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Versions: 01 02 03 04 05 06 07 08 RFC 2478

Internet-Draft                                 Eric Baize, Denis Pinkas
IETF Common Authentication Technology WG                           Bull
<draft-ietf-cat-snego-08.txt>                              4 March 1998



           The Simple and Protected GSS-API Negotiation Mechanism



STATUS OF THIS MEMO

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Comments on this document should be sent to "cat-ietf@mit.edu", the
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2.  ABSTRACT

This draft document specifies a Security Negotiation Mechanism for the
Generic Security Service Application Program Interface (GSS-API) which
is described in [1].

The GSS-API 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 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 which support multiple security mechanisms.


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This allows to negotiate different security mechanisms, different
options within a given security mechanism or different options from
several security mechanisms.

Once the common security mechanism is identified, the security
mechanism may also negotiate mechanism-specific options during its
context establishment. This will be inside the mechanism tokens, and
invisible to the SPNEGO protocol.

The simple and protected GSS-API mechanism negotiation is based on the
following negotiation model : the initiator proposes one security
mechanism or an ordered list of security mechanisms, the target either
accepts the proposed security mechanism, or chooses one from an
offered set, or rejects the proposed value(s). The target then informs
the initiator of its choice.

In its basic form this protocol requires an extra-round trip. Network
connection setup is a critical performance characteristic of any
network infrastructure and extra round trips over WAN links, packet
radio networks, etc. really make a difference. In order to avoid such
an extra round trip the initial security token of the preferred
mechanism for the initiator may be embedded in the initial token.
If the target preferred mechanism matches the initiator's preferred
mechanism, no additional round trips are incurred by using the
negotiation protocol.

The simple and protected GSS-API mechanism negotiation provides a
technique to protect the negotiation that must be used when the
underlying mechanism selected by the target is capable of integrity
protection.

When all the mechanisms proposed by the initiator support integrity
protection or when the selected mechanism supports integrity
protection, then the negotiation mechanism becomes protected since
this guarantees that the appropriate mechanism supported by both
peers has been selected.

The Simple and Protected GSS-API Negotiation Mechanism uses the
concepts developed in the GSS-API specification [1]. 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.

3.  NEGOTIATION MODEL

3.1.  Negotiation description

The model for security mechanism negotiation reuses a subset of the
concepts specified in [2].



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Each OID represents one GSS-API mechanism or one variant of it.

 -  When one security mechanism is proposed by the initiator, it
    represents the only security mechanism supported or
    selected (when the additional APIs defined in the Annex A
    are used) by the initiator.

 -  When several security mechanisms are proposed by the initiator,
    they represent a set of security mechanisms supported or selected
    (when the additional APIs defined in the Annex A are used) by the
    initiator.

The first negotiation token sent by the initiator contains an ordered
list of mechanisms, a set of options (e.g. deleg, replay, conf flags)
that should be supported by the selected mechanism and optionally the
initial security token for the desired mechanism of the initiator
(i.e. the first of the list).

The first negotiation token sent by the target contains the result of
the negotiation (accept_completed, accept_incomplete or reject) and,
in case of accept, the agreed security mechanism. It may also include
the response to the initial security token from the initiator, when
the first proposed mechanism of the initiator has been selected. When
the first mechanism is acceptable to the target,it should respond to
the initial security token for the desired mechanism of the initiator
when it is present. However, if this is not possible, the target can
simply ignore it and omit the responseToken from the first reply.

Implementations that can piggyback the initial token will be rewarded
by faster connection setup.

In case of a successful negotiation, the security mechanism represents
the value suitable for the target, and picked up from the list offered
by the initiator.  The policy by which the target chooses a mechanism
is an implementation-specific local matter.  In the absence of other
policy, the target should chose the first mechanism in the list for
which valid credentials are available.

Once a mechanism has been selected, the tokens specific to the
selected mechanism are carried within the negotiation tokens (in the
mechToken for the initiator and in the responseToken for the target).

3.2.  Negotiation procedure

The negotiation procedure is summarised 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 the negotiation
mechanism) that the Simple and Protected GSS-API Negotiation Mechanism
be used;

(b) the initiator GSS-API implementation emits a negotiation token
containing a list of supported security mechanisms for the credentials

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used for this context establishment, and optionally an initial
security token for the first mechanism from that list (i.e. the
preferred mechanism), and indicates GSS_S_CONTINUE_NEEDED status;

(c) The GSS-API initiator sends the token to the target application;

(d) The GSS-API target deposits the token through invoking
GSS_Accept_sec_context. The target GSS-API implementation emits a
negotiation token containing which if any of the proposed mechanisms
it supports (or has selected).

If the mechanism selected by the target matches the preferred
mechanism identified by the initiator and the initiator provides a
mechToken, the negotiation token response may contain also an initial
security token from that mechanism.

If the preferred mechanism is accepted, GSS_Accept_sec_context()
indicates GSS_S_COMPLETE when unilateral or mutual authentication has
been performed and involves a single token in either direction.

If a proposed mechanism other than the preferred mechanism is
accepted, the negotiation token response may contain also an initial
security token from that mechanism (e.g. to transmit a certificate).

If a proposed mechanism other than the preferred mechanism is accepted,
or the preferred mechanism is accepted but involves multiple exchanges
(e.g. challenge-response authentication), then GSS_Accept_sec_context()
indicates GSS_S_CONTINUE_NEEDED status.

If the proposed mechanism(s) are rejected, GSS_Accept_sec_context()
indicates GSS_S_BAD_MECH status. The security context initialisation
has failed.

(e) The GSS-API target returns the token to the initiator application;

(f) The GSS-API initiator deposits the token through invoking
GSS_Init_sec_context.

GSS_Init_sec_context() may then indicate GSS_S_CONTINUE_NEEDED,
GSS_S_COMPLETE or GSS_S_BAD_MECH status.

     The GSS_S_BAD_MECH status is returned when the negotiation token
     carries a reject result or when the negotiation token carries an
     accept result and the mechanism selected by the target is not
     included in the initial list sent by the initiator.

     The GSS_S_BAD_SIG status is returned when the selected mechanism
     supports a MIC token but the MIC computed over the list of
     mechanisms sent by the initiator is missing or incorrect.

     If the negotiation token carries a reject result, the
     context establishment is impossible. For example, a rejection
     will occur if the target doesn't support the initiator's proposed
     mechanism type(s). Upon failure of the mechanism negotiation

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     procedure, the mech_type output parameter value is the
     negotiation mechanism type.

     The GSS_S_CONTINUE_NEEDED status is returned when the negotiation
     token carries an accept result and further tokens must be
     transferred in order to complete context establishment for the
     selected mechanism. In that case GSS_Init_sec_context() returns
     an initial context token as output_token (with the selected
     mechanism's context token encapsulated within that output_token).

     The initiator then sends the output_token to the target. The
     security context initialisation 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. When GSS_S_CONTINUE_NEEDED is returned, the mech_type
     output parameter is not yet valid.

     When GSS_S_COMPLETE is returned, the mech_type output parameter
     indicates the selected mechanism. When the final negotiation token
     does not contain a MIC, the initiator GSS-API implementation must
     check the returned/selected mechanism is on the originally
     submitted list of mechanisms and also verify that the
     selected mechanism is not able to support a MIC. When the final
     negotiation token contains a MIC over the initial mechanisms list
     sent by the initiator, the MIC must be verified.

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.

The initiator GSS-API calling application may need to know when the
negotiation exchanges were protected or not. For this, when
GSS_S_COMPLETE is returned, it can simply test the integ_avail flag.
When this flag is set it indicates that the negotiation was protected.

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 the negotiation 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).

4.  DATA ELEMENTS

4.1.  Mechanism Type

MechType::= OBJECT IDENTIFIER

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mechType
     Each security mechanism is as defined in [1].

4.2.  Negotiation Tokens

The syntax of the negotiation tokens follows the InitialContextToken
syntax defined in [1]. The security mechanism of the initial
negotiation token is identified by the Object Identifier
iso.org.dod.internet.security.mechanism.snego (1.3.6.1.5.5.2).

4.2.1. Syntax

This section specifies the syntax of the corresponding
"innerContextToken" field for the first token and subsequent
negotiation tokens. During the mechanism negociation,
the "innerContextToken" field contains the ASN.1 structure
"NegociationToken" given below, encoded using the BER/DER encoding
conventions.

NegotiationToken ::= CHOICE {
                              negTokenInit  [0]  NegTokenInit,
                              negTokenTarg  [1]  NegTokenTarg }

MechTypeList ::= SEQUENCE OF MechType

NegTokenInit ::= SEQUENCE {
                            mechTypes       [0] MechTypeList  OPTIONAL,
                            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)
}

negTokenInit
     Negotiation token sent by the initiator to the target, which
     contains, for the first token sent, one or more security
     mechanisms supported by the initiator (as indicated in the field
     mechTypes) and the service options (reqFlags) that are requested
     to establish the context. The context flags should be filled in
     from the req_flags parameter of init_sec_context().

     The mechToken field is optional for the first token sent that
     all target implementations would not have to support. However
     for those targets that do support piggybacking the initial
     mechToken, an optimistic negotiation response is possible.

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     Otherwise the mechToken is used to carry the tokens specific to
     the mechanism selected.

     The mechListMIC is an optional field. In the case that the chosen
     mechanism supports integrity, the initiator may optionally include
     a mechListMIC which is the result of a GetMIC of the MechTypes in
     the initial NegTokenInit and return GSS_S_COMPLETE.

     When the chosen mechanism uses an odd number of messages, the
     final mechanism token will be sent from the initiator to the
     acceptor. In this case, there is a tradeoff between using the
     optimal number of messages, or using an additional message from
     the acceptor to the initiator in order to give the initiator
     assurance that no modification of the initiator's mechanism list
     occurred. The implementation can choose which tradeoff to make
     (see section 4.2.2 for further details for the processing of that
     field).

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

negTokenTarg
     Negotiation token returned by the target to the initiator which
     contains, for the first token returned, a global negotiation
     result and the security mechanism selected (if any).

negResult
     The result accept_completed indicates that a context has been
     successfully established, while the result accept_incomplete
     indicates that additional token exchanges are needed.

          Note: For the case where (a) a single-token context setup
          is used and (b) the preferred mechanism does not support
          the integrity facility which would cause a mechListMIC to be
          generated and enclosed, this feature allows to make a
          difference between a mechToken sent by the initiator
          but not processed by the target (accept_incomplete) and
          a mechToken sent by the initiator and processed by
          the target (accept_completed).

     For those targets that support piggybacking the initial
     mechToken, an optimistic negotiation response is possible
     and includes in that case a responseToken which may continue
     the authentication exchange (e.g. when mutual authentication has
     been requested or when unilateral authentication requires several
     round trips). Otherwise the responseToken is used to carry the
     tokens specific to the mechanism selected.

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     For subsequent tokens (if any) returned by the target, negResult,
     and supportedMech are not present.

     For the last token returned by the target, the mechListMIC, when
     present, is a MIC computed over the MechTypes using the selected
     mechanism.

negResult
     Result of the negotiation exchange, specified by the target.

     This can be either :

          accept_completed
               The target accepts the preferred security mechanism,
                and the context is established for the target or,

          accept_incomplete
               The target accepts one of the proposed security
               mechanisms and further exchanges are necessary, or,

          reject
               The target rejects all the proposed security
               mechanisms.

supportedMech
     This field has to be present when negResult is "accept_completed"
     or "accept_incomplete". It is a choice from the mechanisms offered
     by the initiator.

responseToken
     This field may be used either to transmit the response to the
     mechToken when sent by the initiator and when the first
     mechanism from the list has been selected by the target or
     to carry the tokens specific to the selected security mechanism.

mechListMIC
     If the selected mechanism is capable of integrity protection,
     this field must be present in the last message of the negotiation,
     (i.e., when the underlying mechanism returns a non-empty token
     and a major status of GSS_S_COMPLETE); it contains the result of a
     GetMIC of the MechTypes field in the initial NegTokenInit.
     It allows to verify that the list initially sent by the initiator
     has been received unmodified by the target.

4.2.2. Processing of mechListMIC.

    If the mechanism selected by the negotiation does not support
    integrity, then no mechListMIC is included, otherwise a
    mechListMIC must be used and validated as indicated below.

    If the mechanism supports integrity and uses an even number of
    messages, then the target must compute a MIC as described above,
    and send this in the final NegTokenTarg along with the final
    mechToken. The initiator when receiving the last token must

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    require that the mechListMIC field be present and valid. In the
    absence of a valid mechListMIC, the negotiation must fail as if
    the last context establishment token was invalid.

    In the case that the chosen mechanism supports integrity and uses
    an odd number of messages, the final mechanism token will be sent
    from the initiator to the target. In this case, there is a
    tradeoff between using the optimal number of messages, or using an
    additional message from the target to the initiator in order to
    give the initiator assurance that no modification of the
    initiator's mechanism list occurred. The implementation can choose
    which tradeoff to make.

    When generating the final NegTokenInit message, the NegTokenInit
    may optionally include a mechListMIC which is the result of a
    GetMIC of the MechTypes in the initial NegTokenInit and return
    GSS_S_COMPLETE. The target must check the presence of the MIC
    computed over the mechList sent in the initial NegTokenInit.
    Three cases may then be considered:

         1) If the mechListMIC is present and correct the context is
            established by the target.

         2) If the mechList is present but corrupted, then the context
            establishment must fail.

         3) If the mechListMIC is not included in the final
            NegTokenInit, then GSS_S_CONTINUE_NEEDED must be returned
            to the target. The MIC must then be included in the
            NegTokenTarg as described above, and the NegTokenTarg must
            be sent back to the initiator, which must verify that the
            mechListMIC field is present and valid.

          Note : If the MIC was originally sent by the initiator, but
                 thenafter deleted by an attacker, the target will send
                 back a token according to the description above, but
                 the initiator will be unable to process that returned
                 token and the context establishment must then fail.

5.  EXAMPLES : SECURITY MECHANISM NEGOTIATION

Here are some examples of security mechanism negotiation between an
initiator (I) and a target (T).

5.1.  Initial steps

(I) supports two security mechanism types (GSS-MECH1 and GSS-MECH2).

(I) invokes GSS_Init_sec_context() with :

Input
     mech_type = OID for negotiation mechanism or NULL, if the
     negotiation mechanism is the default mechanism.


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Output
     major_status = GSS_S_CONTINUE_NEEDED
     output_token = negTokenInit

The negotiation token (negTokenInit) contains two security mechanisms
with :
     mechType = GSS-MECH1 or
     mechType = GSS-MECH2

(I) sends to (T) the negotiation token.

5.2  Successful negotiation steps

(T) supports GSS-MECH2
(T) receives the negotiation token (negTokenInit) from (I)
(T) invokes GSS_Accept_sec_context() with :

Input
     input_token = negTokenInit

Output
     major_status = GSS_S_CONTINUE_NEEDED
     output_token = negTokenTarg

The negotiation token (negTokenTarg) contains :
     negResult = accept (the negotiation result)
     supportedMech : mechType = GSS-MECH2

(T) returns the negotiation token (negTokenTarg) to (I)
(I) invokes GSS_Init_sec_context() with :

Input
     input_token = negTokenTarg

Output
     major_status = GSS_S_COMPLETE
     output_token = initialContextToken (initial context token
                                         for GSS-MECH2)
     mech_type = GSS-MECH2

The subsequent steps are security mechanism specific, and work as
specified in [1].  The output tokens from the security mechanism are
encapsulated in a NegTokenTarg message (with the supportedMech field
omitted, and the mechListMIC included with the last token).

5.3.  Failed negotiation steps

(T) supports GSS-MECH3.
(T) receives the negotiation token (negTokenInit) from (I)
(T) invokes GSS_Accept_sec_context() with :

Input
     input_token = negTokenInit


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Output
     major_status = GSS_S_BAD_MECH
     output_token = negTokenTarg

The negotiation token (negTokenTarg) contains :

     negResult = reject (the negotiation result)

(T) returns the negotiation token (negTokenTarg) to (I)
(I) invokes GSS_Init_sec_context() with :

Input
     input_token = negTokenTarg

Output
     major_status = GSS_S_BAD_MECH

The security context establishment has failed.

5.4 Successful Negotiation with preferred mechanism info

(I) supports two security mechanism types (GSS-MECH1 and GSS-MECH2).

(I) invokes GSS_Init_sec_context() with :

Input
     mech_type = OID for negotiation mechanism or NULL, if the
     negotiation mechanism is the default mechanism.

Output
     major_status = GSS_S_CONTINUE_NEEDED
     output_token = negTokenInit

The negotiation token (negTokenInit) contains two security mechanisms
with :
     mechType = GSS-MECH1 or
     mechType = GSS-MECH2

     mechToken = output_token from GSS_Init_sec_context
    ( first mechType) as described in [1]

(I) sends to (T) the negotiation token.

(T) supports GSS-MECH1.
(T) receives the negotiation token (negTokenInit) from (I)
(T) invokes GSS_Accept_sec_context() with :

Input
     input_token = negTokenInit

Output
     major_status = GSS_S_CONTINUE_NEEDED
     output_token = negTokenTarg


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The negotiation token (negTokenTarg) contains :
     negResult = accept (the negotiation result)
     supportedMech : mechType = GSS-MECH1

     mechToken = output_token from
                      GSS_Accept_sec_context(mechToken )

(T) returns the negotiation token (negTokenTarg) to (I)
(I) invokes GSS_Init_sec_context() with :

Input
     input_token = negTokenTarg

Output
     major_status = GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED as needed
     output_token = ContextToken (initial or subsequent context token
                    for GSS-MECH1)
     mech_type = GSS-MECH1

Specific implementations of the protocol can support the optimistic
negotiation by completing the security context establishment using the
agreed upon mechanism as described in [1].  As described above in
section 5.2, the output tokens from the security mechanisms are
encapsulated in a NegTokenTarg message (with the negResult and
supportedMech fields omitted, and the mechListMIC included with the
last token).

6.  ACKNOWLEDGMENTS

Acknowledgments are due to Piers McMahon and Tom Parker of ICL,
Stephen Farrell of SSE, Doug Rosenthal of EINet, John Linn of
RSA Laboratories, and Marc Horowitz of Cygnus Solutions for reviewing
earlier versions of this document and for providing useful inputs.
Acknowledgments are also due to Peter Brundrett of Microsoft for
his proposal for an optimistic negotiation, and for Bill Sommerfeld
of Hewlett-Packard for his proposal for protecting the negotiation.

7.  SECURITY CONSIDERATIONS

When the mechanism selected by the target from the list supplied by
the initiator supports integrity protection, then the negotiation is
protected.

When one of the mechanisms proposed by the initiator does not support
integrity protection, then the negotiation is exposed to all threats
a non secured service is exposed. In particular, an active attacker
can force to use a security mechanism which is not the common
preferred one (when multiple security mechanisms are shared between
peers) but which is acceptable anyway to the target.

In any case, the communicating peers may be exposed to the denial of
service threat.



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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:

GSS_Get_neg_mechs() indicates the set of security mechanisms available
on the local system to the caller for negotiation.

GSS_Set_neg_mechs() specifies the set of security mechanisms to be
used on the local system by the caller for negotiation.

A.1.  GSS_Set_neg_mechs call

Input:
     cred_handle          CREDENTIAL HANDLE
                          - NULL specifies default credentials
     mech_set             SET OF OBJECT IDENTIFIER

Outputs:
     major_status INTEGER,
     minor_status INTEGER,

Return major_status codes :
     GSS_S_COMPLETE indicates that the set of security mechanisms
     available for negotiation has been set to mech_set.
     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 specialised 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 mechanism preference for the target.

A.2.  GSS_Get_neg_mechs call

Input:
     cred_handle    CREDENTIAL HANDLE
                    - NULL specifies default credentials

Outputs:
     major_status INTEGER,
     minor_status INTEGER,
     mech_set     SET OF OBJECT IDENTIFIER


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Return major_status codes :
     GSS_S_COMPLETE indicates that the set of security mechanisms
     available for negotiation has been returned in
     mech_option_set.
     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 specialised 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.


REFERENCES

       [1] Linn, J., "Generic Security Service Application Program
           Interface", RFC 2078, OpenVision, January 1997. Available on
           ftp://ds.internic.net/rfc/rfc2078.txt

       [2] Standard ECMA-206, "Association Context Management including
           Security Context Management", December 1993.  Available on
           http://www.ecma.ch


AUTHORS'S ADDRESSES

   Eric Baize                          Internet email: E.Baize@bull.com
   Bull HN - MA02/211S                          Phone: +1 508 294 61 37
   Technology Park                                Fax: +1 508 294 61 09
   Billerica, MA 01821 - USA

   Denis Pinkas                       Internet email: D.Pinkas@bull.net
   Bull                                        Phone: +33 1 30 80 34 87
   Rue Jean-Jaures                               Fax: +33 1 30 80 33 21
   BP 68
   78340 Les Clayes-sous-Bois - FRANCE












Baize, Pinkas        Document Expiration:  4 September 1998   [Page 14]


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