draft-ietf-kitten-2478bis-01.txt   draft-ietf-kitten-2478bis-02.txt 
NETWORK WORKING GROUP L. Zhu NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach Internet-Draft P. Leach
Obsoletes: 2478 (if approved) K. Jaganathan Obsoletes: 2478 (if approved) K. Jaganathan
Expires: May 26, 2005 Microsoft Corporation Expires: June 1, 2005 Microsoft Corporation
W. Ingersoll W. Ingersoll
Sun Microsystems Sun Microsystems
November 25, 2004 December 1, 2004
The Simple and Protected GSS-API Negotiation Mechanism The Simple and Protected GSS-API Negotiation Mechanism
draft-ietf-kitten-2478bis-01 draft-ietf-kitten-2478bis-02
Status of this Memo Status of this Memo
This document is an Internet-Draft and is subject to all provisions This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of 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 is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with which he or she become aware will be disclosed, in accordance with
RFC 3668. RFC 3668.
skipping to change at page 1, line 38 skipping to change at page 1, line 38
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 26, 2005. This Internet-Draft will expire on June 1, 2005.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). Copyright (C) The Internet Society (2004).
Abstract Abstract
This document specifies a negotiation mechanism for the Generic This document specifies a negotiation mechanism for the Generic
Security Service Application Program Interface (GSS-API) which is Security Service Application Program Interface (GSS-API) which is
described in RFC 2743. described in RFC 2743.
skipping to change at page 2, line 31 skipping to change at page 2, line 31
6. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 16 6. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
10. Normative References . . . . . . . . . . . . . . . . . . . . 19 10. Normative References . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19
A. GSS-API Negotiation Support API . . . . . . . . . . . . . . . 21 A. GSS-API Negotiation Support API . . . . . . . . . . . . . . . 21
A.1 GSS_Set_neg_mechs call . . . . . . . . . . . . . . . . . . 21 A.1 GSS_Set_neg_mechs call . . . . . . . . . . . . . . . . . . 21
A.2 GSS_Get_neg_mechs call . . . . . . . . . . . . . . . . . . 21 A.2 GSS_Get_neg_mechs call . . . . . . . . . . . . . . . . . . 21
B. Changes since RFC2478 . . . . . . . . . . . . . . . . . . . . 23 B. Changes since RFC2478 . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . 24 Intellectual Property and Copyright Statements . . . . . . . . 25
1. Introduction 1. Introduction
The GSS-API [RFC2743] provides a generic interface which can be The GSS-API [RFC2743] provides a generic interface which can be
layered atop different security mechanisms such that if communicating layered atop different security mechanisms such that if communicating
peers acquire GSS-API credentials for the same security mechanism, peers acquire GSS-API credentials for the same security mechanism,
then a security context may be established between them (subject to then a security context may be established between them (subject to
policy). However, GSS-API doesn't prescribe the method by which policy). However, GSS-API does not prescribe the method by which
GSS-API peers can establish whether they have a common security GSS-API peers can establish whether they have a common security
mechanism. mechanism.
The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism
defined here is a pseudo security mechanism, represented by the defined here is a pseudo security mechanism, represented by the
Object Identifier iso.org.dod.internet.security.mechanism.snego 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 (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), whether their credentials share common GSS-API security mechanism(s),
and if so, to invoke normal security context establishment for a and if so, to invoke normal security context establishment for a
selected common security mechanism. This is most useful for selected common security mechanism. This is most useful for
applications that are based on GSS-API implementations and multiple applications which are based on GSS-API implementations and share
mechanisms are shared between the peers. multiple mechanisms between the peers.
The SPNEGO mechanism negotiation is based on the following The SPNEGO mechanism negotiation is based on the following
negotiation model: the initiator proposes a list of security negotiation model: the initiator proposes a list of security
mechanism(s), in its preference order (favorite choice first), the mechanism(s), in decreasing preference order (favorite choice first),
acceptor (also known as the target) either accepts the initiator's the acceptor (also known as the target) either accepts the
preferred security mechanism (the first in the list), or chooses one initiator's preferred security mechanism (the first in the list), or
that is available from the offered list, or rejects the proposed chooses one that is available from the offered list, or rejects the
value(s). The target then informs the initiator of its choice. proposed value(s). The target then informs the initiator of its
choice.
Once a common security mechanism is chosen, it MAY also negotiate Once a common security mechanism is chosen, mechanism-specific
mechanism-specific options during its context establishment, but that options MAY be negotiated as part of the selected mechanism's context
will be inside the mechanism tokens and invisible to this protocol. 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 If per-message integrity services are available on the established
mechanism security context, the peers can then exchange MIC tokens to mechanism security context, then the peers can exchange MIC tokens to
ensure that the mechanism list was not tampered with. This MIC token ensure that the mechanism list was not tampered with. This MIC token
exchange is OPTIONAL if no interference could have material impact on exchange is OPTIONAL if the selected mechanism is the most preferred
the negotiation, i.e., when the selected mechanism is the first choice of both peers (see Section 5).
choice for both peers.
In order to avoid an extra round trip, the first security token of In order to avoid an extra round trip, the first security token of
the preferred mechanism SHOULD be embedded in the initial negotiation the initiator's preferred mechanism SHOULD be embedded in the initial
message (as defined in Section 4.2). This mechanism token is negotiation message (as defined in Section 4.2). This mechanism
referred to as the optimistic token in this document. If the token is referred to as the optimistic mechanism token in this
selected mechanism matches the initiator's preferred mechanism, no document. If the selected mechanism matches the initiator's
additional round trips need to be incurred by using this protocol. preferred mechanism, no additional round trips need be incurred by
In addition, by using the optimistic token, the initiator can recover using this protocol. In addition, using the optimistic mechanism
from a non-fatal error in producing the first token before a token allows the initiator to recover from non-fatal errors while
mechanism can be selected. Implementations, however, MAY omit the producing the first mechanism token before a mechanism can be
optimistic token, to avoid the cost of generating it in cases where selected. Implementations MAY omit the optimistic mechanism token to
the initiator's preferred mechanism is not selected by the acceptor. 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 SPNEGO relies the concepts developed in the GSS-API specification
[RFC2743]. The negotiation data is encapsulated in context-level [RFC2743]. The negotiation data is encapsulated in context-level
tokens. Therefore, callers of the GSS-API do not need to be aware of 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 the existence of the negotiation tokens but only of the new
pseudo-security mechanism. A failure in the negotiation phase causes pseudo-security mechanism. A failure in the negotiation phase causes
a major status code to be returned: GSS_S_BAD_MECH. a major status code to be returned: GSS_S_BAD_MECH.
2. Conventions Used in This Document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Negotiation Protocol 3. Negotiation Protocol
When the established mechanism context provides for integrity When the established mechanism context provides integrity protection,
protection, the mechanism negotiation can be protected. When the mechanism negotiation can be protected. When acquiring
acquiring negotiated security mechanism tokens, per-message integrity negotiated security mechanism tokens, per-message integrity services
services are always requested by the SPNEGO mechanism. are always requested by the SPNEGO mechanism.
When the established mechanism context supports per-message integrity When the established mechanism context supports per-message integrity
services, SPNEGO guarantees that the selected mechanism is mutually services, SPNEGO guarantees that the selected mechanism is mutually
preferred. preferred.
This section describes the negotiation process of this protocol. This section describes the negotiation process of this protocol.
3.1 Negotiation Description 3.1 Negotiation Description
The first negotiation token sent by the initiator contains an ordered The first negotiation token sent by the initiator contains an ordered
list of mechanisms (in preference order, favorite choice first), and list of mechanisms (in decreasing preference order, favorite
optionally the initial security token for the preferred mechanism of mechanism first), and optionally the initial mechanism token for the
the initiator (i.e., the first in the list). The list of security preferred mechanism of the initiator (i.e., the first in the list).
mechanisms available for negotiation is based on the credentials The list of security mechanisms available for negotiation is based on
being used. the credentials being used.
The target then processes the token from the initiator. This will 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): result in one of four possible states (as defined in Section 4.2.2):
accept_completed, accept_incomplete, reject, or request_mic. A accept_completed, accept_incomplete, reject, or request_mic. A
reject state will terminate the negotiation; an accept_completed reject state will terminate the negotiation; an accept_completed
state indicates that not only was the initiator-selected mechanism state indicates that not only was the initiator-selected mechanism
acceptable to the target, but that the initial token was sufficient acceptable to the target, but also that the initial mechanism token
to complete the authentication; an accept_incomplete state indicates was sufficient to complete the authentication; an accept_incomplete
that further message exchange is needed but the MIC token exchange as state indicates that further message exchange is needed but the MIC
described in Section 5 is OPITONAL; a request_mic state (this state token exchange as described in Section 5 is OPTIONAL; a request_mic
can only be present in the first reply message from the target) state (this state can only be present in the first reply message from
indicates the MIC token exchange is REQUIRED if per-message integrity the target) indicates the MIC token exchange is REQUIRED if
services are available. per-message integrity services are available.
Unless the preference order is specified by the application (see Unless the preference order is specified by the application (see
Appendix A), the policy by which the target chooses a mechanism is an Appendix A), the policy by which the target chooses a mechanism is an
implementation-specific local matter. In the absence of application implementation-specific local matter. In the absence of an
specified preference order or other policy, the target SHALL choose application specified preference order or other policy, the target
the first mechanism in the initiator proposed list for which it has SHALL choose the first mechanism in the initiator proposed list for
valid credentials. which it has valid credentials.
In case of a successful negotiation, the security mechanism in the In case of a successful negotiation, the security mechanism in the
first reply message represents the value suitable for the target, and first reply message represents the value suitable for the target,
picked up from the list offered by the initiator. A context level picked up from the list offered by the initiator. A context level
token for a reject state is OPTIONAL. token for a reject state is OPTIONAL.
Once a mechanism has been selected, the tokens specific to the Once a mechanism has been selected, the tokens specific to the
selected mechanism are carried within the negotiation tokens. selected mechanism are carried within the negotiation tokens.
Lastly, MIC tokens MAY be exchanged to ensure the authenticity of the Lastly, MIC tokens MAY be exchanged to ensure the authenticity of the
mechanism list as seen by the target. mechanism list received by the target.
To avoid conflicts with the use of MIC tokens by SPNEGO, To avoid conflicts with the use of MIC tokens by SPNEGO,
partially-established contexts are not used for per-message calls: partially-established contexts are not used for per-message calls:
the prot_ready_state [RFC2743] will be false even if the underlying the prot_ready_state [RFC2743] will be false even if the underlying
mechanism would return true natively. mechanism would return true natively.
3.2 Negotiation Procedure 3.2 Negotiation Procedure
The basic form of the procedure assumes that per-message integrity The basic form of the procedure assumes that per-message integrity
services are available on the established mechanism context, and it services are available on the established mechanism context, and it
is summarized as follows: is summarized as follows:
(a) The GSS-API initiator invokes GSS_Init_sec_context() as normal, (a) The GSS-API initiator invokes GSS_Init_sec_context() as normal,
but requests (either explicitly, with the negotiation mechanism, but requests that SPNEGO be used. SPNEGO can either be explicity
or through accepting a default, when the default is this requested or accepted as the default mechanism.
negotiation mechanism) that SPNEGO is used.
(b) The initiator GSS-API implementation emits a negotiation token (b) The initiator GSS-API implementation emits a negotiation token
containing a list of supported security mechanisms (possible just containing a list of one or more security mechanisms that are
one mechanism) for the credentials used for this context available based on the credentials used for this context
establishment, and optionally an initial security token for the establishment, and optionally the initial mechanism token for the
first mechanism from that list. first mechanism in the list.
(c) The GSS-API initiator application sends the token to the target (c) The GSS-API initiator application sends the token to the target
application. The GSS-API target application deposits the token application. The GSS-API target application deposits the token by
through invoking GSS_Accept_sec_context(). The acceptor will do invoking GSS_Accept_sec_context(). The acceptor will do one of
one of the following: the following:
(I) No proposed mechanism is acceptable, the negotiation SHALL be (I) If none of the proposed mechanisms are acceptable, the
terminated. GSS_Accept_sec_context indicates GSS_S_BAD_MECH. negotiation SHALL be terminated. GSS_Accept_sec_context
The acceptor MAY output a negotiation token containing a reject indicates GSS_S_BAD_MECH. The acceptor MAY output a
state. negotiation token containing a reject state.
(II) If either the initiator's preferred mechanism is not accepted (II) If either the initiator's preferred mechanism is not accepted
by the target, or this mechanism is accepted but it is not the by the target or this mechanism is accepted but it is not the
most preferred mechanism available for the acceptor (see acceptor's most preferred mechanism (see Section 3.1 and
Section 3.1 and Section 5), GSS_Accept_sec_context() indicates Section 5), GSS_Accept_sec_context() indicates
GSS_S_CONTINUE_NEEDED. The acceptor MUST output a negotiation GSS_S_CONTINUE_NEEDED. The acceptor MUST output a negotiation
token containing a request_mic state. token containing a request_mic state.
(III) Otherwise, GSS_Accept_sec_conext() indicates GSS_S_COMPLETE (III) Otherwise, GSS_Accept_sec_conext() indicates GSS_S_COMPLETE
or GSS_S_CONTINUE_NEEDED, depending on if at least one or GSS_S_CONTINUE_NEEDED depending on if at least one
additional negotiation token from the initiator is needed to additional negotiation token from the initiator is needed to
establish this context. The acceptor outputs a negotiation establish this context. The acceptor outputs a negotiation
token containing an accept_complete or accept_incomplete state, token containing an accept_complete or accept_incomplete state,
respectively. respectively.
If the initiator's preferred mechanism is accepted, and an If the initiator's preferred mechanism is accepted, and an
optimistic mechanism token was included, this mechanism token MUST optimistic mechanism token was included, this mechanism token MUST
be deposited to the selected mechanism through invoking be deposited to the selected mechanism by invoking
GSS_Accept_sec_context() and if a response mechanism token is GSS_Accept_sec_context() and if a response mechanism token is
emitted, it MUST be included in the response negotiation token. emitted, it MUST be included in the response negotiation token.
Otherwise, the target will not emit a response mechanism token in Otherwise, the target will not emit a response mechanism token in
the first reply. the first reply.
(d) The GSS-API target application returns the negotiation token to (d) The GSS-API target application returns the negotiation token to
the initiator application. The GSS-API initiator application the initiator application. The GSS-API initiator application
deposits the token through invoking GSS_Init_sec_context(). The deposits the token by invoking GSS_Init_sec_context(). The
security context initialization is then continued according to the security context initialization is then continued according to the
standard GSS-API conventions for the selected mechanism, where the standard GSS-API conventions for the selected mechanism, where the
tokens of the selected mechanism are encapsulated until the tokens of the selected mechanism are encapsulated until the
GSS_S_COMPLETE is returned for both the initiator and the target GSS_S_COMPLETE is returned for both the initiator and the target
by the selected security mechanism. by the selected security mechanism.
(e) MIC tokens are then either skipped or exchanged according to (e) MIC tokens are then either skipped or exchanged according to
Section 5. Section 5.
Note that the *_req_flag input parameters for context establishment Note that the *_req_flag input parameters for context establishment
are relative to the selected mechanism, as are the *_state output are relative to the selected mechanism, as are the *_state output
parameters. i.e., these parameters are not applicable to the parameters. i.e., these parameters are not applicable to the
negotiation process per se. negotiation process per se.
On receipt of a negotiation token on the target side, a GSS-API On receipt of a negotiation token on the target side, a GSS-API
implementation that does not support negotiation would indicate the implementation that does not support negotiation would indicate the
GSS_S_BAD_MECH status as if a particular basic security mechanism had GSS_S_BAD_MECH status as if a particular basic security mechanism had
been requested but was not supported. been requested and was not supported.
When GSS_Acquire_cred is invoked with this SPNEGO mechanism as When GSS_Acquire_cred is invoked with this negotiation mechanism in
desired_mechs, an implementation-specific default credential is used the desired_mechs, an implementation-specific default credential is
to carry on the negotiation. A set of mechanisms as specified used to carry on the negotiation. A set of mechanisms as specified
locally by the system administrator is then available for locally by the system administrator is then available for
negotiation. If there is a desire for the caller to make its own 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). choice, then an additional API has to be used (see Appendix A).
4. Token Definitions 4. Token Definitions
The type definitions in this section assume an ASN.1 module The type definitions in this section assume an ASN.1 module
definition of the following form: definition of the following form:
SPNEGOASNOneSpec { SPNEGOASNOneSpec {
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4.2 Negotiation Tokens 4.2 Negotiation Tokens
The syntax of the initial negotiation tokens follows the The syntax of the initial negotiation tokens follows the
initialContextToken syntax defined in Section 3.1 of [RFC2743]. The initialContextToken syntax defined in Section 3.1 of [RFC2743]. The
SPNEGO pseudo mechanism is identified by the Object Identifier SPNEGO pseudo mechanism is identified by the Object Identifier
specified in Section 1. Subsequent tokens are not encapsulated in specified in Section 1. Subsequent tokens are not encapsulated in
this GSS-API generic token framing. this GSS-API generic token framing.
This section specifies the syntax of the inner token for the initial This section specifies the syntax of the inner token for the initial
message, and the syntax of subsequent context establishment tokens. message and the syntax of subsequent context establishment tokens.
NegotiationToken ::= CHOICE { NegotiationToken ::= CHOICE {
negTokenInit [0] NegTokenInit, negTokenInit [0] NegTokenInit,
negTokenResp [1] negTokenResp negTokenResp [1] negTokenResp
} }
4.2.1 negTokenInit 4.2.1 negTokenInit
NegTokenInit ::= SEQUENCE { NegTokenInit ::= SEQUENCE {
mechTypes [0] MechTypeList, mechTypes [0] MechTypeList,
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confFlag (5), confFlag (5),
integFlag (6) integFlag (6)
} }
This is the syntax for the inner token of the initial negotiation This is the syntax for the inner token of the initial negotiation
message. message.
mechTypes mechTypes
This field contains one or more security mechanisms available This field contains one or more security mechanisms available
for the initiator in preference order (favorite choice first). for the initiator in decreasing preference order (favorite
choice first).
reqFlags reqFlags
This field, if present, contains the service options that are This field, if present, contains the service options that are
requested to establish the context. The context flags SHOULD requested to establish the context. The context flags SHOULD
be filled in from the req_flags parameter of be filled in from the req_flags parameter of
GSS_Init_sec_context(). This field SHALL NOT have impact on GSS_Init_sec_context(). This field SHALL NOT have impact on
the negotiation. the negotiation.
mechToken mechToken
This field, is present, contains the optimistic security This field, if present, contains the optimistic mechanism
mechanism token. token.
mechlistMIC mechlistMIC
This field, is present, contains a MIC token, which is computed This field, if present, contains a MIC token for the mechanism
according to Section 5, for the mechanism list in the initial list in the initial negotiation message. This MIC token is
negotiation message. computed according to Section 5.
4.2.2 negTokenResp 4.2.2 negTokenResp
NegTokenResp ::= SEQUENCE { NegTokenResp ::= SEQUENCE {
negResult [0] ENUMERATED { negResult [0] ENUMERATED {
accept_completed (0), accept_completed (0),
accept_incomplete (1), accept_incomplete (1),
reject (2), reject (2),
request_mic (3) request_mic (3)
} OPTIONAL, } OPTIONAL,
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integrity services are available on the mechanism context to integrity services are available on the mechanism context to
be established. This value SHALL only be present in the be established. This value SHALL only be present in the
first reply from the target. first reply from the target.
This field is REQUIRED in the first reply from the target, and This field is REQUIRED in the first reply from the target, and
it is OPTIONAL thereafter. it is OPTIONAL thereafter.
supportedMech supportedMech
This field SHALL only be present in the first reply from the This field SHALL only be present in the first reply from the
target. It is a choice from the mechanism(s) offered by the target. It MUST be one of the mechanism(s) offered by the
initiator. initiator.
ResponseToken ResponseToken
The field, if present, contains tokens specific to the This field, if present, contains tokens specific to the
mechanism selected. mechanism selected.
mechlistMIC mechlistMIC
This field, is present, contains a MIC token, which is computed This field, if present, contains a MIC token for the mechanism
according to Section 5, for the mechanism list in the initial list in the initial negotiation message. This MIC token is
negotiation message. computed according to Section 5.
5. Processing of mechListMIC 5. Processing of mechListMIC
If the mechanism selected by the negotiation does not support If the mechanism selected by the negotiation does not support
integrity protection, then no mechlistMIC token is used. Otherwise integrity protection, then no mechlistMIC token is used.
if the initiator's preferred mechanism is accepted and it is also the
most preferred mechanism available for the acceptor (there is no Otherwise if the accepted mechanism is the most preferred mechanism
mechanism which, had it been present in the mechanism list, the of both the initiator and the acceptor, then the MIC token exchange,
acceptor would have preferred over the accepted mechanism), then the as described later in this section, is OPTIONAL. A mechanism is the
MIC token exchange, as described later in this section, is OPTIONAL. 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 In all other cases, MIC tokens MUST be exchanged after the mechanism
context is fully established. context is fully established.
It is assumed that per-message integrity services are available on It is assumed that per-message integrity services are available on
the established mechanism context in the following procedure for the established mechanism context in the following procedure for
processing MIC tokens of the initiator's mechanism list. processing MIC tokens of the initiator's mechanism list.
a) The mechlistMIC token (or simply the MIC token) is computed a) The mechlistMIC token (or simply the MIC token) is computed by
through invoking GSS_GetMIC(): the input context_handle is the invoking GSS_GetMIC(): the input context_handle is the established
established mechanism context, the input qop_req is 0, and the mechanism context, the input qop_req is 0, and the input message
input message is the mechTypes field in the initial negotiation is the mechTypes field in the initial negotiation message (only
message (only the DER encoding of type MechTypeList is included). the DER encoding of the type MechTypeList is included).
b) If the selected mechanism uses an even number of mechanism tokens b) If the selected mechanism uses an even number of mechanism tokens
(namely the acceptor sends the last mechanism token), the acceptor (namely the acceptor sends the last mechanism token), the acceptor
does the following when emitting the negotiation message does the following when emitting the negotiation message
containing the last mechanism token: if the MIC token exchange is containing the last mechanism token: if the MIC token exchange is
not required, GSS_Accept_sec_context() either indicates not required, GSS_Accept_sec_context() either indicates
GSS_S_COMPLETE and does not include a mechlistMIC token, or GSS_S_COMPLETE and does not include a mechlistMIC token, or
indicates GSS_S_CONTINUE_NEEDED and includes a mechlistMIC token indicates GSS_S_CONTINUE_NEEDED and includes a mechlistMIC token
and an accept_incomplete state; if the MIC token exchange is and an accept_incomplete state; if the MIC token exchange is
required, GSS_Accept_sec_context() indicates required, GSS_Accept_sec_context() indicates
GSS_S_CONTINUE_NEEDED, and includes a mechlistMIC token. GSS_S_CONTINUE_NEEDED, and includes a mechlistMIC token.
Acceptors who wish to be compatible with legacy Windows SPNEGO Acceptors that wish to be compatible with legacy Windows SPNEGO
implementations as described in Appendix B shall not generate a implementations as described in Appendix B shall not generate a
mechlistMIC token when the MIC token exchange is not required. mechlistMIC token when the MIC token exchange is not required.
The initiator then processes the last mechanism token, and does The initiator then processes the last mechanism token, and does
one of the following: one of the following:
(I) If a mechlistMIC token was included, and is correctly (I) If a mechlistMIC token was included, and is correctly
verified, GSS_Init_sec_context() indicates GSS_S_COMPLETE. The verified, GSS_Init_sec_context() indicates GSS_S_COMPLETE. The
output negotiation message contains a mechlistMIC token, and an output negotiation message contains a mechlistMIC token, and an
accept_complete state. The acceptor MUST then verify this accept_complete state. The acceptor MUST then verify this
mechlistMIC token. mechlistMIC token.
skipping to change at page 14, line 23 skipping to change at page 14, line 27
c) In the case that the chosen mechanism uses an odd number of c) In the case that the chosen mechanism uses an odd number of
mechanism tokens (namely the initiator sends the last mechanism mechanism tokens (namely the initiator sends the last mechanism
token), the initiator does the following when emitting the token), the initiator does the following when emitting the
negotiation message containing the last mechanism token: if the negotiation message containing the last mechanism token: if the
negResult state was request_mic in the first reply from the negResult state was request_mic in the first reply from the
target, a mechlistMIC token MUST be included, otherwise the target, a mechlistMIC token MUST be included, otherwise the
mechlistMIC token is OPTIONAL. In the case that the optimistic mechlistMIC token is OPTIONAL. In the case that the optimistic
mechanism token is the only mechanism token for the initiator's mechanism token is the only mechanism token for the initiator's
preferred mechanism, the mechlistMIC token is OPTIONAL. preferred mechanism, the mechlistMIC token is OPTIONAL.
GSS_Init_sec_context() indicates GSS_S_CONTINUE_NEEDED. GSS_Init_sec_context() indicates GSS_S_CONTINUE_NEEDED.
Initiators who wish to be compatible with legacy Windows SPNEGO Initiators that wish to be compatible with legacy Windows SPNEGO
implementations as described in Appendix B shall not generate a implementations as described in Appendix B shall not generate a
mechlistMIC token when the MIC token exchange is not required. mechlistMIC token when the MIC token exchange is not required.
The acceptor then processes the last mechanism token, and does one The acceptor then processes the last mechanism token and does one
of the following: of the following:
(I) If a mechlistMIC token was included, and is correctly (I) If a mechlistMIC token was included and is correctly verified,
verified, GSS_Accept_sec_context() indicates GSS_S_COMPLETE. GSS_Accept_sec_context() indicates GSS_S_COMPLETE. The output
The output negotiation message contains a mechlistMIC token, negotiation message contains a mechlistMIC token and an
and an accept_complete state. The initiator MUST then verify accept_complete state. The initiator MUST then verify this
this mechlistMIC token. mechlistMIC token.
(II) If a mechlistMIC token was included but is incorrect, the (II) If a mechlistMIC token was included but is incorrect, the
negotiation SHALL be terminated. GSS_Accept_sec_context() negotiation SHALL be terminated. GSS_Accept_sec_context()
indicates GSS_S_DEFECTIVE_TOKEN. indicates GSS_S_DEFECTIVE_TOKEN.
(III) If no mechlistMIC token was included and the mechlistMIC (III) If no mechlistMIC token was included but the mechlistMIC
token exchange is not required, GSS_Accept_sec_context() token exchange is not required, GSS_Accept_sec_context()
indicates GSS_S_COMPLETE. The output negotiation message indicates GSS_S_COMPLETE. The output negotiation message
contains an accept_complete state. contains an accept_complete state.
(IV) In the case that the optimistic mechanism token is also the (IV) In the case that the optimistic mechanism token is also the
last mechanism token (when the initiator's preferred mechanism last mechanism token (when the initiator's preferred mechanism
is accepted by the target), and the target sends a request_mic is accepted by the target) and the target sends a request_mic
state, but the initiator did not send a mechlistMIC token, the state but the initiator did not send a mechlistMIC token, the
target then MUST include a mechlistMIC token in that first target then MUST include a mechlistMIC token in that first
reply. GSS_Accept_sec_context() indicates reply. GSS_Accept_sec_context() indicates
GSS_S_CONTINUE_NEEDED. The initiator MUST verify the received GSS_S_CONTINUE_NEEDED. The initiator MUST verify the received
mechlistMIC token, and generate a mechlistMIC token to send mechlistMIC token and generate a mechlistMIC token to send back
back to the target, who SHALL in turn verify the returned to the target. The target SHALL in turn verify the returned
mechlistMIC token and complete the negotiation. mechlistMIC token and complete the negotiation.
(V) If no mechlistMIC token was included and the acceptor sent a (V) If no mechlistMIC token was included and the acceptor sent a
request_mic state in the first reply message (the exchange of request_mic state in the first reply message (the exchange of
MIC tokens is required), the negotiation SHALL be terminated. MIC tokens is required), the negotiation SHALL be terminated.
GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN. GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN.
6. Extensibility 6. Extensibility
Two mechanisms are provided by extensibility. First, the ASN.1 Two mechanisms are provided for extensibility. First, the ASN.1
structures in this specification MAY be expanded by IETF standards structures in this specification MAY be expanded by IETF standards
action. Implementations receiving unknown fields MUST ignore these action. Implementations receiving unknown fields MUST ignore these
fields. fields.
Secondly, OIDs corresponding to a desired mechanism attribute may be Secondly, OIDs corresponding to a desired mechanism attribute may be
included in the set of preferred mechanisms by an initiator. The included in the set of preferred mechanisms by an initiator. The
acceptor can choose to honor this request by preferring mechanisms acceptor can choose to honor this request by preferring mechanisms
that have that attribute. Future work within the Kitten working that have the included attributes. Future work within the Kitten
group is expected to standardize common attributes that SPNEGO working group is expected to standardize common attributes that
mechanisms may wish to support. At this time it is sufficient to say SPNEGO mechanisms may wish to support. At this time it is sufficient
that initiators MAY include OIDs that do not correspond to mechanisms to say that initiators MAY include OIDs that do not correspond to
but instead correspond to desired mechanism attributes in their mechanisms but instead correspond to desired mechanism attributes in
requests. Such OIDs MAY influence the acceptor's choice of their requests. Such OIDs MAY influence the acceptor's choice of
mechanism. As discussed in Section 5, if there are mechanisms that mechanism. As discussed in Section 5, if there are mechanisms that
if present in the initiator's list of mechanisms might be preferred if present in the initiator's list of mechanisms might be preferred
by the acceptor to the initiator's preferred mechanism, the acceptor by the acceptor to the initiator's preferred mechanism, the acceptor
MUST demand the MIC token exchange. As a consequence, acceptors MUST MUST demand the MIC token exchange. As a consequence, acceptors MUST
demand the MIC token exchange if they support negotiation of demand the MIC token exchange if they support negotiation of
attributes not available in the initiator's preferred mechanism attributes not available in the initiator's preferred mechanism
regardless of whether the initiator actually requested these regardless of whether the initiator actually requested these
attributes. attributes.
7. Security Considerations 7. Security Considerations
In order to produce the MIC token for the mechanism list, the In order to produce the MIC token for the mechanism list, the
mechanism must provide integrity protection. When the selected mechanism must provide integrity protection. When the selected
mechanism does not support integrity protection, then the negotiation mechanism does not support integrity protection, the negotiation is
is vulnerable: an active attacker can force it to use a security vulnerable: an active attacker can force it to use a security
mechanism that is not mutually preferred but is acceptable anyway to mechanism that is not mutually preferred but is acceptable to the
the target. target.
When per-message integrity services are available on the established This protocol provides the following guarantees when per-message
mechanism context, and there was an alteration of the mechanism list integrity services are available on the established mechanism context
by an adversary such that a common mechanism that is not mutually and the mechanism list was altered by an adversary such that a
preferred could be selected, this protocol provides the following mechanism which is not mutually preferred could be selected:
guarantees: if the last mechanism token is sent by the initiator,
both peers shall fail; if the last mechanism token is sent by the o if the last mechanism token is sent by the initiator, both peers
acceptor, the acceptor shall not complete and the initiator at worst shall fail;
shall complete with its preferred mechanism being selected. The o if the last mechanism token is sent by the acceptor, the acceptor
negotiation may not be terminated if an alteration was made but it shall not complete and the initiator at worst shall complete with
had no material impact. 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 The protection of the negotiation depends on the strength of the
integrity protection. In particular, the strength of SPNEGO is no integrity protection. In particular, the strength of SPNEGO is no
stronger than the integrity protection of the weakest mechanism stronger than the integrity protection of the weakest mechanism
acceptable to GSS-API peers. acceptable to GSS-API peers.
In all cases, the communicating peers are exposed to the denial of In all cases, the communicating peers are exposed to the denial of
service threat. service threat.
8. IANA Considerations 8. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
9. Acknowledgments 9. Acknowledgments
The authors wish to thank Sam Hartman, Nicolas Williams, Ken Raeburn, The authors wish to thank Sam Hartman, Nicolas Williams, Ken Raeburn,
Jeff Altman, Tom Yu, Cristian Ilac and Martin Rex for their comments Jeff Altman, Tom Yu, Cristian Ilac and Martin Rex for their comments
and suggestions on earlier versions of this document. and suggestions during development of this document.
Luke Howard provided a prototype of this protocol in Heimdal, and Luke Howard provided a prototype of this protocol in Heimdal and
resolved several issues in the initial draft. resolved several issues in the initial draft.
Eric Baize and Denis Pinkas wrote the original SPNEGO specification Eric Baize and Denis Pinkas wrote the original SPNEGO specification
[RFC2478], of which some of the text has been retained in this [RFC2478] of which some of the text has been retained in this
document. document.
10 Normative References 10 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2478] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API [RFC2478] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998. Negotiation Mechanism", RFC 2478, December 1998.
[RFC2743] Linn, J., "Generic Security Service Application Program [RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000. Interface Version 2, Update 1", RFC 2743, January 2000.
[X690] ASN.1 encoding rules: Specification of Basic Encoding Rules [X690] ASN.1 encoding rules: Specification of Basic Encoding
(BER), Canonical Encoding Rules (CER) and Distinguished Rules (BER), Canonical Encoding Rules (CER) and
Encoding Rules (DER), ITU-T Recommendation X.690 (1997) | Distinguished Encoding Rules (DER), ITU-T Recommendation
ISO/IEC International Standard 8825-1:1998. X.690 (1997) | ISO/IEC International Standard 8825-1:1998.
Authors' Addresses Authors' Addresses
Larry Zhu Larry Zhu
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
US US
EMail: lzhu@microsoft.com EMail: lzhu@microsoft.com
skipping to change at page 23, line 8 skipping to change at page 23, line 8
available credentials). available credentials).
Note: The GSS_Indicate_mechs() function indicates the full set of Note: The GSS_Indicate_mechs() function indicates the full set of
mechanism types available on the local system. Since this call has mechanism types available on the local system. Since this call has
no input parameter, the returned set is not necessarily available for no input parameter, the returned set is not necessarily available for
all credentials. all credentials.
Appendix B. Changes since RFC2478 Appendix B. Changes since RFC2478
SPNEGO implementations in Windows 2000/Windows XP/Windows Server SPNEGO implementations in Windows 2000/Windows XP/Windows Server
2003 have the following behavior: no mechlistMIC is produced, and 2003 have the following behavior: no mechlistMIC is produced and
mechlistMIC is not processed if one is provided; if the initiator mechlistMIC is not processed if one is provided; if the initiator
sends the last mechanism token, the acceptor will send back a sends the last mechanism token, the acceptor will send back a
negotiation token with an accept_complete state and no mechlistMIC 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 token. In addition, the OID (1.2.840.48018.1.2.2) can be used to
identify the GSS-API Kerberos Version 5 mechanism. identify the GSS-API Kerberos Version 5 mechanism.
The following changes have been made to be compatible with these The following changes have been made to be compatible with these
legacy implementations. legacy implementations.
* NegTokenTarg is changed to negTokenResp and it is the message * NegTokenTarg is changed to negTokenResp and it is the message
format for all subsequent negotiation tokens. format for all subsequent negotiation tokens.
* NegTokenInit is the message for the initial token and that * NegTokenInit is the message for the initial negotiation message
token only. and that message only.
* mechTypes in negTokenInit is not optional. * mechTypes in negTokenInit is not optional.
* Two MIC tokens are exchanged, one in each direction. * Two MIC tokens are exchanged, one in each direction.
* If the selected mechanism is also the most preferred mechanism * If the selected mechanism is also the most preferred mechanism
for both peers, it is safe to omit the MIC tokens. for both peers, it is safe to omit the MIC tokens.
If at least one of the two peers implements the pseudo mechanism If at least one of the two peers implements the pseudo mechanism
in this document, the negotiation is protected. in this document, the negotiation is protected.
The following changes are to address the problems in RFC 2478. The following changes are to address the problems in RFC 2478.
* reqFlags is not protected therefore it should not impact the * reqFlags is not protected therefore it should not impact the
negotiation. negotiation.
* DER encoding is required. * DER encoding is required.
* GSS_GetMIC() input is clarified. * GSS_GetMIC() input is clarified.
* Per-message integrity services are requested for the negotiated * Per-message integrity services are requested for the negotiated
mechanism. 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
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.
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
 End of changes. 

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