xref: /macosx-10.10.1/Heimdal-398.1.2/doc/standardisation/draft-ietf-kitten-2478bis-02.txt

NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Obsoletes: 2478 (if approved)                              K. Jaganathan
Expires: June 1, 2005                              Microsoft Corporation
                                                            W. Ingersoll
                                                        Sun Microsystems
                                                        December 1, 2004


         The Simple and Protected GSS-API Negotiation Mechanism
                       draft-ietf-kitten-2478bis

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.

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   This Internet-Draft will expire on June 1, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document specifies a negotiation mechanism for the Generic
   Security Service Application Program Interface (GSS-API) which is
   described in RFC 2743.




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   GSS-API peers can use this negotiation mechanism to choose from a
   common set of security mechanisms.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  5
   3.  Negotiation Protocol . . . . . . . . . . . . . . . . . . . . .  6
     3.1   Negotiation Description  . . . . . . . . . . . . . . . . .  6
     3.2   Negotiation Procedure  . . . . . . . . . . . . . . . . . .  7
   4.  Token Definitions  . . . . . . . . . . . . . . . . . . . . . .  9
     4.1   Mechanism Types  . . . . . . . . . . . . . . . . . . . . .  9
     4.2   Negotiation Tokens . . . . . . . . . . . . . . . . . . . .  9
       4.2.1   negTokenInit . . . . . . . . . . . . . . . . . . . . . 10
       4.2.2   negTokenResp . . . . . . . . . . . . . . . . . . . . . 11
   5.  Processing of mechListMIC  . . . . . . . . . . . . . . . . . . 13
   6.  Extensibility  . . . . . . . . . . . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   10.   Normative References . . . . . . . . . . . . . . . . . . . . 19
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19
   A.  GSS-API Negotiation Support API  . . . . . . . . . . . . . . . 21
     A.1   GSS_Set_neg_mechs call . . . . . . . . . . . . . . . . . . 21
     A.2   GSS_Get_neg_mechs call . . . . . . . . . . . . . . . . . . 21
   B.  Changes since RFC2478  . . . . . . . . . . . . . . . . . . . . 23
       Intellectual Property and Copyright Statements . . . . . . . . 25
























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1.  Introduction

   The GSS-API [RFC2743] provides a generic interface which can be
   layered atop different security mechanisms such that if communicating
   peers acquire GSS-API credentials for the same security mechanism,
   then a security context may be established between them (subject to
   policy).  However, GSS-API does not prescribe the method by which
   GSS-API peers can establish whether they have a common security
   mechanism.

   The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism
   defined here is a pseudo security mechanism, represented by the
   Object Identifier iso.org.dod.internet.security.mechanism.snego
   (1.3.6.1.5.5.2), which enables GSS-API peers to determine in-band
   whether their credentials share common GSS-API security mechanism(s),
   and if so, to invoke normal security context establishment for a
   selected common security mechanism.  This is most useful for
   applications which are based on GSS-API implementations and share
   multiple mechanisms between the peers.

   The SPNEGO mechanism negotiation is based on the following
   negotiation model: the initiator proposes a list of security
   mechanism(s), in decreasing preference order (favorite choice first),
   the acceptor (also known as the target) either accepts the
   initiator's preferred security mechanism (the first in the list), or
   chooses one that is available from the offered list, or rejects the
   proposed value(s).  The target then informs the initiator of its
   choice.

   Once a common security mechanism is chosen, mechanism-specific
   options MAY be negotiated as part of the selected mechanism's context
   establishment.  These negotiations (if any) are internal to the
   mechanism and opaque to the SPNEGO protocol.  As such they are
   outside the scope of this document.

   If per-message integrity services are available on the established
   mechanism security context, then the peers can exchange MIC tokens to
   ensure that the mechanism list was not tampered with.  This MIC token
   exchange is OPTIONAL if the selected mechanism is the most preferred
   choice of both peers (see Section 5).

   In order to avoid an extra round trip, the first security token of
   the initiator's preferred mechanism SHOULD be embedded in the initial
   negotiation message (as defined in Section 4.2).  This mechanism
   token is referred to as the optimistic mechanism token in this
   document.  If the selected mechanism matches the initiator's
   preferred mechanism, no additional round trips need be incurred by
   using this protocol.  In addition, using the optimistic mechanism



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   token allows the initiator to recover from non-fatal errors while
   producing the first mechanism token before a mechanism can be
   selected.  Implementations MAY omit the optimistic mechanism token to
   avoid the cost of generating it in cases where the initiator's
   preferred mechanism is not selected by the acceptor.

   SPNEGO relies the concepts developed in the GSS-API specification
   [RFC2743].  The negotiation data is encapsulated in context-level
   tokens.  Therefore, callers of the GSS-API do not need to be aware of
   the existence of the negotiation tokens but only of the new
   pseudo-security mechanism.  A failure in the negotiation phase causes
   a major status code to be returned: GSS_S_BAD_MECH.







































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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

   When the established mechanism context provides integrity protection,
   the mechanism negotiation can be protected.  When acquiring
   negotiated security mechanism tokens, per-message integrity services
   are always requested by the SPNEGO mechanism.

   When the established mechanism context supports per-message integrity
   services, SPNEGO guarantees that the selected mechanism is mutually
   preferred.

   This section describes the negotiation process of this protocol.

3.1  Negotiation Description

   The first negotiation token sent by the initiator contains an ordered
   list of mechanisms (in decreasing preference order, favorite
   mechanism first), and optionally the initial mechanism token for the
   preferred mechanism of the initiator (i.e., the first in the list).
   The list of security mechanisms available for negotiation is based on
   the credentials being used.

   The target then processes the token from the initiator.  This will
   result in one of four possible states (as defined in Section 4.2.2):
   accept_completed, accept_incomplete, reject, or request_mic.  A
   reject state will terminate the negotiation;  an accept_completed
   state indicates that not only was the initiator-selected mechanism
   acceptable to the target, but also that the initial mechanism token
   was sufficient to complete the authentication;  an accept_incomplete
   state indicates that further message exchange is needed but the MIC
   token exchange as described in Section 5 is OPTIONAL;  a request_mic
   state (this state can only be present in the first reply message from
   the target) indicates the MIC token exchange is REQUIRED if
   per-message integrity services are available.

   Unless the preference order is specified by the application (see
   Appendix A), the policy by which the target chooses a mechanism is an
   implementation-specific local matter.  In the absence of an
   application specified preference order or other policy, the target
   SHALL choose the first mechanism in the initiator proposed list for
   which it has valid credentials.

   In case of a successful negotiation, the security mechanism in the
   first reply message represents the value suitable for the target,
   picked up from the list offered by the initiator.  A context level
   token for a reject state is OPTIONAL.

   Once a mechanism has been selected, the tokens specific to the



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   selected mechanism are carried within the negotiation tokens.

   Lastly, MIC tokens MAY be exchanged to ensure the authenticity of the
   mechanism list received by the target.

   To avoid conflicts with the use of MIC tokens by SPNEGO,
   partially-established contexts are not used for per-message calls:
   the prot_ready_state [RFC2743] will be false even if the underlying
   mechanism would return true natively.

3.2  Negotiation Procedure

   The basic form of the procedure assumes that per-message integrity
   services are available on the established mechanism context, and it
   is summarized as follows:

   (a) The GSS-API initiator invokes GSS_Init_sec_context() as normal,
      but requests that SPNEGO be used.  SPNEGO can either be explicity
      requested or accepted as the default mechanism.

   (b) The initiator GSS-API implementation emits a negotiation token
      containing a list of one or more security mechanisms that are
      available based on the credentials used for this context
      establishment, and optionally the initial mechanism token for the
      first mechanism in the list.

   (c) The GSS-API initiator application sends the token to the target
      application.  The GSS-API target application deposits the token by
      invoking GSS_Accept_sec_context().  The acceptor will do one of
      the following:

      (I) If none of the proposed mechanisms are acceptable, the
         negotiation SHALL be terminated.  GSS_Accept_sec_context
         indicates GSS_S_BAD_MECH.  The acceptor MAY output a
         negotiation token containing a reject state.

      (II) If either the initiator's preferred mechanism is not accepted
         by the target or this mechanism is accepted but it is not the
         acceptor's most preferred mechanism (see Section 3.1 and
         Section 5), GSS_Accept_sec_context() indicates
         GSS_S_CONTINUE_NEEDED.  The acceptor MUST output a negotiation
         token containing a request_mic state.

      (III) Otherwise, GSS_Accept_sec_conext() indicates GSS_S_COMPLETE
         or GSS_S_CONTINUE_NEEDED depending on if at least one
         additional negotiation token from the initiator is needed to
         establish this context.  The acceptor outputs a negotiation
         token containing an accept_complete or accept_incomplete state,



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         respectively.

      If the initiator's preferred mechanism is accepted, and an
      optimistic mechanism token was included, this mechanism token MUST
      be deposited to the selected mechanism by invoking
      GSS_Accept_sec_context() and if a response mechanism token is
      emitted, it MUST be included in the response negotiation token.
      Otherwise, the target will not emit a response mechanism token in
      the first reply.

   (d) The GSS-API target application returns the negotiation token to
      the initiator application.  The GSS-API initiator application
      deposits the token by invoking GSS_Init_sec_context().  The
      security context initialization is then continued according to the
      standard GSS-API conventions for the selected mechanism, where the
      tokens of the selected mechanism are encapsulated until the
      GSS_S_COMPLETE is returned for both the initiator and the target
      by the selected security mechanism.

   (e) MIC tokens are then either skipped or exchanged according to
      Section 5.

   Note that the *_req_flag input parameters for context establishment
   are relative to the selected mechanism, as are the *_state output
   parameters.  i.e., these parameters are not applicable to the
   negotiation process per se.

   On receipt of a negotiation token on the target side, a GSS-API
   implementation that does not support negotiation would indicate the
   GSS_S_BAD_MECH status as if a particular basic security mechanism had
   been requested and was not supported.

   When GSS_Acquire_cred is invoked with this negotiation mechanism in
   the desired_mechs, an implementation-specific default credential is
   used to carry on the negotiation.  A set of mechanisms as specified
   locally by the system administrator is then available for
   negotiation.  If there is a desire for the caller to make its own
   choice, then an additional API has to be used (see Appendix A).













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4.  Token Definitions

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


      SPNEGOASNOneSpec {
          iso(1) identified-organization(3) dod(6) internet(1)
          security(5) mechanism(5) snego (2) modules(4) spec2(2)
      } DEFINITIONS EXPLICIT TAGS ::= BEGIN

      -- rest of definitions here

      END


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

   The encoding of SPNEGO protocol messages shall obey the Distinguished
   Encoding Rules (DER) of ASN.1 as described in [X690].

4.1  Mechanism Types

   In this negotiation model, each OID represents one GSS-API mechanism
   or one variant (see Section 6) of it according to [RFC2743].


       MechType ::= OBJECT IDENTIFIER
           -- OID represents each security mechanism as suggested by
           -- [RFC2743]

       MechTypeList ::= SEQUENCE OF MechType


4.2  Negotiation Tokens

   The syntax of the initial negotiation tokens follows the
   initialContextToken syntax defined in Section 3.1 of [RFC2743].  The
   SPNEGO pseudo mechanism is identified by the Object Identifier
   specified in Section 1.  Subsequent tokens are not encapsulated in
   this GSS-API generic token framing.

   This section specifies the syntax of the inner token for the initial
   message and the syntax of subsequent context establishment tokens.

       NegotiationToken ::= CHOICE {
           negTokenInit    [0] NegTokenInit,



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           negTokenResp    [1] negTokenResp
       }



4.2.1  negTokenInit

       NegTokenInit ::= SEQUENCE {
           mechTypes       [0] MechTypeList,
           reqFlags        [1] ContextFlags  OPTIONAL,
           mechToken       [2] OCTET STRING  OPTIONAL,
           mechListMIC     [3] OCTET STRING  OPTIONAL,
           ...
       }
       ContextFlags ::= BIT STRING {
           delegFlag       (0),
           mutualFlag      (1),
           replayFlag      (2),
           sequenceFlag    (3),
           anonFlag        (4),
           confFlag        (5),
           integFlag       (6)
       }

   This is the syntax for the inner token of the initial negotiation
   message.

   mechTypes

         This field contains one or more security mechanisms available
         for the initiator in decreasing preference order (favorite
         choice first).

   reqFlags

         This field, if present, contains the service options that are
         requested to establish the context.  The context flags SHOULD
         be filled in from the req_flags parameter of
         GSS_Init_sec_context().  This field SHALL NOT have impact on
         the negotiation.

   mechToken

         This field, if present, contains the optimistic mechanism
         token.






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   mechlistMIC

         This field, if present, contains a MIC token for the mechanism
         list in the initial negotiation message.  This MIC token is
         computed according to Section 5.


4.2.2  negTokenResp

       NegTokenResp ::= SEQUENCE {
           negResult       [0] ENUMERATED {
               accept_completed    (0),
               accept_incomplete   (1),
               reject              (2),
               request_mic         (3)
           }                                 OPTIONAL,
             -- REQUIRED in the first reply from the target
           supportedMech   [1] MechType      OPTIONAL,
             -- present only in the first reply from the target
           responseToken   [2] OCTET STRING  OPTIONAL,
           mechListMIC     [3] OCTET STRING  OPTIONAL,
           ...
       }

   This is the syntax for all subsequent negotiation messages.

   negResult

         This field, if present, contains the state of the negotiation.
         This can be:

         accept_completed

            No further negotiation message from the peer is expected,
            and the security context is established for the sender.

         accept_incomplete

            At least one more negotiation message from the peer is
            needed to establish the security context.

         reject

            The sender terminates the negotiation.







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         request_mic

            The sender indicates that the exchange of MIC tokens, as
            described in Section 5, will be REQUIRED if per-message
            integrity services are available on the mechanism context to
            be established.  This value SHALL only be present in the
            first reply from the target.

         This field is REQUIRED in the first reply from the target, and
         it is OPTIONAL thereafter.

   supportedMech

         This field SHALL only be present in the first reply from the
         target.  It MUST be one of the mechanism(s) offered by the
         initiator.

   ResponseToken

         This field, if present, contains tokens specific to the
         mechanism selected.

   mechlistMIC

         This field, if present, contains a MIC token for the mechanism
         list in the initial negotiation message.  This MIC token is
         computed according to Section 5.
























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5.  Processing of mechListMIC

   If the mechanism selected by the negotiation does not support
   integrity protection, then no mechlistMIC token is used.

   Otherwise if the accepted mechanism is the most preferred mechanism
   of both the initiator and the acceptor, then the MIC token exchange,
   as described later in this section, is OPTIONAL.  A mechanism is the
   acceptor's most preferred mechanism if there is no other mechanism
   which would have been preferred over the accepted mechanism if it had
   been present in the received mechanism list.

   In all other cases, MIC tokens MUST be exchanged after the mechanism
   context is fully established.

   It is assumed that per-message integrity services are available on
   the established mechanism context in the following procedure for
   processing MIC tokens of the initiator's mechanism list.

   a) The mechlistMIC token (or simply the MIC token) is computed by
      invoking GSS_GetMIC(): the input context_handle is the established
      mechanism context, the input qop_req is 0, and the input message
      is the mechTypes field in the initial negotiation message (only
      the DER encoding of the type MechTypeList is included).

   b) If the selected mechanism uses an even number of mechanism tokens
      (namely the acceptor sends the last mechanism token), the acceptor
      does the following when emitting the negotiation message
      containing the last mechanism token: if the MIC token exchange is
      not required, GSS_Accept_sec_context() either indicates
      GSS_S_COMPLETE and does not include a mechlistMIC token, or
      indicates GSS_S_CONTINUE_NEEDED and includes a mechlistMIC token
      and an accept_incomplete state; if the MIC token exchange is
      required, GSS_Accept_sec_context() indicates
      GSS_S_CONTINUE_NEEDED, and includes a mechlistMIC token.
      Acceptors that wish to be compatible with legacy Windows SPNEGO
      implementations as described in Appendix B shall not generate a
      mechlistMIC token when the MIC token exchange is not required.
      The initiator then processes the last mechanism token, and does
      one of the following:

      (I) If a mechlistMIC token was included, and is correctly
         verified, GSS_Init_sec_context() indicates GSS_S_COMPLETE.  The
         output negotiation message contains a mechlistMIC token, and an
         accept_complete state.  The acceptor MUST then verify this
         mechlistMIC token.





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      (II) If a mechlistMIC token was included but is incorrect, the
         negotiation SHALL be terminated.  GSS_Accept_sec_context()
         indicates GSS_S_DEFECTIVE_TOKEN.

      (III) If no mechlistMIC token was included, and the MIC token
         exchange is not required, GSS_Init_sec_context() indicates
         GSS_S_COMPLETE with no output token.

      (IV) If no mechlistMIC token was included, but the MIC token
         exchange is required, the negotiation SHALL be terminated.
         GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN.

   c) In the case that the chosen mechanism uses an odd number of
      mechanism tokens (namely the initiator sends the last mechanism
      token), the initiator does the following when emitting the
      negotiation message containing the last mechanism token: if the
      negResult state was request_mic in the first reply from the
      target, a mechlistMIC token MUST be included, otherwise the
      mechlistMIC token is OPTIONAL.  In the case that the optimistic
      mechanism token is the only mechanism token for the initiator's
      preferred mechanism, the mechlistMIC token is OPTIONAL.
      GSS_Init_sec_context() indicates GSS_S_CONTINUE_NEEDED.
      Initiators that wish to be compatible with legacy Windows SPNEGO
      implementations as described in Appendix B shall not generate a
      mechlistMIC token when the MIC token exchange is not required.
      The acceptor then processes the last mechanism token and does one
      of the following:

      (I) If a mechlistMIC token was included and is correctly verified,
         GSS_Accept_sec_context() indicates GSS_S_COMPLETE.  The output
         negotiation message contains a mechlistMIC token and an
         accept_complete state.  The initiator MUST then verify this
         mechlistMIC token.

      (II) If a mechlistMIC token was included but is incorrect, the
         negotiation SHALL be terminated.  GSS_Accept_sec_context()
         indicates GSS_S_DEFECTIVE_TOKEN.

      (III) If no mechlistMIC token was included but the mechlistMIC
         token exchange is not required, GSS_Accept_sec_context()
         indicates GSS_S_COMPLETE.  The output negotiation message
         contains an accept_complete state.

      (IV) In the case that the optimistic mechanism token is also the
         last mechanism token (when the initiator's preferred mechanism
         is accepted by the target) and the target sends a request_mic
         state but the initiator did not send a mechlistMIC token, the
         target then MUST include a mechlistMIC token in that first



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         reply.  GSS_Accept_sec_context() indicates
         GSS_S_CONTINUE_NEEDED.  The initiator MUST verify the received
         mechlistMIC token and generate a mechlistMIC token to send back
         to the target.  The target SHALL in turn verify the returned
         mechlistMIC token and complete the negotiation.

      (V) If no mechlistMIC token was included and the acceptor sent a
         request_mic state in the first reply message (the exchange of
         MIC tokens is required), the negotiation SHALL be terminated.
         GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN.









































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6.  Extensibility

   Two mechanisms are provided for extensibility.  First, the ASN.1
   structures in this specification MAY be expanded by IETF standards
   action.  Implementations receiving unknown fields MUST ignore these
   fields.

   Secondly, OIDs corresponding to a desired mechanism attribute may be
   included in the set of preferred mechanisms by an initiator.  The
   acceptor can choose to honor this request by preferring mechanisms
   that have the included attributes.  Future work within the Kitten
   working group is expected to standardize common attributes that
   SPNEGO mechanisms may wish to support.  At this time it is sufficient
   to say that initiators MAY include OIDs that do not correspond to
   mechanisms but instead correspond to desired mechanism attributes in
   their requests.  Such OIDs MAY influence the acceptor's choice of
   mechanism.  As discussed in Section 5, if there are mechanisms that
   if present in the initiator's list of mechanisms might be preferred
   by the acceptor to the initiator's preferred mechanism, the acceptor
   MUST demand the MIC token exchange.  As a consequence, acceptors MUST
   demand the MIC token exchange if they support negotiation of
   attributes not available in the initiator's preferred mechanism
   regardless of whether the initiator actually requested these
   attributes.



























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7.  Security Considerations

   In order to produce the MIC token for the mechanism list, the
   mechanism must provide integrity protection.  When the selected
   mechanism does not support integrity protection, the negotiation is
   vulnerable: an active attacker can force it to use a security
   mechanism that is not mutually preferred but is acceptable to the
   target.

   This protocol provides the following guarantees when per-message
   integrity services are available on the established mechanism context
   and the mechanism list was altered by an adversary such that a
   mechanism which is not mutually preferred could be selected:

   o  if the last mechanism token is sent by the initiator, both peers
      shall fail;
   o  if the last mechanism token is sent by the acceptor, the acceptor
      shall not complete and the initiator at worst shall complete with
      its preferred mechanism being selected.

   The negotiation may not be terminated if an alteration was made but
   it had no material impact.

   The protection of the negotiation depends on the strength of the
   integrity protection.  In particular, the strength of SPNEGO is no
   stronger than the integrity protection of the weakest mechanism
   acceptable to GSS-API peers.

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





















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8.  IANA Considerations

   This document has no actions for IANA.
















































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9.  Acknowledgments

   The authors wish to thank Sam Hartman, Nicolas Williams, Ken Raeburn,
   Jeff Altman, Tom Yu, Cristian Ilac and Martin Rex for their comments
   and suggestions during development of this document.

   Luke Howard provided a prototype of this protocol in Heimdal and
   resolved several issues in the initial draft.

   Eric Baize and Denis Pinkas wrote the original SPNEGO specification
   [RFC2478] of which some of the text has been retained in this
   document.

10  Normative References

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

   [RFC2478]  Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
              Negotiation Mechanism", RFC 2478, December 1998.

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

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

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


   Wyllys Ingersoll
   Sun Microsystems
   1775 Wiehle Avenue, 2nd Floor
   Reston, VA  20190
   US

   EMail: wyllys.ingersoll@sun.com



































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Appendix A.  GSS-API Negotiation Support API

   In order to provide to a GSS-API caller (either the initiator or the
   target or both) the ability to choose among the set of supported
   mechanisms a reduced set of mechanisms for negotiation, two
   additional APIs are defined:

   o  GSS_Get_neg_mechs() indicates the set of security mechanisms
      available on the local system to the caller for negotiation, based
      on the credentials being used.
   o  GSS_Set_neg_mechs() specifies the set of security mechanisms to be
      used on the local system by the caller for negotiation, for the
      given credentials.

A.1  GSS_Set_neg_mechs call

   Inputs:

   o  cred_handle CREDENTIAL HANDLE, -- NULL specifies default
      -- credentials
   o  mech_set SET OF OBJECT IDENTIFIER

   Outputs:

   o  major_status INTEGER,
   o  minor_status INTEGER

   Return major_status codes:

   o  GSS_S_COMPLETE indicates that the set of security mechanisms
      available for negotiation has been set to mech_set.
   o  GSS_S_FAILURE indicates that the requested operation could not be
      performed for reasons unspecified at the GSS-API level.

   Allows callers to specify the set of security mechanisms that may be
   negotiated with the credential identified by cred_handle.  This call
   is intended for support of specialized callers who need to restrict
   the set of negotiable security mechanisms from the set of all
   security mechanisms available to the caller (based on available
   credentials).  Note that if more than one mechanism is specified in
   mech_set, the order in which those mechanisms are specified implies a
   relative preference.

A.2  GSS_Get_neg_mechs call

   Input:





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   o  cred_handle CREDENTIAL HANDLE -- NULL specifies default
      -- credentials

   Outputs:

   o  major_status INTEGER,
   o  minor_status INTEGER,
   o  mech_set SET OF OBJECT IDENTIFIER

   Return major_status codes:

   o  GSS_S_COMPLETE indicates that the set of security mechanisms
      available for negotiation has been returned in mech_set.
   o  GSS_S_FAILURE indicates that the requested operation could not be
      performed for reasons unspecified at the GSS-API level.

   Allows callers to determine the set of security mechanisms available
   for negotiation with the credential identified by cred_handle.  This
   call is intended for support of specialized callers who need to
   reduce the set of negotiable security mechanisms from the set of
   supported security mechanisms available to the caller (based on
   available credentials).

   Note: The GSS_Indicate_mechs() function indicates the full set of
   mechanism types available on the local system.  Since this call has
   no input parameter, the returned set is not necessarily available for
   all credentials.
























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Appendix B.  Changes since RFC2478

      SPNEGO implementations in Windows 2000/Windows XP/Windows Server
      2003 have the following behavior: no mechlistMIC is produced and
      mechlistMIC is not processed if one is provided; if the initiator
      sends the last mechanism token, the acceptor will send back a
      negotiation token with an accept_complete state and no mechlistMIC
      token.  In addition, the OID (1.2.840.48018.1.2.2) can be used to
      identify the GSS-API Kerberos Version 5 mechanism.

      The following changes have been made to be compatible with these
      legacy implementations.

      *  NegTokenTarg is changed to negTokenResp and it is the message
         format for all subsequent negotiation tokens.
      *  NegTokenInit is the message for the initial negotiation message
         and that message only.
      *  mechTypes in negTokenInit is not optional.
      *  Two MIC tokens are exchanged, one in each direction.
      *  If the selected mechanism is also the most preferred mechanism
         for both peers, it is safe to omit the MIC tokens.

      If at least one of the two peers implements the pseudo mechanism
      in this document, the negotiation is protected.

      The following changes are to address the problems in RFC 2478.

      *  reqFlags is not protected therefore it should not impact the
         negotiation.
      *  DER encoding is required.
      *  GSS_GetMIC() input is clarified.
      *  Per-message integrity services are requested for the negotiated
         mechanism.

   An implementation that conforms to this specification  will not
   interoperate with a strict 2748 implementation.  Even if the new
   implementation always sends a mechlistMIC token, it will still fail
   to interoperate.  If it is a server, it will fail because it requests
   a mechlistMIC token using an option that older implementations simply
   do not support.  Clients will tend to fail as well.

   As an alternative to the approach chosen in this specification, we
   could have documented a correct behavior that is fully backward
   compatible with RFC 2478 and included an appendix on how to
   interoperate with existing incorrect implementations of RFC 2478.

   As a practical matter, the SPNEGO implementers within the IETF have
   valued interoperability with the Microsoft implementations.  We were



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   unable to choose to maintain reasonable security guarantees, maintain
   interoperability with the Microsoft implementations and maintain
   interoperability with correct implementations of RFC 2478.  The
   working group was not aware of any RFC 2478 implementations.  Even if
   there are RFC 2478 implementations, it is unlikely that they will
   interoperate because of a critical flaw in the description of the
   encoding of the mechanism list in RFC 2478.

   With the approach taken in this specification, we get security
   between new implementations all the time while maintaining
   interoperability with the implementations we have within the IETF
   community.  The working group believes that this justifies breaking
   compatibility with a correct implementation of RFC 2478.






































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