draft-ietf-kitten-2478bis-02.txt [plain text]
NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Obsoletes: 2478 (if approved) K. Jaganathan
Expires: June 1, 2005 Microsoft Corporation
W. Ingersoll
Sun Microsystems
December 1, 2004
The Simple and Protected GSS-API Negotiation Mechanism
draft-ietf-kitten-2478bis
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document specifies a negotiation mechanism for the Generic
Security Service Application Program Interface (GSS-API) which is
described in RFC 2743.
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GSS-API peers can use this negotiation mechanism to choose from a
common set of security mechanisms.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Negotiation Protocol . . . . . . . . . . . . . . . . . . . . . 6
3.1 Negotiation Description . . . . . . . . . . . . . . . . . 6
3.2 Negotiation Procedure . . . . . . . . . . . . . . . . . . 7
4. Token Definitions . . . . . . . . . . . . . . . . . . . . . . 9
4.1 Mechanism Types . . . . . . . . . . . . . . . . . . . . . 9
4.2 Negotiation Tokens . . . . . . . . . . . . . . . . . . . . 9
4.2.1 negTokenInit . . . . . . . . . . . . . . . . . . . . . 10
4.2.2 negTokenResp . . . . . . . . . . . . . . . . . . . . . 11
5. Processing of mechListMIC . . . . . . . . . . . . . . . . . . 13
6. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
10. Normative References . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19
A. GSS-API Negotiation Support API . . . . . . . . . . . . . . . 21
A.1 GSS_Set_neg_mechs call . . . . . . . . . . . . . . . . . . 21
A.2 GSS_Get_neg_mechs call . . . . . . . . . . . . . . . . . . 21
B. Changes since RFC2478 . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . 25
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1. Introduction
The GSS-API [RFC2743] provides a generic interface which can be
layered atop different security mechanisms such that if communicating
peers acquire GSS-API credentials for the same security mechanism,
then a security context may be established between them (subject to
policy). However, GSS-API does not prescribe the method by which
GSS-API peers can establish whether they have a common security
mechanism.
The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism
defined here is a pseudo security mechanism, represented by the
Object Identifier iso.org.dod.internet.security.mechanism.snego
(1.3.6.1.5.5.2), which enables GSS-API peers to determine in-band
whether their credentials share common GSS-API security mechanism(s),
and if so, to invoke normal security context establishment for a
selected common security mechanism. This is most useful for
applications which are based on GSS-API implementations and share
multiple mechanisms between the peers.
The SPNEGO mechanism negotiation is based on the following
negotiation model: the initiator proposes a list of security
mechanism(s), in decreasing preference order (favorite choice first),
the acceptor (also known as the target) either accepts the
initiator's preferred security mechanism (the first in the list), or
chooses one that is available from the offered list, or rejects the
proposed value(s). The target then informs the initiator of its
choice.
Once a common security mechanism is chosen, mechanism-specific
options MAY be negotiated as part of the selected mechanism's context
establishment. These negotiations (if any) are internal to the
mechanism and opaque to the SPNEGO protocol. As such they are
outside the scope of this document.
If per-message integrity services are available on the established
mechanism security context, then the peers can exchange MIC tokens to
ensure that the mechanism list was not tampered with. This MIC token
exchange is OPTIONAL if the selected mechanism is the most preferred
choice of both peers (see Section 5).
In order to avoid an extra round trip, the first security token of
the initiator's preferred mechanism SHOULD be embedded in the initial
negotiation message (as defined in Section 4.2). This mechanism
token is referred to as the optimistic mechanism token in this
document. If the selected mechanism matches the initiator's
preferred mechanism, no additional round trips need be incurred by
using this protocol. In addition, using the optimistic mechanism
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token allows the initiator to recover from non-fatal errors while
producing the first mechanism token before a mechanism can be
selected. Implementations MAY omit the optimistic mechanism token to
avoid the cost of generating it in cases where the initiator's
preferred mechanism is not selected by the acceptor.
SPNEGO relies the concepts developed in the GSS-API specification
[RFC2743]. The negotiation data is encapsulated in context-level
tokens. Therefore, callers of the GSS-API do not need to be aware of
the existence of the negotiation tokens but only of the new
pseudo-security mechanism. A failure in the negotiation phase causes
a major status code to be returned: GSS_S_BAD_MECH.
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2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Negotiation Protocol
When the established mechanism context provides integrity protection,
the mechanism negotiation can be protected. When acquiring
negotiated security mechanism tokens, per-message integrity services
are always requested by the SPNEGO mechanism.
When the established mechanism context supports per-message integrity
services, SPNEGO guarantees that the selected mechanism is mutually
preferred.
This section describes the negotiation process of this protocol.
3.1 Negotiation Description
The first negotiation token sent by the initiator contains an ordered
list of mechanisms (in decreasing preference order, favorite
mechanism first), and optionally the initial mechanism token for the
preferred mechanism of the initiator (i.e., the first in the list).
The list of security mechanisms available for negotiation is based on
the credentials being used.
The target then processes the token from the initiator. This will
result in one of four possible states (as defined in Section 4.2.2):
accept_completed, accept_incomplete, reject, or request_mic. A
reject state will terminate the negotiation; an accept_completed
state indicates that not only was the initiator-selected mechanism
acceptable to the target, but also that the initial mechanism token
was sufficient to complete the authentication; an accept_incomplete
state indicates that further message exchange is needed but the MIC
token exchange as described in Section 5 is OPTIONAL; a request_mic
state (this state can only be present in the first reply message from
the target) indicates the MIC token exchange is REQUIRED if
per-message integrity services are available.
Unless the preference order is specified by the application (see
Appendix A), the policy by which the target chooses a mechanism is an
implementation-specific local matter. In the absence of an
application specified preference order or other policy, the target
SHALL choose the first mechanism in the initiator proposed list for
which it has valid credentials.
In case of a successful negotiation, the security mechanism in the
first reply message represents the value suitable for the target,
picked up from the list offered by the initiator. A context level
token for a reject state is OPTIONAL.
Once a mechanism has been selected, the tokens specific to the
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selected mechanism are carried within the negotiation tokens.
Lastly, MIC tokens MAY be exchanged to ensure the authenticity of the
mechanism list received by the target.
To avoid conflicts with the use of MIC tokens by SPNEGO,
partially-established contexts are not used for per-message calls:
the prot_ready_state [RFC2743] will be false even if the underlying
mechanism would return true natively.
3.2 Negotiation Procedure
The basic form of the procedure assumes that per-message integrity
services are available on the established mechanism context, and it
is summarized as follows:
(a) The GSS-API initiator invokes GSS_Init_sec_context() as normal,
but requests that SPNEGO be used. SPNEGO can either be explicity
requested or accepted as the default mechanism.
(b) The initiator GSS-API implementation emits a negotiation token
containing a list of one or more security mechanisms that are
available based on the credentials used for this context
establishment, and optionally the initial mechanism token for the
first mechanism in the list.
(c) The GSS-API initiator application sends the token to the target
application. The GSS-API target application deposits the token by
invoking GSS_Accept_sec_context(). The acceptor will do one of
the following:
(I) If none of the proposed mechanisms are acceptable, the
negotiation SHALL be terminated. GSS_Accept_sec_context
indicates GSS_S_BAD_MECH. The acceptor MAY output a
negotiation token containing a reject state.
(II) If either the initiator's preferred mechanism is not accepted
by the target or this mechanism is accepted but it is not the
acceptor's most preferred mechanism (see Section 3.1 and
Section 5), GSS_Accept_sec_context() indicates
GSS_S_CONTINUE_NEEDED. The acceptor MUST output a negotiation
token containing a request_mic state.
(III) Otherwise, GSS_Accept_sec_conext() indicates GSS_S_COMPLETE
or GSS_S_CONTINUE_NEEDED depending on if at least one
additional negotiation token from the initiator is needed to
establish this context. The acceptor outputs a negotiation
token containing an accept_complete or accept_incomplete state,
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respectively.
If the initiator's preferred mechanism is accepted, and an
optimistic mechanism token was included, this mechanism token MUST
be deposited to the selected mechanism by invoking
GSS_Accept_sec_context() and if a response mechanism token is
emitted, it MUST be included in the response negotiation token.
Otherwise, the target will not emit a response mechanism token in
the first reply.
(d) The GSS-API target application returns the negotiation token to
the initiator application. The GSS-API initiator application
deposits the token by invoking GSS_Init_sec_context(). The
security context initialization is then continued according to the
standard GSS-API conventions for the selected mechanism, where the
tokens of the selected mechanism are encapsulated until the
GSS_S_COMPLETE is returned for both the initiator and the target
by the selected security mechanism.
(e) MIC tokens are then either skipped or exchanged according to
Section 5.
Note that the *_req_flag input parameters for context establishment
are relative to the selected mechanism, as are the *_state output
parameters. i.e., these parameters are not applicable to the
negotiation process per se.
On receipt of a negotiation token on the target side, a GSS-API
implementation that does not support negotiation would indicate the
GSS_S_BAD_MECH status as if a particular basic security mechanism had
been requested and was not supported.
When GSS_Acquire_cred is invoked with this negotiation mechanism in
the desired_mechs, an implementation-specific default credential is
used to carry on the negotiation. A set of mechanisms as specified
locally by the system administrator is then available for
negotiation. If there is a desire for the caller to make its own
choice, then an additional API has to be used (see Appendix A).
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4. Token Definitions
The type definitions in this section assume an ASN.1 module
definition of the following form:
SPNEGOASNOneSpec {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanism(5) snego (2) modules(4) spec2(2)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
-- rest of definitions here
END
This specifies that the tagging context for the module will be
explicit and non-automatic.
The encoding of SPNEGO protocol messages shall obey the Distinguished
Encoding Rules (DER) of ASN.1 as described in [X690].
4.1 Mechanism Types
In this negotiation model, each OID represents one GSS-API mechanism
or one variant (see Section 6) of it according to [RFC2743].
MechType ::= OBJECT IDENTIFIER
-- OID represents each security mechanism as suggested by
-- [RFC2743]
MechTypeList ::= SEQUENCE OF MechType
4.2 Negotiation Tokens
The syntax of the initial negotiation tokens follows the
initialContextToken syntax defined in Section 3.1 of [RFC2743]. The
SPNEGO pseudo mechanism is identified by the Object Identifier
specified in Section 1. Subsequent tokens are not encapsulated in
this GSS-API generic token framing.
This section specifies the syntax of the inner token for the initial
message and the syntax of subsequent context establishment tokens.
NegotiationToken ::= CHOICE {
negTokenInit [0] NegTokenInit,
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negTokenResp [1] negTokenResp
}
4.2.1 negTokenInit
NegTokenInit ::= SEQUENCE {
mechTypes [0] MechTypeList,
reqFlags [1] ContextFlags OPTIONAL,
mechToken [2] OCTET STRING OPTIONAL,
mechListMIC [3] OCTET STRING OPTIONAL,
...
}
ContextFlags ::= BIT STRING {
delegFlag (0),
mutualFlag (1),
replayFlag (2),
sequenceFlag (3),
anonFlag (4),
confFlag (5),
integFlag (6)
}
This is the syntax for the inner token of the initial negotiation
message.
mechTypes
This field contains one or more security mechanisms available
for the initiator in decreasing preference order (favorite
choice first).
reqFlags
This field, if present, contains the service options that are
requested to establish the context. The context flags SHOULD
be filled in from the req_flags parameter of
GSS_Init_sec_context(). This field SHALL NOT have impact on
the negotiation.
mechToken
This field, if present, contains the optimistic mechanism
token.
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mechlistMIC
This field, if present, contains a MIC token for the mechanism
list in the initial negotiation message. This MIC token is
computed according to Section 5.
4.2.2 negTokenResp
NegTokenResp ::= SEQUENCE {
negResult [0] ENUMERATED {
accept_completed (0),
accept_incomplete (1),
reject (2),
request_mic (3)
} OPTIONAL,
-- REQUIRED in the first reply from the target
supportedMech [1] MechType OPTIONAL,
-- present only in the first reply from the target
responseToken [2] OCTET STRING OPTIONAL,
mechListMIC [3] OCTET STRING OPTIONAL,
...
}
This is the syntax for all subsequent negotiation messages.
negResult
This field, if present, contains the state of the negotiation.
This can be:
accept_completed
No further negotiation message from the peer is expected,
and the security context is established for the sender.
accept_incomplete
At least one more negotiation message from the peer is
needed to establish the security context.
reject
The sender terminates the negotiation.
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request_mic
The sender indicates that the exchange of MIC tokens, as
described in Section 5, will be REQUIRED if per-message
integrity services are available on the mechanism context to
be established. This value SHALL only be present in the
first reply from the target.
This field is REQUIRED in the first reply from the target, and
it is OPTIONAL thereafter.
supportedMech
This field SHALL only be present in the first reply from the
target. It MUST be one of the mechanism(s) offered by the
initiator.
ResponseToken
This field, if present, contains tokens specific to the
mechanism selected.
mechlistMIC
This field, if present, contains a MIC token for the mechanism
list in the initial negotiation message. This MIC token is
computed according to Section 5.
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5. Processing of mechListMIC
If the mechanism selected by the negotiation does not support
integrity protection, then no mechlistMIC token is used.
Otherwise if the accepted mechanism is the most preferred mechanism
of both the initiator and the acceptor, then the MIC token exchange,
as described later in this section, is OPTIONAL. A mechanism is the
acceptor's most preferred mechanism if there is no other mechanism
which would have been preferred over the accepted mechanism if it had
been present in the received mechanism list.
In all other cases, MIC tokens MUST be exchanged after the mechanism
context is fully established.
It is assumed that per-message integrity services are available on
the established mechanism context in the following procedure for
processing MIC tokens of the initiator's mechanism list.
a) The mechlistMIC token (or simply the MIC token) is computed by
invoking GSS_GetMIC(): the input context_handle is the established
mechanism context, the input qop_req is 0, and the input message
is the mechTypes field in the initial negotiation message (only
the DER encoding of the type MechTypeList is included).
b) If the selected mechanism uses an even number of mechanism tokens
(namely the acceptor sends the last mechanism token), the acceptor
does the following when emitting the negotiation message
containing the last mechanism token: if the MIC token exchange is
not required, GSS_Accept_sec_context() either indicates
GSS_S_COMPLETE and does not include a mechlistMIC token, or
indicates GSS_S_CONTINUE_NEEDED and includes a mechlistMIC token
and an accept_incomplete state; if the MIC token exchange is
required, GSS_Accept_sec_context() indicates
GSS_S_CONTINUE_NEEDED, and includes a mechlistMIC token.
Acceptors that wish to be compatible with legacy Windows SPNEGO
implementations as described in Appendix B shall not generate a
mechlistMIC token when the MIC token exchange is not required.
The initiator then processes the last mechanism token, and does
one of the following:
(I) If a mechlistMIC token was included, and is correctly
verified, GSS_Init_sec_context() indicates GSS_S_COMPLETE. The
output negotiation message contains a mechlistMIC token, and an
accept_complete state. The acceptor MUST then verify this
mechlistMIC token.
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(II) If a mechlistMIC token was included but is incorrect, the
negotiation SHALL be terminated. GSS_Accept_sec_context()
indicates GSS_S_DEFECTIVE_TOKEN.
(III) If no mechlistMIC token was included, and the MIC token
exchange is not required, GSS_Init_sec_context() indicates
GSS_S_COMPLETE with no output token.
(IV) If no mechlistMIC token was included, but the MIC token
exchange is required, the negotiation SHALL be terminated.
GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN.
c) In the case that the chosen mechanism uses an odd number of
mechanism tokens (namely the initiator sends the last mechanism
token), the initiator does the following when emitting the
negotiation message containing the last mechanism token: if the
negResult state was request_mic in the first reply from the
target, a mechlistMIC token MUST be included, otherwise the
mechlistMIC token is OPTIONAL. In the case that the optimistic
mechanism token is the only mechanism token for the initiator's
preferred mechanism, the mechlistMIC token is OPTIONAL.
GSS_Init_sec_context() indicates GSS_S_CONTINUE_NEEDED.
Initiators that wish to be compatible with legacy Windows SPNEGO
implementations as described in Appendix B shall not generate a
mechlistMIC token when the MIC token exchange is not required.
The acceptor then processes the last mechanism token and does one
of the following:
(I) If a mechlistMIC token was included and is correctly verified,
GSS_Accept_sec_context() indicates GSS_S_COMPLETE. The output
negotiation message contains a mechlistMIC token and an
accept_complete state. The initiator MUST then verify this
mechlistMIC token.
(II) If a mechlistMIC token was included but is incorrect, the
negotiation SHALL be terminated. GSS_Accept_sec_context()
indicates GSS_S_DEFECTIVE_TOKEN.
(III) If no mechlistMIC token was included but the mechlistMIC
token exchange is not required, GSS_Accept_sec_context()
indicates GSS_S_COMPLETE. The output negotiation message
contains an accept_complete state.
(IV) In the case that the optimistic mechanism token is also the
last mechanism token (when the initiator's preferred mechanism
is accepted by the target) and the target sends a request_mic
state but the initiator did not send a mechlistMIC token, the
target then MUST include a mechlistMIC token in that first
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reply. GSS_Accept_sec_context() indicates
GSS_S_CONTINUE_NEEDED. The initiator MUST verify the received
mechlistMIC token and generate a mechlistMIC token to send back
to the target. The target SHALL in turn verify the returned
mechlistMIC token and complete the negotiation.
(V) If no mechlistMIC token was included and the acceptor sent a
request_mic state in the first reply message (the exchange of
MIC tokens is required), the negotiation SHALL be terminated.
GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN.
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6. Extensibility
Two mechanisms are provided for extensibility. First, the ASN.1
structures in this specification MAY be expanded by IETF standards
action. Implementations receiving unknown fields MUST ignore these
fields.
Secondly, OIDs corresponding to a desired mechanism attribute may be
included in the set of preferred mechanisms by an initiator. The
acceptor can choose to honor this request by preferring mechanisms
that have the included attributes. Future work within the Kitten
working group is expected to standardize common attributes that
SPNEGO mechanisms may wish to support. At this time it is sufficient
to say that initiators MAY include OIDs that do not correspond to
mechanisms but instead correspond to desired mechanism attributes in
their requests. Such OIDs MAY influence the acceptor's choice of
mechanism. As discussed in Section 5, if there are mechanisms that
if present in the initiator's list of mechanisms might be preferred
by the acceptor to the initiator's preferred mechanism, the acceptor
MUST demand the MIC token exchange. As a consequence, acceptors MUST
demand the MIC token exchange if they support negotiation of
attributes not available in the initiator's preferred mechanism
regardless of whether the initiator actually requested these
attributes.
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7. Security Considerations
In order to produce the MIC token for the mechanism list, the
mechanism must provide integrity protection. When the selected
mechanism does not support integrity protection, the negotiation is
vulnerable: an active attacker can force it to use a security
mechanism that is not mutually preferred but is acceptable to the
target.
This protocol provides the following guarantees when per-message
integrity services are available on the established mechanism context
and the mechanism list was altered by an adversary such that a
mechanism which is not mutually preferred could be selected:
o if the last mechanism token is sent by the initiator, both peers
shall fail;
o if the last mechanism token is sent by the acceptor, the acceptor
shall not complete and the initiator at worst shall complete with
its preferred mechanism being selected.
The negotiation may not be terminated if an alteration was made but
it had no material impact.
The protection of the negotiation depends on the strength of the
integrity protection. In particular, the strength of SPNEGO is no
stronger than the integrity protection of the weakest mechanism
acceptable to GSS-API peers.
In all cases, the communicating peers are exposed to the denial of
service threat.
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8. IANA Considerations
This document has no actions for IANA.
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9. Acknowledgments
The authors wish to thank Sam Hartman, Nicolas Williams, Ken Raeburn,
Jeff Altman, Tom Yu, Cristian Ilac and Martin Rex for their comments
and suggestions during development of this document.
Luke Howard provided a prototype of this protocol in Heimdal and
resolved several issues in the initial draft.
Eric Baize and Denis Pinkas wrote the original SPNEGO specification
[RFC2478] of which some of the text has been retained in this
document.
10 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2478] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[X690] ASN.1 encoding rules: Specification of Basic Encoding
Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER), ITU-T Recommendation
X.690 (1997) | ISO/IEC International Standard 8825-1:1998.
Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: lzhu@microsoft.com
Paul Leach
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: paulle@microsoft.com
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Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: karthikj@microsoft.com
Wyllys Ingersoll
Sun Microsystems
1775 Wiehle Avenue, 2nd Floor
Reston, VA 20190
US
EMail: wyllys.ingersoll@sun.com
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Appendix A. GSS-API Negotiation Support API
In order to provide to a GSS-API caller (either the initiator or the
target or both) the ability to choose among the set of supported
mechanisms a reduced set of mechanisms for negotiation, two
additional APIs are defined:
o GSS_Get_neg_mechs() indicates the set of security mechanisms
available on the local system to the caller for negotiation, based
on the credentials being used.
o GSS_Set_neg_mechs() specifies the set of security mechanisms to be
used on the local system by the caller for negotiation, for the
given credentials.
A.1 GSS_Set_neg_mechs call
Inputs:
o cred_handle CREDENTIAL HANDLE, -- NULL specifies default
-- credentials
o mech_set SET OF OBJECT IDENTIFIER
Outputs:
o major_status INTEGER,
o minor_status INTEGER
Return major_status codes:
o GSS_S_COMPLETE indicates that the set of security mechanisms
available for negotiation has been set to mech_set.
o GSS_S_FAILURE indicates that the requested operation could not be
performed for reasons unspecified at the GSS-API level.
Allows callers to specify the set of security mechanisms that may be
negotiated with the credential identified by cred_handle. This call
is intended for support of specialized callers who need to restrict
the set of negotiable security mechanisms from the set of all
security mechanisms available to the caller (based on available
credentials). Note that if more than one mechanism is specified in
mech_set, the order in which those mechanisms are specified implies a
relative preference.
A.2 GSS_Get_neg_mechs call
Input:
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o cred_handle CREDENTIAL HANDLE -- NULL specifies default
-- credentials
Outputs:
o major_status INTEGER,
o minor_status INTEGER,
o mech_set SET OF OBJECT IDENTIFIER
Return major_status codes:
o GSS_S_COMPLETE indicates that the set of security mechanisms
available for negotiation has been returned in mech_set.
o GSS_S_FAILURE indicates that the requested operation could not be
performed for reasons unspecified at the GSS-API level.
Allows callers to determine the set of security mechanisms available
for negotiation with the credential identified by cred_handle. This
call is intended for support of specialized callers who need to
reduce the set of negotiable security mechanisms from the set of
supported security mechanisms available to the caller (based on
available credentials).
Note: The GSS_Indicate_mechs() function indicates the full set of
mechanism types available on the local system. Since this call has
no input parameter, the returned set is not necessarily available for
all credentials.
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Appendix B. Changes since RFC2478
SPNEGO implementations in Windows 2000/Windows XP/Windows Server
2003 have the following behavior: no mechlistMIC is produced and
mechlistMIC is not processed if one is provided; if the initiator
sends the last mechanism token, the acceptor will send back a
negotiation token with an accept_complete state and no mechlistMIC
token. In addition, the OID (1.2.840.48018.1.2.2) can be used to
identify the GSS-API Kerberos Version 5 mechanism.
The following changes have been made to be compatible with these
legacy implementations.
* NegTokenTarg is changed to negTokenResp and it is the message
format for all subsequent negotiation tokens.
* NegTokenInit is the message for the initial negotiation message
and that message only.
* mechTypes in negTokenInit is not optional.
* Two MIC tokens are exchanged, one in each direction.
* If the selected mechanism is also the most preferred mechanism
for both peers, it is safe to omit the MIC tokens.
If at least one of the two peers implements the pseudo mechanism
in this document, the negotiation is protected.
The following changes are to address the problems in RFC 2478.
* reqFlags is not protected therefore it should not impact the
negotiation.
* DER encoding is required.
* GSS_GetMIC() input is clarified.
* Per-message integrity services are requested for the negotiated
mechanism.
An implementation that conforms to this specification will not
interoperate with a strict 2748 implementation. Even if the new
implementation always sends a mechlistMIC token, it will still fail
to interoperate. If it is a server, it will fail because it requests
a mechlistMIC token using an option that older implementations simply
do not support. Clients will tend to fail as well.
As an alternative to the approach chosen in this specification, we
could have documented a correct behavior that is fully backward
compatible with RFC 2478 and included an appendix on how to
interoperate with existing incorrect implementations of RFC 2478.
As a practical matter, the SPNEGO implementers within the IETF have
valued interoperability with the Microsoft implementations. We were
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unable to choose to maintain reasonable security guarantees, maintain
interoperability with the Microsoft implementations and maintain
interoperability with correct implementations of RFC 2478. The
working group was not aware of any RFC 2478 implementations. Even if
there are RFC 2478 implementations, it is unlikely that they will
interoperate because of a critical flaw in the description of the
encoding of the mechanism list in RFC 2478.
With the approach taken in this specification, we get security
between new implementations all the time while maintaining
interoperability with the implementations we have within the IETF
community. The working group believes that this justifies breaking
compatibility with a correct implementation of RFC 2478.
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