

NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Expires: June 4, 2005                                      K. Jaganathan
                                                   Microsoft Corporation
                                                           December 2004


              Kerberos Cryptosystem Negotiation Extension
                     draft-zhu-kerb-enctype-nego-00

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
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   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on June 4, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document specifies an extension by Kerberos to negotiate new
   encryption types between the client-server peers.






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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Conventions Used in This Document  . . . . . . . . . . . . .   4
   3.   Negotiation Protocol . . . . . . . . . . . . . . . . . . . .   5
   4.   Security Considerations  . . . . . . . . . . . . . . . . . .   6
   5.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .   7
   6.   Normative References . . . . . . . . . . . . . . . . . . . .   7
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .   7
   A.   Leveraging this Enctype Negotiation in Windows SPNEGO
        Implementations  . . . . . . . . . . . . . . . . . . . . . .   9
        Intellectual Property and Copyright Statements . . . . . . .  10







































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1.  Introduction

   Under the current mechanism [CLAR], the KDC must limit the ticket
   session key enctype chosen for a given service to one it believes is
   supported by both the client and the server.  If both the client and
   server understand a stronger enctype than is selected by the KDC,
   they can not negotiate it.  As the result, the protection of
   application traffic is often weaker than necessary when different
   application software that support different set of enctypes can be
   used by the same server principal.

   This document specifies an extension to Kerberos to allow clients and
   servers to negotiate a different and possible stronger cryptosystem
   to be used in subsequent communication.

   This extension utilizes an authorization data element in the
   authenticator of the KRB_AP_REQ message [CLAR].  The client sends the
   list of enctypes that it supports to the server, the server then
   informs the client its choice.  The negotiated subkey is sent in the
   KRB_AP_REP.































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2.  Conventions Used in This Document

   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 [RFC2119].














































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3.  Negotiation Protocol

   If the client prefers an enctype over that of the service ticket
   session key, then it MUST send the list of enctypes it supports
   (including the one selected by the KDC), in decreasing preference
   order.

   The client sends the enctype list via the authorization-data of the
   authenticator in the KRB_AP_REQ [CLAR].  A new authorization data
   element type AD-ETYPE-NEGOTIATION (129) is defined.  This
   authorization data element itself is enclosed in the AD-IF-RELEVANT
   container, thus a correctly implemented server that does not
   understand this element should ignore it [CLAR].  The value of this
   authorization element contains the DER [X60] encoding of the
   following ASN.1 type:

           EtypeList ::= SEQUENCE OF Int32
              -- the client's proposed enctype list in decreasing
              -- preference order, favorite choice first

   If the EtypeList is present and the server prefers an enctype from
   the client's enctype list over that of the KRB_AP_REQ authenticator
   subkey (if that is present) or the service ticket session key, the
   server MUST create a subkey using that enctype.  This negotiated
   subkey is sent in the subkey field of KRB_AP_REP message and it MUST
   be used for subsequent communication.

   Note that to preserve the quality of randomness provided by the KDC,
   implementations of this protocol SHOULD consider using the service
   ticket session key value as a source of additional entropy when
   generating the negotiated subkey.  If the KRB_AP_REQ authenticator
   subkey is present, it MAY also be used as a source of entropy.

   The policy by which the client or the server chooses an enctype
   (i.e., how the preference order for the supported enctypes is
   selected) is an implementation-specific local matter.















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4.  Security Considerations

   The client's enctype list and the server's reply enctype are part of
   encrypted data, thus the security considerations are the same as
   those of the Kerberos encrypted data.

   In all cases, the communicating peers are exposed to the denial of
   service threat.











































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5.  IANA Considerations

   No IANA actions are required for this document.

6.  Normative References

   [CLAR]     Neuman, B., Yu, Y., Hartman, S. and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", August
              2004.

   [GSS-CFX]  Zhu, L., Jaganathan, K. and S. Hartman, "The Kerberos
              Version 5 GSS-API Mechanism: Version 2", November 2004.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [SPNEGOBIS]
              Zhu, L., Leach, P., Jaganathan, K., Hartman, S. and W.
              Ingersoll, "The Simple and Protected GSS-API Negotiation
              Mechanism", November 2004.


Authors' Addresses

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: lzhu@microsoft.com


   Paul Leach
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: paulle@microsoft.com








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   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: karthikj@microsoft.com












































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Appendix A.  Leveraging this Enctype Negotiation in Windows SPNEGO
            Implementations

   The SPNEGO implementations in Windows 2000, Windows XP and Windows
   2003 do not generate or verify the mechlistMIC field when it is
   required [SPNEGOBIS].

   When the SPNEGO implementations that are updated according to
   [SPNEGOBIS], an SSPI initiator or acceptor needs to determine if the
   peer is updated, so that it can generate the mechlistMIC token when
   the peer can process it.  With the bidirectional negotiation, the
   updated SPNEGO implementation can achieve the following two goals:

   o  It can remain backward compatible with legacy implementations, if
      local policy allows unsafe and unprotected negotiation with
      downlevel implementations when the mechlistMIC token exchange
      would otherwise be required by [SPNEGOBIS].
   o  The mechanism negotiation is protected according to [SPNEGOBIS]
      when both peers are updated.

   However, the updated SPNEGO implementation itself can not securely
   inform the peer whether the local implementation is updated, thus it
   has to obtain such information from the negotiated mechanism.

   For Windows SPNEGO implementations, both the initiator and the
   acceptor are assumed to have been updated if a "newer" [CLAR] or
   different enctype is negotiated for use by the Kerberos GSS-API
   mechanism.























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Intellectual Property Statement

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Acknowledgment

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   Internet Society.




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