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|
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
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The list of current Internet-Drafts can be accessed at
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http://www.ietf.org/shadow.html.
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
Zhu, et al. Expires May 22, 2005 [Page 1]
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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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