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[shishi.git] / doc / specifications / draft-kamada-krb-client-friendly-cross-00.txt

Network Working Group                                    Ken'ichi Kamada
INTERNET-DRAFT                                            Shoichi Sakane
Expires: August 25, 2007                   Yokogawa Electric Corporation
                                                       February 21, 2007


            Client-Friendly Cross-Realm Model for Kerberos 5
             draft-kamada-krb-client-friendly-cross-00.txt




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   Copyright (C) The IETF Trust (2007).










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Abstract

   This document proposes a cross-realm transition model, which is
   friendly to resource-limited clients.  This model provides a way to
   move the cost of consecutive Ticket-Granting Service (TGS) exchanges
   from the clients to the KDC and a way to reduce the transition cost
   by generating a direct inter-realm relationship between two realms.
   The document itself does not specify any protocols, which need to be
   specified separately.


Table of Contents

    1. Introduction .................................................  3
    2. Problems on Client Performance ...............................  3
       2.1. Long Authentication Path ................................  4
       2.2. Client-Centric Ticketing ................................  4
    3. Proposal of Client-Friendly Cross-Realm Model ................  4
       3.1. Temporary Inter-Realm Relationship Generation Mode ......  5
       3.2. Attorney Ticketing Mode .................................  6
       3.3. Combination of the Two Modes ............................  7
    4. Generality and Orthogonality of These Modes ..................  8
    5. Related Protocols Currently Proposed .........................  9
       5.1. PKCROSS .................................................  9
       5.2. XTGS ....................................................  9
    6. Interoperability Considerations ..............................  9
    7. Security Considerations ...................................... 10
       7.1. Denial of Service (DoS) ................................. 10
       7.2. Ticketing Policy Enforcement ............................ 10
    8. IANA Considerations .......................................... 10
    9. Acknowledgments .............................................. 10
   10. References ................................................... 10
   Authors' Addresses ............................................... 11
   Full Copyright Statement ......................................... 11
   Intellectual Property Statement .................................. 11
















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1.  Introduction

   Kerberos Version 5 [RFC4120] has a concept of cross-realm
   authentication so that principals in different realms can
   authenticate each other.  However in the current cross-realm model,
   each client has to traverse the authentication path, and the load of
   the traversal is not negligible for clients with limited resources,
   e.g., computational speed or power supply [CRPS].

   In the current cross-realm operation, a client obtains a service
   ticket for a remote principal in the following steps:

   1)  (Number of transit) TGSes to get cross-realm TGTs in order to
       traverse the intermediate realms, and

   2)  One TGS to get the final service ticket.

   That is, a client need to perform (number of transits) + 1
   transactions to obtain a ticket for a remote service.

   This document proposes a new cross-realm model, which consists of
   "temporary inter-realm relationship generation mode" and "attorney
   ticketing mode".  The former is intended to reduce the transition
   cost itself, on the other hand, the latter is to move the cost from
   clients to KDCs.  The document does not specify any protocols, which
   need to be specified separately.

   Terms defined in section 1.7 of RFC 4120 are used throughout this
   document.


2.  Problems on Client Performance

   In the current model of cross-realm operation, a client has to
   transit all realms on the path to the destination realm.  If the
   source realm and the destination realm have a direct inter-realm
   relationship, a client is able to obtain a service ticket with two
   TGS transactions (one for a cross-realm TGT and one for the service
   ticket), but if there is an only multi-hop relationship between the
   realms, client's job increases in proportion to the distance of the
   relationship.

   Two issues can be observed here behind the client load, which are
   described in the following subsections.







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2.1.  Long Authentication Path

   When a client wants to get a service ticket for a remote realm, it
   must transit to the remote realm by traversing the intermediate
   realms on the authentication path to the remote realm.  The result of
   a transition is cached as a cross-realm TGT, nonetheless, it is a
   per-client optimization.  Thus all clients accessing a remote realm
   must pay the cost separately, even if their resource are limited.
   For a long authentication path, the cost of the whole system becomes
   large.


2.2.  Client-Centric Ticketing

   In Kerberos, any service tickets or cross-realm TGTs are issued via
   TGS, where a client present a ticket for the TGS and obtains a next
   ticket.  Currently, all TGS transactions are initiated by the client
   and it need to get all necessary cross-realm TGTs iteratively before
   the final service ticket.  This process is a burden to a resource-
   limited client.


3.  Proposal of Client-Friendly Cross-Realm Model

   In this section, two modes of operation are introduced, "Temporary
   Inter-Realm Relationship Generation Mode" and "Attorney Ticketing
   Mode", to solve the issues described in the previous section.

   Temporary Inter-Realm Relationship Generation Mode
      This is introduced to solve the issue of the long authentication
      path.  In this mode, if the source realm and the destination realm
      do not have a direct inter-realm relationship, the source KDC
      traverses the authentication path by itself, contacts with the
      remote KDC, and generates a direct inter-realm relationship
      between them.  After that, the source KDC can issue direct inter-
      realm TGTs for the destination realm.  The purpose of this mode is
      to reduce the transition cost itself.

   Attorney Ticketing Mode
      This is introduced to solve the issue of the client-centric
      ticketing.  Consecutive TGS transactions to get cross-realm TGTs
      and/or a final service ticket are initiated by a client in the
      traditional Kerberos, but in this mode, a KDC undertake it.  The
      purpose of this mode is to shift the cost of TGSes from a client
      to a KDC.  This does not reduce the cost itself.

   These two modes are designed to be independent, that is, can be used
   separately or in combination.



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3.1.  Temporary Inter-Realm Relationship Generation Mode

   Temporary inter-realm relationship generation mode enables a KDC to
   issue an inter-realm TGT directly to a remote KDC with which the KDC
   doesn't preshare an inter-realm key.  To issue an inter-realm TGT
   directly, a temporary inter-realm key needs to be provided somehow.
   To achieve that, the local KDC obtains a special ticket for the
   remote KDC and use its session key as an inter-realm key.  This
   methodology was introduced by PKCROSS [PKCROSS].  In this document,
   that special ticket is called as an "inter-KDC ticket", and an inter-
   realm TGT generated using an inter-KDC ticket as a "direct inter-
   realm TGT".

   How does the local KDC reach the remote KDC is out of scope of this
   model, but we can easily come up with 1) traversing a long
   authentication path if available or 2) using PKINIT.  That is,
   PKCROSS is interpreted as a combination of this model and PKINIT.

   This document does not standardize a specific protocol, but a inter-
   KDC ticket will have the following form:

   -  its sname has a special form (PKCROSS proposes
      "pkcross/realm@REALM", but it would be better to use a more
      general name than "pkcross"),

   and a direct inter-realm TGT will have the following form:

   -  its TicketExtensions field [KRBEXT] contains the inter-KDC ticket,
      and

   -  it is protected by the session key (or the sub-session key) of the
      inter-KDC ticket.



















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         client C                KDC-L          KDC-I          KDC-R
         --------                -----          -----          -----

      1.    --------TGS-REQ-------->
      2.                           [Reach to KDC-R in any way.]
                                   [Below is an example of PKCROSS.]
                                   ------------PKINIT------------>
                                   <----------XTKT(L,R)-----------
      3.    <--TKT(C,R)w/XTKT(L,R)--
      4.    ----------------------TGS-REQ------------------------>
      5.    <---------------------TKT(C,S)------------------------

      [Note: TKT(x,y) means a ticket whose cname is x and sname is y.  ]
      [      XTKT is an inter-KDC ticket.  See PKCROSS.                ]
      [      The client C and KDC-L belong to the local realm L.       ]
      [      The KDC-I is a KDC of an intermediate realm I.            ]
      [      The KDC-R is a KDC of the remote realm R.                 ]

      1. The client C sends a normal TGS-REQ to KDC-L, requesting
         a cross-realm TGT to KDC-R.
      2. KDC-L reaches KDC-R in any way and obtains a XTKT.
         There is no standardized way to achieve this step yet.
         PKCROSS is one candidate.  We could also standardize a way
         in which KDC-L normally transits to KDC-R and obtains an XTKT.
      3. KDC-L generates a cross-realm TGT that is from C to KDC-R
         and returns to it to C.  [PKCROSS] [KRBEXT]
      4. The same with the traditional cross-realm TGS.
      5. The same with the traditional cross-realm TGS.

           Figure 1: Message Flow of Temporary Inter-Realm Relationship
                     Generation


3.2.  Attorney Ticketing Mode

   Traditionally, a Kerberos client repeats TGS transactions until it
   gets the final ticket.  For example, it has a TGT for its own realm
   and wants to get a ticket for a service in 3-hop neighbor realm, then
   it will:

   1)  Present the TGT and get a cross-realm TGT for the next realm,

   2)  Present the 1st cross-realm TGT and get a cross-realm TGT for the
       2nd next realm,

   3)  Present the 2nd cross-realm TGT and get a cross-realm TGT for the
       final realm, and




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   4)  Present the final cross-realm TGT and get a service ticket.

   Attorney ticketing mode enables the client to delegate the KDC to
   perform all transactions listed above on behalf of the client.  The
   client entrust the KDC with its TGT.  The KDC "impersonates" the
   client and perform necessary TGS transactions, and returns the final
   ticket to the client.

         client C                KDC-L          KDC-I          KDC-R
         --------                -----          -----          -----

      1.    --------TGS-REQ-------->
      2.                        TKT(C,I)
      3.                           ----TGS-REQ---->
                                   <---TKT(C,R)----
      4.                           ------------TGS-REQ----------->
                                   <-----------TKT(C,S)-----------
      5.    <-------TKT(C,S)--------

      1. The client C sends a special TGS-REQ, which indicates attorney
         ticketing mode requesting a service ticket for S instead of a
         cross-realm TGT, to KDC-L.
      2. KDC-L internally generates a cross-realm TGT that is from C
         to KDC-I, but does not return it to C.
      3. KDC-L uses generated cross-realm TGT by itself, and sends
         a TGS-REQ to KDC-I, which requests a cross-realm TGT from C
         to KDC-R.
      4. KDC-L use the obtained cross-realm TGT by itself, and sends
         a TGS-REQ to KDC-R, which requests a service ticket from C
         to S.
      5. KDC-L returns the final service ticket to C.

            Figure 2: Message Flow of Attorney Ticketing Mode


3.3.  Combination of the Two Modes















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         client C                KDC-L          KDC-I          KDC-R
         --------                -----          -----          -----

      1.    --------TGS-REQ-------->
      2.                           [Below is PKCROSS for example.]
                                   ------------PKINIT------------>
                                   <----------XTKT(L,R)-----------
      3.                   TKT(C,R)w/XTKT(L,R)
      4.                           ------------TGS-REQ----------->
                                   <-----------TKT(C,S)-----------
      5.    <-------TKT(C,S)--------

           Figure 3: Message Flow When Temporary Inter-Realm
                     Relationship Generation Mode and Attorney Ticketing
                     Mode Are Combined


4.  Generality and Orthogonality of These Modes

   -  Compatibility with Traditional Kerberos Deployment

      Temporary inter-realm relationship generation involves only KDCs.
      From the viewpoint of a client (and a server), it seems that there
      is a direct inter-realm relationship between two realms.

      In short, temporary inter-realm relationship generation mode needs
      to be deployed only in KDCs.  The merit of this property is large,
      because it does not affect large installed base of clients.

      Attorney ticketing mode involves only a client and the local KDC.
      From the viewpoint of the remote KDC, TGS-REQs from a KDC as an
      attorney cannot be distinguished from those from a "genuine"
      client.
      [except caddr]
      Resulting service ticket is identical to the traditional one, so
      the remote server has nothing to do this mode.

      In short, attorney ticketing mode can be deployed in local realm,
      independently of the remote deployment.  The merit of this
      property is large, because remote realms are often in different
      administration.

   -  Orthogonality of Temporary Inter-Realm Relationship Generation
      Mode and Attorney Ticketing Mode

      Temporary inter-realm relationship generation mode and attorney
      ticketing mode are independent concepts.  Both can be implemented
      separately or can be used in combination.  When they are combined,



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      load of clients are shifted to KDCs and additional load of KDCs
      are minimized, thus efficient cross-realm environment is achieved.


5.  Related Protocols Currently Proposed


5.1.  PKCROSS

   PKCROSS will be usable as a protocol for temporary inter-realm
   relationship generation mode.


5.2.  XTGS

   The purpose of XTGS protocol is similar to that of this model, but
   the behavior is somewhat different [XTGS].  If we dare to look at
   XTGS from the viewpoint of this model, it blended the two mode
   indivisibly and abandoned the abstraction of cross-realm TGTs and
   inter-KDC tickets in compensation for reducing the number of messages
   between KDCs as far as possible.

   Once a front-end (i.e., between a client and a KDC) protocol to
   request attorney ticketing mode is standardized, XTGS can be used as
   an opaque back-end.


6.  Interoperability Considerations

   [To be more considered...]

   [anything to do with u2u?]

   [negotiation of various parameters]

   [when there are multiple transit paths...]
   [if authz data is involved halfway...?]

   [something to do with referral?]

   [transited field?]

   [when a KDC in attorney mode receives a KRB-ERROR?]








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7.  Security Considerations


7.1.  Denial of Service (DoS)

   A KDC that implements attorney ticketing mode need to initiate
   multiple TGS-REQ upon a request from a client.  This means that the
   KDC will have some states in it and may suffer from DoS attacks.

   Fortunately, attorney ticketing mode can be requested in TGS-REQ,
   which is only available to authenticated clients, thus, any untrusted
   party cannot exploit this statefulness.


7.2.  Ticketing Policy Enforcement

   [attorney: no problem?]

   [tmp inter-realm rel gen: ?]

   [learn from PKCROSS]


8.  IANA Considerations

   This document has no actions for IANA.


9.  Acknowledgments

   The authors would like to acknowledge Saber Zrelli, Masahiro
   Ishiyama, Atsushi Inoue, Kazunori Miyazawa, and Nobuo Okabe for
   contributions to this document.


10.  References

   [RFC4120]     Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
                 Kerberos Network Authentication Service (V5)", RFC
                 4120, July 2005.

   [CRPS]        Sakane, S., Zrelli, S., M. Ishiyama, "Problem statement
                 on the cross-realm operation of Kerberos in a specific
                 system", draft-sakane-krb-cross-problem-statement-01,
                 Work in Progress, October 2006.

   [KRBEXT]      Yu, T., "The Kerberos Network Authentication Service
                 (Version 5)", draft-ietf-krb-wg-rfc1510ter-03, Work in



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                 Progress, October 2006.

   [PKCROSS]     Hur, M. et al., "Public Key Cryptography for Cross-
                 Realm Authentication in Kerberos", draft-ietf-cat-
                 kerberos-pk-cross-08 (expired), Work in Progress,
                 November 2001.

   [XTGS]        Zrelli, S. et al., "XTGSP, the Inter-TGS protocol for
                 cross-realm operations in Kerberos", draft-zrelli-krb-
                 xtgsp-00, Work in Progress, November 2006.

Authors' Addresses

   Ken'ichi Kamada
   Shoichi Sakane
   Yokogawa Electric Corporation
   2-9-32 Nakacho, Musashino-shi,
   Tokyo 180-8750 Japan
   E-mail: Ken-ichi.Kamada@jp.yokogawa.com,
           Shouichi.Sakane@jp.yokogawa.com


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