path: root/source4/heimdal/doc/standardisation/draft-ietf-kitten-2478bis-00.txt

NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Obsoletes: 2478 (if approved)                              K. Jaganathan
Expires: May 22, 2005                              Microsoft Corporation
                                                               S. Harman
                                                                     MIT
                                                            W. Ingersoll
                                                        Sun Microsystems
                                                       November 21, 2004


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

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   This Internet-Draft will expire on May 22, 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



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   described in RFC 2743.

   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  . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
   A.  GSS-API Negotiation Support API  . . . . . . . . . . . . . . . 19
     A.1   GSS_Set_neg_mechs call . . . . . . . . . . . . . . . . . . 19
     A.2   GSS_Get_neg_mechs call . . . . . . . . . . . . . . . . . . 19
   B.  Changes since RFC2478  . . . . . . . . . . . . . . . . . . . . 21
       Intellectual Property and Copyright Statements . . . . . . . . 22























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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 doesn't 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 that are based on GSS-API implementations and multiple
   mechanisms are shared between the peers.

   The SPNEGO mechanism negotiation is based on the following
   negotiation model: the initiator proposes a list of security
   mechanism(s), in its 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, it MAY also negotiate
   mechanism-specific options during its context establishment, but that
   will be inside the mechanism tokens and invisible to this protocol.

   If per-message integrity services are available on the established
   mechanism security context, the peers can then exchange MIC tokens to
   ensure that the mechanism list was not tampered with.  This MIC token
   exchange is OPTIONAL if no interference could have material impact on
   the negotiation, i.e., when the selected mechanism is the first
   choice for both peers.

   In order to avoid an extra round trip, the first security token of
   the 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 token in this document.  If the
   selected mechanism matches the initiator's preferred mechanism, no
   additional round trips need to be incurred by using this protocol.
   In addition, by using the optimistic token, the initiator can recover
   from a non-fatal error in producing the first token before a
   mechanism can be selected.  Implementations, however, MAY omit the



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   optimistic token, to avoid the cost of generating it in cases where
   the initiator's preferred mechanism is not selected by the acceptor.

   SPNEGO uses 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 for 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 preference order, favorite choice first), and
   optionally the initial security 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 that the initial 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 OPITONAL;  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 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, and
   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 as seen 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 (either explicitly, with the negotiation mechanism,
      or through accepting a default, when the default is this
      negotiation mechanism) that SPNEGO is used.

   (b) The initiator GSS-API implementation emits a negotiation token
      containing a list of supported security mechanisms (possible just
      one mechanism) for the credentials used for this context
      establishment, and optionally an initial security token for the
      first mechanism from that list.

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

      (I) No proposed mechanism is 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
         most preferred mechanism available for the acceptor (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



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         token containing an accept_complete or accept_incomplete state,
         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 through 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 through 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 but was not supported.

   When GSS_Acquire_cred is invoked with this SPNEGO mechanism as
   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 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 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, is present, contains the optimistic security
         mechanism token.







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   mechlistMIC

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


4.2.2  negTokenResp

       NegTokenResp ::= SEQUENCE {
           negResult       [0] ENUMERATED {
               accept_completed    (0),
               accept_incomplete   (1),
               reject              (2),
               request_mic         (3)
           },
           supportedMech   [1] MechType      OPTIONAL,
           responseToken   [2] OCTET STRING  OPTIONAL,
           mechListMIC     [3] OCTET STRING  OPTIONAL,
           ...
       }

   This is the syntax for all subsequent negotiation messages.

   negResult

         This field 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.

         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.






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   supportedMech

         This field SHALL only be present in the first reply from the
         target.  It is a choice from the mechanism(s) offered by the
         initiator.

   ResponseToken

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

   mechlistMIC

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



































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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 initiator's preferred mechanism is accepted and it is also the
   most preferred mechanism available for the acceptor (there is no
   mechanism which, had it been present in the mechanism list, the
   acceptor would have preferred over the accepted mechanism), then the
   MIC token exchange, as described later in this section, is OPTIONAL.
   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
      through 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 "value" portion, omitting the tag and length, of
      the ASN.1 encoding for that field 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 who 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.  GSS_Init_sec_context() indicates
      GSS_S_CONTINUE_NEEDED.  Initiators who 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 and 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) 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 by 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 that attribute.  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, then the negotiation
   is vulnerable: an active attacker can force it to use a security
   mechanism that is not mutually preferred but is acceptable anyway to
   the target.

   When per-message integrity services are available on the established
   mechanism context, and there was an alteration of the mechanism list
   by an adversary such that a common mechanism that is not mutually
   preferred could be selected, this protocol provides the following
   guarantees: if the last mechanism token is sent by the initiator,
   both peers shall fail; 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.  Acknowledgments

   The authors wish to thank Nicolas Williams, Ken Raeburn, Jeff Altman,
   Cristian Ilac and Martin Rex for their comments and suggestions on
   earlier versions of this document.

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

9  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


   Sam Hartman
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA  02139
   US

   EMail: hartmans@mit.edu


   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 token and that
         token only.
      *  mechTypes in negTokenInit is not optional.
      *  negResult is not optional in the negTokenResp token.
      *  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.

















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