including how the challenges and responses
     are encoded, how the server indicates completion or failure of the
     exchange, how the client aborts an exchange, and how the exchange
     method interacts with any line length limits in the protocol.

     The exchange method SHOULD allow the server to include an optional
     data ("optional challenge") with a success notification.  This
     allows the server to avoid a round trip when using a mechanism
     which is defined to have the server send additional data along with
     the indication of successful completion.  Note that if additional
     data is sent with success, it can not be empty. See section 6.2 of
     this document for further information.

     4) A protocol profile SHOULD specify a mechanism through which a



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     client may obtain the names of the SASL mechanisms available to it.
     This is typically done through the protocol's extensions or
     capabilities mechanism.

     5) Identification of the octet where any negotiated security layer
     starts to take effect, in both directions.

     6) Specify if the protocol profile supports "multiple
     authentications" (see section 6.3).

     7) If both a Transport Layer Security [TLS] and a SASL security
     layer are allowed to be negotiated by the protocol, the protocol
     profile MUST define in which order they are applied to a cleartext
     data sent over the connection.

     8) A protocol profile MAY further refine the definition of an
     authorization identity by adding additional syntactic restrictions
     and protocol-specific semantics. A protocol profile MUST specify
     the form of the authorization identity (since it is protocol-
     specific, as opposed to the authentication identity, which is
     mechanism-specific) and how authorization identities are to be
     compared. Profiles whose authorization identities are simple user
     names (e.g. IMAP [RFC 3501]) SHOULD use "SASLprep" profile
     [SASLprep] of the "stringprep" algorithm [StringPrep] to prepare
     these names for matching. The profiles MAY use a stringprep profile
     that is more strict than SASLprep.

     9) Where the application-layer protocol does not precisely state
     how identities established through SASL relate to identities used
     elsewhere (e.g., access controls) in the application-layer
     protocol, it may be useful for the application-layer protocol to
     provide a facility which the client may use to discover the
     identity used.


   A protocol profile SHOULD NOT attempt to amend the definition of
   mechanisms or create mechanism-specific encodings.  This breaks the
   separation between protocol and mechanism that is fundamental to the
   design of SASL. (Likewise, SASL mechanisms are intended to be profile
   neutral.)

5.    Mechanism profile guidelines


   Designers of new SASL mechanism should be aware of the following
   issues:

   1) Authorization identity



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   While some legacy mechanisms are incapable of transmitting an
   authorization identity (which means that for these mechanisms the
   authorization identity is always the empty string), newly defined
   mechanisms SHOULD be capable of transmitting a non-empty
   authorization identity. See also section 3.2.

   2) Character string issues

   Authentication mechanisms SHOULD encode character strings in UTF-8
   [UTF-8] (see [CHARSET-POLICY] for IETF policy regarding character
   sets in IETF protocols).  In order to avoid interoperability problems
   due to differing normalizations, when a mechanism specifies that
   character data is to be used as input to a cryptographic and/or
   comparison function, the mechanism specification MUST detail how the
   data is to be represented, including any normalizations or other
   preparations, to ensure proper function.  Designers of mechanisms
   SHOULD use the "SASLprep" profile [SASLprep] of the "stringprep"
   algorithm [StringPrep] where applicable.  This recommendation does
   not apply to authorization identities as their handling is protocol-
   specific.

   The preparation can be potentially performed on the client side (upon
   getting user input or retrieving a value from configuration) or on
   the server side (upon receiving the value from the client, retrieving
   a value from its authentication database or generating a new value in
   order to store in in the authentication database).  SASL mechanisms
   MUST define which entity (or entities) must perform the preparation.
   If preparation fails or turns a non-empty string into the empty
   string, the entity doing the preparation MUST fail the authentication
   exchange.

   Implementation note: A server side can be represented by multiple
   processes. For example, the server side may consist of the server
   process itself that communicated with a client and a utility (a
   server agent) that is able to store passwords/hashes (or derivitives)
   in a database that can be later used by the server. For the server
   agent the requirement to "fail the authentication exchange" should be
   interpreted as a requirement to refuse to store the data in the
   database.

   3) The mechanism specification MUST detail whether or not it offers a
   security layer.  If it does, it MUST detail the security and other
   services offered in the layer as well as how these services are to be
   implemented.

   4) If the underlying cryptographic technology used by a mechanism
   supports data integrity, then the mechanism specification MUST
   integrity protect the transmission of an authorization identity and



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   the negotiation of the security layer.

   5) The mechanism SHOULD NOT use the authorization identity in
   generation of any long-term cryptographic keys/hashes.  The reason is
   that different protocols (and sometimes even different
   implementations of the same protocol) may use multiple forms of an
   authorization identity that are semantically equivalent and some
   clients may use one form while other clients use a different form.

   6) SASL mechanisms should be designed to minimize the number of round
   trips required, because SASL can be used with protocols where
   connections are short-lived.

   7) SASL mechanisms SHOULD be profile neutral.

6.    Specific issues

6.1.  Client sends data first

   Some mechanisms specify that the first data sent in the
   authentication exchange is from the client to the server.

   If a protocol's profile permits the command which initiates an
   authentication exchange to contain an initial client response, this
   parameter SHOULD be used with such mechanisms.

   If the initial client response parameter is not given, or if a
   protocol's profile does not permit the command which initiates an
   authentication exchange to contain an initial client response, then
   the server issues a challenge with no data.  The client's response to
   this challenge is then used as the initial client response.  (The
   server then proceeds to send the next challenge, indicates
   completion, or indicates failure.)

6.1.1.  Client sends data first examples


   The following are two examples of the SECURID authentication [SASL-
   SECURID] in the SMTP protocol [SMTP].  In the first example below,
   the client is trying fast reauthentication by sending the initial
   response:

      S: 220-smtp.example.com ESMTP Server
      C: EHLO client.example.com
      S: 250-smtp.example.com Welcome client.example.com
      S: 250-AUTH GSSAPI SECURID
      S: 250 DSN
      C: AUTH SECURID AG1hZ251cwAxMjM0NTY3OAA=



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      S: 235 Authentication successful

   The example below is almost identical to the previous, but here the
   client chooses not to use the initial response parameter.

      S: 220-smtp.example.com ESMTP Server
      C: EHLO client.example.com
      S: 250-smtp.example.com Welcome client.example.com
      S: 250-AUTH GSSAPI SECURID
      S: 250 DSN
      C: AUTH SECURID
      S: 334
      C: AG1hZ251cwAxMjM0NTY3OAA=
      S: 235 Authentication successful

   Additonal examples that show usage of initial response can be found
   in section 7.2.


6.2.  Server returns success with additional data

   Some mechanisms may specify that additional data be sent to the
   client along with an indication of successful completion of the
   exchange.  This data would, for example, authenticate the server to
   the client.

   If a protocol's profile does not permit this additional data to be
   returned with a success indication, then the server issues the data
   as a server challenge, without an indication of successful
   completion.  The client then responds with no data.  After receiving
   this empty response, the server then indicates successful completion
   (with no additional data).

   Client implementors should be aware of an additional failure case
   that might occur when the profile supports sending the additional
   data with success. Imagine that an active attacker is trying to
   impersonate the server and sends faked data, which should be used to
   authenticate the server to the client, with success.  (A similar
   situation can happen when either the server and/or the client has a
   bug and they calculate different responses.) After checking the data,
   the client will think that the authentication exchange has failed,
   however the server will think that the authentication exchange has
   completed successfully.  At this point the client can not abort the
   authentication exchange; it SHOULD close the connection instead.
   However, if the profile did not support sending of additional data
   with success, the client could have aborted the exchange at the very
   last step of the authentication exchange.




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6.2.1.  Server returns success with additional data examples


   The following are two examples of a DIGEST-MD5 authentication [SASL-
   DIGEST] in the Extensible Messaging and Presence Protocol [XMPP]. In
   the first example below, the server is sending mutual authentication
   data with success.

      C: <stream:stream
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          to='example.com'
          version='1.0'>
      S: <stream:stream
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          id='c2s_234'
          from='example.com'
          version='1.0'>
      S: <stream:features>
           <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
             <mechanism>DIGEST-MD5</mechanism>
             <mechanism>CRAM-MD5</mechanism>
           </mechanisms>
         </stream:features>
      C: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
               mechanism='DIGEST-MD5'/>
      S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
         cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9
         ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
         </challenge>
      C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
         dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
         T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
         MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
         LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
         YXJzZXQ9dXRmLTgK
         </response>
      S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
         cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
         </success>

      The example below is almost identical to the previous, but here
      the server chooses not to use the additional data with success.

      C: <stream:stream
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'



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          to='example.com'
          version='1.0'>
      S: <stream:stream
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
          id='c2s_234'
          from='example.com'
          version='1.0'>
      S: <stream:features>
           <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
             <mechanism>DIGEST-MD5</mechanism>
             <mechanism>CRAM-MD5</mechanism>
           </mechanisms>
         </stream:features>
      C: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
               mechanism='DIGEST-MD5'/>
      S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
         cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9
         ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
         </challenge>
      C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
         dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
         T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
         MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
         LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
         YXJzZXQ9dXRmLTgK
         </response>
      S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
         cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
         </challenge>
      C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
      S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

6.3.  Multiple authentications

   Unless otherwise stated by the protocol's profile, only one
   successful SASL negotiation may occur in a protocol session.  In this
   case, once an authentication exchange has successfully completed,
   further attempts to initiate an authentication exchange fail.

   If a profile explicitly permits multiple successful SASL negotiations
   to occur, then in no case may multiple security layers be
   simultaneously in effect.  If a security layer is in effect and a
   subsequent SASL negotiation selects a second security layer, then the
   second security layer replaces the first.  If a security layer is in
   effect and a subsequent SASL negotiation selects no security layer,
   the original security layer remains in effect.




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   Where a protocol profile permits multiple successful SASL
   negotiations, the profile MUST detail the effect of a failed SASL
   negotiation upon the previously established authentication state.
   In particular, it MUST state whether the previously established
   authenticated state remains in force or whether the connection is to
   revert to an non-authenticated state. Regardless of the specified
   effect upon authentication state, the previously negotiated security
   layer remains in effect.

7.    The EXTERNAL mechanism

   The mechanism name associated with external authentication is
   "EXTERNAL".

   The client sends a single message containing the UTF-8 encoding of
   the requested authorization identity. The message may be empty. The
   form of the authorization identity may be restricted by the
   application protocol's SASL profile.

   Some system external to SASL must authenticate the client.  If that
   succeeds, the server determines the authentication identity from
   information from this system.  If the requested authorization
   identity is empty, the authorization identity is derived from the
   authentication identity.  The server determines if the authentication
   identity is allowed to act as the authorization identity.  If all
   that succeeds, the server indicates successful completion of the
   authentication exchange; otherwise it indicates failure.

   The system providing this external information may be, for example,
   IPSec [IPSec] or TLS [TLS]. However, the client can make no
   assumptions as to what information the server can use in determining
   client authorization.  For example, just because TLS was established,
   doesn't mean that the server will use the information provided by
   TLS.

7.1.  Formal syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) notation as specified in [ABNF].  Non-terminals referenced
   but not defined below are as defined by [UTF-8].

   The "extern-resp" rule below defines the message sent from client to
   server.

   extern-resp       = *( UTF8-char-no-nul )

   UTF8-char-no-nul  = UTF8-1-no-nul / UTF8-2 / UTF8-3 / UTF8-4




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   UTF8-1-no-nul     = %x01-7F


7.2.  Examples of SASL EXTERNAL

   The following is an example of an EXTERNAL authentication in the SMTP
   protocol [SMTP]. In this example, the client is proxy authenticating,
   sending the authorization identity "fred@example.com" in the
   (optional) initial response. The server has obtained the client's
   (authentication) identity from an external service, such as IPsec,
   and has a security policy that permits that identity to assume the
   identity of the asserted authorization identity.

   To the protocol profile, the sequence "fred@example.com" is an opaque
   binary data. The SASL protocol profile for SMTP [SMTP-AUTH] specifies
   that server challenges and client responses are encoded in BASE64
   [BASE64, section 3]; the BASE64 encoding of "fred@example.com" is
   "ZnJlZEBleGFtcGxlLmNvbQ==".

      S: 220 smtp.example.com ESMTP server ready
      C: EHLO jgm.example.com
      S: 250-smtp.example.com
      S: 250 AUTH DIGEST-MD5 EXTERNAL
      C: AUTH EXTERNAL ZnJlZEBleGFtcGxlLmNvbQ==
      S: 235 Authentication successful.

   The following example is almost identical to the one above, but the
   client doesn't request proxy authentication.

      S: 220 smtp.example.com ESMTP server ready
      C: EHLO jgm.example.com
      S: 250-smtp.example.com
      S: 250 AUTH DIGEST-MD5 EXTERNAL
      C: AUTH EXTERNAL
      S: 235 Authentication successful.

      The following is an example of an EXTERNAL authentication in the
      IMAP4 protocol [IMAP]. IMAP4 doesn't support the initial response
      feature of SASL.  As in the previous example, the client doesn't
      request proxy authentication.

      S: * OK IMAP4rev1 Server
      C: C01 CAPABILITY
      S: * CAPABILITY IMAP4 IMAP4rev1 AUTH=DIGEST-MD5 AUTH=EXTERNAL
      [...]
      C: A01 AUTHENTICATE EXTERNAL
      (note that there is a space following the "+" in the following line)
      S: +



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      C:
      S: A01 OK Success


8.    IANA Considerations


8.1.  Guidelines for IANA


   It is requested that IANA updates the SASL mechanisms registry as
   follows:


      Change the "Intended usage" of the KERBEROS_V4 and SKEY mechanism
      registrations to OBSOLETE.  Change the "Published specification"
      of the EXTERNAL mechanism to this document. Updated registration
      information is provided in Section 8.6.

8.2.  Registration procedure


   Registration of a SASL mechanism is done by filling in the template
   in section 8.5 and sending it via electronic mail to <iana@iana.org>.
   IANA has the right to reject obviously bogus registrations, but will
   perform no review of claims made in the registration form.  SASL
   mechanism registrations are currently available at the URL
   <http://www.iana.org/assignments/sasl-mechanisms>.

   There is no naming convention for SASL mechanisms; any name that
   conforms to the syntax of a SASL mechanism name can be registered.
   However an IETF Standards Track document may reserve a portion of the
   SASL mechanism namespace ("family of SASL mechanisms") for its own
   use, amending the registration rules for that portion of the
   namespace.  Each family of SASL mechanisms MUST be identified by a
   prefix.

   While the registration procedures do not require expert review,
   authors of SASL mechanisms are encouraged to seek community review
   and comment whenever that is feasible.  Authors may seek community
   review by posting a specification of their proposed mechanism as an
   Internet-Draft.  SASL mechanisms intended for widespread use should
   be standardized through the normal IETF process, when appropriate.

8.3.  Comments on SASL mechanism registrations

   Comments on registered SASL mechanisms should first be sent to the
   "owner" of the mechanism and/or to the SASL WG mailing list.



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   Submitters of comments may, after a reasonable attempt to contact the
   owner, request IANA to attach their comment to the SASL mechanism
   registration itself.  If IANA approves of this, the comment will be
   made accessible in conjunction with the SASL mechanism registration
   itself.

8.4.  Change control

   Once a SASL mechanism registration has been published by IANA, the
   author may request a change to its definition.  The change request
   follows the same procedure as the registration request.

   The owner of a SASL mechanism may pass responsibility for the SASL
   mechanism to another person or agency by informing IANA; this can be
   done without discussion or review.

   The IESG may reassign responsibility for a SASL mechanism. The most
   common case of this will be to enable changes to be made to
   mechanisms where the author of the registration has died, moved out
   of contact or is otherwise unable to make changes that are important
   to the community.

   SASL mechanism registrations may not be deleted; mechanisms which are
   no longer believed appropriate for use can be declared OBSOLETE by a
   change to their "intended usage" field; such SASL mechanisms will be
   clearly marked in the lists published by IANA.

   The IESG is considered to be the owner of all SASL mechanisms which
   are on the IETF standards track.

8.5.  Registration template


     Subject: Registration of SASL mechanism X

     Family of SASL mechanisms: (YES or NO)

     SASL mechanism name (or prefix for the family):

     Security considerations:

     Published specification (optional, recommended):

     Person & email address to contact for further information:

     Intended usage:

     (One of COMMON, LIMITED USE or OBSOLETE)



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     Owner/Change controller:

     (Any other information that the author deems interesting may be
     added below this line.)


8.6.  The EXTERNAL mechanism registration

   It is requested that the SASL Mechanism registry [IANA-SASL] entry
   for the EXTERNAL mechanism be updated to reflect that this document
   now provides its technical specification.


      Subject: Updated Registration of SASL mechanism EXTERNAL

      Family of SASL mechanisms: NO

      SASL mechanism name: EXTERNAL

      Security considerations: See RFC XXXX, section 9.

      Published specification (optional, recommended): RFC XXXX

      Person & email address to contact for further information:
        Alexey Melnikov <Alexey.Melnikov@isode.com>

      Intended usage: COMMON

      Owner/Change controller: IESG <iesg@ietf.org>

      Note: Updates existing entry for EXTERNAL

9.   Security considerations

   Security issues are discussed throughout this memo.

   When the client selects a security layer with at least integrity
   protection, this protects against an active attacker hijacking the
   connection and modifying the authentication exchange to negotiate a
   plaintext connection.

   When a server or client supports multiple authentication mechanisms,
   each of which has a different security strength, it is possible for
   an active attacker to cause a party to use the least secure mechanism
   supported.  To protect against this sort of attack, a client or
   server which supports mechanisms of different strengths should have a
   configurable minimum strength that it will use.  It is not sufficient
   for this minimum strength check to only be on the server, since an



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   active attacker can change which mechanisms the client sees as being
   supported, causing the client to send authentication credentials for
   its weakest supported mechanism.

   The client's selection of a SASL mechanism is done in the clear and
   may be modified by an active attacker.  It is important for any new
   SASL mechanisms to be designed such that an active attacker cannot
   obtain an authentication with weaker security properties by modifying
   the SASL mechanism name and/or the challenges and responses.

   In order to detect Man-in-the-middle (MITM) attacks the client MAY
   list available SASL mechanisms both before and after the SASL
   security layer is negotiated.  This allows the client to detect
   active attacks that remove mechanisms from the server's list of
   supported mechanisms, and allows the client to ensure that it is
   using the best mechanism supported by both client and server.  New
   protocol profiles SHOULD require servers to make the list of SASL
   mechanisms available for the initial authentication available to the
   client after security layers are established.  Some older protocols
   do not require this (or don't support listing of SASL mechanisms once
   authentication is complete); for these protocols clients MUST NOT
   treat an empty list of SASL mechanisms after authentication as a MITM
   attack.

   Any protocol interactions prior to authentication are performed in
   the clear and may be modified by an active attacker.  In the case
   where a client selects integrity protection, it is important that any
   security-sensitive protocol negotiations be performed after
   authentication is complete.  Protocols should be designed such that
   negotiations performed prior to authentication should be either
   ignored or revalidated once authentication is complete.

   Clients should be admonished to validate TLS server IDs to prevent
   MITM attacks when using SASL-over-TLS.  The same recommendation
   applies to other protocols providing security services.

   When use of a security layer is negotiated by the authentication
   protocol exchange, the receiver should handle gracefully any
   protected data buffer larger than the defined/negotiated maximal
   size. In particular, it must not blindly allocate the amount of
   memory specified in the buffer size field, as this might cause the
   "out of memory" condition. If the receiver detects a large block, it
   SHOULD close the connection.

   Most of the currently available mechanisms that provide security
   layers only provide basic data security services, such as data
   integrity and data privacy services.  It is hoped that future
   mechanisms will provide more advance data security services like re-



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   keying and truncation attack detection.

   Distributed server implementations need to be careful in how they
   trust other parties.  In particular, authentication secrets should
   only be disclosed to other parties that are trusted to manage and use
   those secrets in manner acceptable to disclosing party.  Applications
   using SASL assume that SASL security layers providing data
   confidentiality are secure even when an attacker chooses the text to
   be protected by the security layer. Similarly applications assume
   that the SASL security layer is secure even if the attacker can
   manipulate the ciphertext output of the security layer. New SASL
   mechanisms MUST meet these assumptions.

   "stringprep" and Unicode security considerations apply to
   authentication identities, authorization identities and passwords.

   The EXTERNAL mechanism provides no security protection; it is
   vulnerable to spoofing by either client or server, active attack, and
   eavesdropping.  It should only be used when external security
   mechanisms are present and have sufficient strength.


9.1.  Re-keying


   The secure or administratively permitted lifetimes of SASL
   mechanisms' security layers are finite.  Cryptographic keys weaken as
   they are used and as time passes; the more time and/or ciphertext
   that a cryptanalyst has after the first use of the a key, the easier
   it is for the cryptanalyst to mount attacks on the key.

   Administrative limits on security layers lifetime may take the form
   of time limits expressed in x.509 certificates, Kerberos V tickets,
   or in directories, and are often desired.  In practice one likely
   effect of administrative security layers lifetime limits is that
   applications may find that security layers stop working in the middle
   of application protocol operation, such as, perhaps, during large
   data transfers.  As the result of this the connection will be closed
   (see section 3.3), which will result in unpleasant user experience.

   Re-keying (key renegotiation process) is a way of addressing the
   weakening of cryptographic keys. SASL framework does not itself
   provide for re-keying.  SASL mechanisms may. Designers of future SASL
   mechanisms should consider providing re-keying services.

   Applications that wish to re-key SASL security layers where the
   mechanism does not provide for re-keying should reauthenticate the
   same IDs and replace the expired or soon-to-expire security layers.



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   This approach requires support for reauthentication in the
   application protocols.  See section 6.3.


10.    References

10.1.  Normative References

   [ABNF] Crocker, D. (Ed.), Overell, P., "Augmented BNF for Syntax
   Specifications: ABNF", RFC 2234, November 1997

   [ASCII] American National Standards Institute, "Code Extension
   Techniques for Use with the 7-bit Coded Character Set of American
   National Standard Code (ASCII) for Information Interchange", FIPS PUB
   35, 1974

   [CHARSET-POLICY] Alvestrand, H., "IETF Policy on Character Sets and
   Languages", RFC 2277, BCP 18, January 1998

   [GSSAPI] Linn, J., "Generic Security Service Application Program
   Interface, Version 2, Update 1", RFC 2743, January 2000

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, BCP 19, March 1997

   [Unicode] The Unicode Consortium, "The Unicode Standard, Version
   3.2.0" is defined by "The Unicode Standard, Version 3.0" (Reading,
   MA, Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the
   "Unicode Standard Annex #27: Unicode 3.1"
   (http://www.unicode.org/reports/tr27/) and by the "Unicode Standard
   Annex #28: Unicode 3.2" (http://www.unicode.org/reports/tr28/).

   [Stringprep] Hoffman, P., Blanchet, M., "Preparation of
   Internationalized Strings ("stringprep")", RFC 3454, December 2002.

   [SASLprep] Zeilenga, K., "SASLprep: Stringprep profile for user names
   and passwords", Work in progress, draft-ietf-sasl-saslprep-XX.txt.

   [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
   RFC 3629, STD 63, November 2003.

10.2.  Informative References


   [SASL-GSSAPI] Melnikov, A., "SASL GSSAPI mechanisms", work in
   progress, draft-ietf-sasl-gssapi-XX.txt, November 2003

   [SASL-DIGEST] Leach, P., Newman, C., Melnikov, A., "Using Digest



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   Authentication as a SASL Mechanism", work in progress, draft-ietf-
   sasl-rfc2831bis-XX.txt, replaces RFC 2831

   [SASL-OTP] Newman, C., "The One-Time-Password SASL Mechanism", RFC
   2444, October 1998.

   [SASL-SECURID] Nystrom, M., "The SecurID(r) SASL Mechanism", RFC
   2808, April 2000.

   [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
   2001.

   [SMTP-AUTH] Myers, J., "SMTP Service Extension for Authentication",
   RFC 2554, March 1999.

   Being revised by Siemborski, R., "SMTP Service Extension for
   Authentication", work in progress, draft-siemborski-rfc2554bis-
   XX.txt.

   [XMPP] Saint-Andre, P., "Extensible Messaging and Presence Protocol
   (XMPP): Core", work in progress, draft-ietf-xmpp-core-XX.txt.

   [BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
   Encodings", RFC 3548, July 2003.

   [RFC-INSTRUCTIONS] Postel, J., Reynolds, J., "Instructions to RFC
   Authors", RFC 2223, October 1997.

   [IANA-SASL]  IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
   MECHANISMS", http://www.iana.org/assignments/sasl-mechanisms.

   [TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
   2246, January 1999.

   [IPSec] Kent, S., and R.  Atkinson, "Security Architecture for the
   Internet Protocol", RFC 2401, November 1998.

   [Sec-Glossary] Shirey, R., "Internet Security Glossary", RFC 2828,
   May 2000.


11.   Editor's Address

     Alexey Melnikov
     Isode Limited
     5 Castle Business Village
     36 Station Road
     Hampton, Middlesex,



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     TW12 2BX, United Kingdom

     Email: Alexey.Melnikov@isode.com
     URI:   http://www.melnikov.ca/

12.   Acknowledgments

   This document is a revision of RFC 2222 written by John G. Myers.  He
   also contributed significantly to this revision.

   Contributions of many members of the SASL mailing list are gratefully
   acknowledged, in particular that of Kurt Zeilenga, Peter Saint-Andre,
   Rob Siemborski, Magnus Nystrom, Jeffrey Hutzelman, Hallvard B
   Furuseth, Tony Hansen, Simon Josefsson, Abhijit Menon-Sen, RL 'Bob'
   Morgan, Sam Hartman, Nicolas Williams, Tim Alsop and Luke Howard.

Appendix A. Relation of SASL to transport security

   Questions have been raised about the relationship between SASL and
   various services (such as IPsec and TLS) which provide a secured
   connection.

   Two of the key features of SASL are:

      The separation of the authorization identity from the identity in
      the client's credentials.  This permits agents such as proxy
      servers to authenticate using their own credentials, yet request
      the access privileges of the identity for which they are proxying.

      Upon successful completion of an authentication exchange, the
      server knows the authorization identity the client wishes to use.
      This allows servers to move to a "user is authenticated" state in
      the protocol.

   These features are extremely important to some application protocols,
   yet Transport Security services do not always provide them.  To
   define SASL mechanisms based on these services would be a very messy
   task, as the framing of these services would be redundant with the
   framing of SASL and some method of providing these important SASL
   features would have to be devised.

   Sometimes it is desired to enable within an existing connection the
   use of a security service which does not fit the SASL model.  (TLS is
   an example of such a service.)  This can be done by adding a command,
   for example "STARTTLS", to the protocol.  Such a command is outside
   the scope of SASL, and should be different from the command which
   starts a SASL authentication protocol exchange.




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   In certain situations, it is reasonable to use SASL underneath one of
   these Transport Security services.  The transport service would
   secure the connection, either service would authenticate the client,
   and SASL would negotiate the authorization identity.  The SASL
   negotiation would be what moves the protocol from "unauthenticated"
   to "authenticated" state.  The "EXTERNAL" SASL mechanism is
   explicitly intended to handle the case where the transport service
   secures the connection and authenticates the client and SASL
   negotiates the authorization identity.

Appendix B. Changes since RFC 2222

   The GSSAPI mechanism was removed.  It is now specified in a separate
   document [SASL-GSSAPI].

   The "KERBEROS_V4" mechanism defined in RFC 2222 is obsolete and has
   been removed.

   The "SKEY" mechanism described in RFC 2222 is obsolete and has been
   removed.  It has been replaced by the OTP mechanism [SASL-OTP].

   The introduction has been substantially reorganized and clarified.

   Clarified the definition and semantics of the authorization identity.

   Prohibited the NUL character in authorization identities.

   Added a section on character string issues.

   The word "must" in the first paragraph of the "Protocol profile
   requirements" section was changed to "MUST".

   Specified that protocol profiles SHOULD provide a way for clients to
   discover available SASL mechanisms.

   Made the requirement that protocol profiles specify the semantics of
   the authorization identity optional to the protocol profile.
   Clarified that such a specification is a refinement of the definition
   in the base SASL spec.

   Added a requirement discouraging protocol profiles from breaking the
   separation between protocol and mechanism.

   Mentioned that standards track documents may carve out their own
   portions of the SASL mechanism namespace and may amend registration
   rules for the portion. However registration of individual SASL
   mechanisms is still required.




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   Clarified that authorization identity should be encoded in UTF-8.

   Specified that the authorization identity in the EXTERNAL mechanism
   is encoded in UTF-8.

   Added a statement that a protocol profile SHOULD allow challenge data
   to be sent with a success indication.

   Added a security consideration for the EXTERNAL mechanism.

   Clarified sections concerning success with additional data.

   Cleaned up IANA considerations/registrations and assembled them in
   one place.

   Updated references and split them into Informative and Normative.

   Added text to the Security considerations section regarding handling
   of extremely large SASL blocks.

   Added text about SASLprep for authentication identities and
   passwords.  Described where SASLprep preparation should take place.

   Added paragraph about verifying authorization identities.

   Added a protocol profile requirement to specify interaction between
   SASL and TLS security layers.

   Added a protocol profile requirement to specify if it supports
   reauthentication.

   Removed the text that seemed to suggest that SASL security layer must
   not be used when TLS is available.

   Created two subsections in 3.2 to talk separately about proxy
   authorization and format of the authorization identities.

   Made requirement to verify that an authorization identity is correct
   by performing SASLprep.

   Clarified that each SASL mechanism must decide where SASLprep is
   taking place.

   Added 4 new examples for initial response and additional data with
   success.

   Added text on checking the list of available SASL mechanisms after
   negotiating a security layer.



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   Added definition of "integrity protection" and "confidentiality
   protection".

   Added warning about negotiating no layer once a security layer is
   negotiated.

   Added new section with guidelines to a SASL mechanism designer.

   Added a requirement to specify how an empty initial challenge is
   encoded if initial response is supported by a protocol.

   Clarified that empty "additional data with success" is not allowed.

   Replaced "buffers of cipher-text" with "buffers of protected data"
   for clarity.

   Clarified that SASL EXTERNAL can be used even with SASL profiles that
   don't support initial client response.

   Changed "authentication protocol exchange" to "authentication
   exchange" everywhere.

   Added some text about re-keying and other services that can be
   provided by a security layer.


Appendix C. Full Copyright Statement and Intellectual Property Statement

Full 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 translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
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A. Melnikov                                            FORMFEED[Page 28]





Internet DRAFT                    SASL                  16 February 2005


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A. Melnikov                                            FORMFEED[Page 29]





Internet DRAFT                    SASL                  16 February 2005


   Status of this Memo .......................................... i
   Abstract ..................................................... 2
   1.  Conventions used in this document ........................ 2
   2.    Introduction ........................................... 2
   2.1.  Relationship to other documents ........................ 4
   3.    Authentication mechanisms .............................. 5
   3.1.  Authentication Exchange ................................ 5
   3.2.  Identity Concepts ...................................... 6
   3.2.1.  Authorization identities and proxy authentication .... 7
   3.2.2.  Authorization Identity Format ........................ 7
   3.3.  Security layers ........................................ 8
   4.    Protocol profile requirements .......................... 9
   5.    Mechanism profile guidelines .......................... 10
   6.    Specific issues ....................................... 12
   6.1.  Client sends data first ............................... 12
   6.1.1.  Client sends data first examples .................... 12
   6.2.  Server returns success with additional data ........... 13
   6.2.1.  Server returns success with additional data examples. 14
   6.3.  Multiple authentications .............................. 15
   7.    The EXTERNAL mechanism ................................ 16
   7.1.  Formal syntax ......................................... 16
   7.2.  Examples of SASL EXTERNAL ............................. 17
   8.    IANA Considerations ................................... 18
   8.1.  Guidelines for IANA ................................... 18
   8.2.  Registration procedure ................................ 18
   8.3.  Comments on SASL mechanism registrations .............. 18
   8.4.  Change control ........................................ 19
   8.5.  Registration template ................................. 19
   8.6.  The EXTERNAL mechanism registration ................... 20
   9.   Security considerations ................................ 20
   9.1.  Re-keying ............................................. 22
   10.    References ........................................... 23
   10.1.  Normative References ................................. 23
   10.2.  Informative References ............................... 23
   11.   Editor's Address ...................................... 24
   12.   Acknowledgments ....................................... 25
   Appendix A. Relation of SASL to transport security .......... 25
   Appendix B. Changes since RFC 2222 .......................... 26
   Appendix C. Full Copyright Statement and Intellectual
               Property Statement .............................. 28
   Full Copyright Statement .................................... 28
   Intellectual Property ....................................... 29









A. Melnikov                                            FORMFEED[Page ii]