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+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
+   which he or she is aware have been or will be disclosed, and any of
+   which he or she become aware will be disclosed, in accordance with
+   RFC 3668.
+
+   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.
+
+   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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+Zhu, et al.               Expires June 4, 2005                  [Page 1]
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+Internet-Draft             Enctype Negotiation             December 2004
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+Table of Contents
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+   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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+Zhu, et al.               Expires June 4, 2005                  [Page 2]
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+Internet-Draft             Enctype Negotiation             December 2004
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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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+Zhu, et al.               Expires June 4, 2005                  [Page 3]
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+Internet-Draft             Enctype Negotiation             December 2004
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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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+Zhu, et al.               Expires June 4, 2005                  [Page 4]
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+Internet-Draft             Enctype Negotiation             December 2004
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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.
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+   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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+Zhu, et al.               Expires June 4, 2005                  [Page 5]
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+Internet-Draft             Enctype Negotiation             December 2004
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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.
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+   In all cases, the communicating peers are exposed to the denial of
+   service threat.
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+Zhu, et al.               Expires June 4, 2005                  [Page 6]
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+Internet-Draft             Enctype Negotiation             December 2004
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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.
+
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+Authors' Addresses
+
+   Larry Zhu
+   Microsoft Corporation
+   One Microsoft Way
+   Redmond, WA  98052
+   US
+
+   Email: lzhu@microsoft.com
+
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+   Paul Leach
+   Microsoft Corporation
+   One Microsoft Way
+   Redmond, WA  98052
+   US
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+   Email: paulle@microsoft.com
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+Zhu, et al.               Expires June 4, 2005                  [Page 7]
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+Internet-Draft             Enctype Negotiation             December 2004
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+   Karthik Jaganathan
+   Microsoft Corporation
+   One Microsoft Way
+   Redmond, WA  98052
+   US
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+   Email: karthikj@microsoft.com
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+Zhu, et al.               Expires June 4, 2005                  [Page 8]
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+Internet-Draft             Enctype Negotiation             December 2004
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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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+Zhu, et al.               Expires June 4, 2005                  [Page 9]
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+Internet-Draft             Enctype Negotiation             December 2004
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+Intellectual Property Statement
+
+   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.
+
+
+Disclaimer of Validity
+
+   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 PARTICULAR PURPOSE.
+
+
+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.
+
+
+Acknowledgment
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
+
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+Zhu, et al.               Expires June 4, 2005                 [Page 10]
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