Request for Comments: 4178 P. Leach
Obsoletes: 2478 K. Jaganathan
Category: Standards Track Microsoft Corporation
W. Ingersoll
Sun Microsystems
October 2005
The Simple and Protected
Generic Security Service Application Program Interface (GSS-API)
Negotiation Mechanism
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document specifies a negotiation mechanism for the Generic
Security Service Application Program Interface (GSS-API), which is
described in RFC 2743. GSS-API peers can use this negotiation
mechanism to choose from a common set of security mechanisms. If
per-message integrity services are available on the established
mechanism context, then the negotiation is protected against an
attacker that forces the selection of a mechanism not desired by the
peers.
This mechanism replaces RFC 2478 in order to fix defects in that
specification and to describe how to inter-operate with
implementations of that specification that are commonly deployed on
the Internet.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
3. Negotiation Protocol ............................................3
3.1. Negotiation Description ....................................4
3.2. Negotiation Procedure ......................................5
4. Token Definitions ...............................................7
4.1. Mechanism Types ............................................7
4.2. Negotiation Tokens .........................................7
4.2.1. negTokenInit ........................................8
4.2.2. negTokenResp ........................................9
5. Processing of mechListMIC ......................................10
6. Extensibility ..................................................13
7. Security Considerations ........................................13
8. Acknowledgments ................................................14
9. References .....................................................14
9.1. Normative References ......................................14
9.2. Informative References ....................................15
Appendix A. SPNEGO ASN.1 Module ..................................16
Appendix B. GSS-API Negotiation Support API ......................17
B.1. GSS_Set_neg_mechs Call ...................................17
B.2. GSS_Get_neg_mechs Call ...................................18
Appendix C. Changes since RFC 2478 ...............................18
Appendix D. mechListMIC Computation Example ......................20
1. Introduction
The GSS-API [RFC2743] provides a generic interface that 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 that enables GSS-API
peers to determine in-band whether their credentials support a common
set of one or more GSS-API security mechanisms; if so, it invokes the
normal security context establishment for a selected common security
mechanism. This is most useful for applications that depend on GSS-
API implementations and share multiple mechanisms between the peers.
The SPNEGO mechanism negotiation is based on the following 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 of the available
mechanisms from the offered list; if neither is acceptable, the
acceptor 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 [RFC2743] are available on the
established mechanism security context, then the negotiation is
protected to ensure that the mechanism list has not been modified.
In cases where an attacker could have materially influenced the
negotiation, peers exchange message integrity code (MIC) tokens to
confirm that the mechanism list has not been modified. If no action
of an attacker could have materially modified the outcome of the
negotiation, the exchange of MIC tokens is optional (see Section 5).
Allowing MIC tokens to be optional in this case provides
interoperability with existing implementations while still protecting
the negotiation. This interoperability comes at the cost of
increased complexity.
SPNEGO relies on 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.
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].
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). (Note that
the list MUST NOT contain this SPNEGO mechanism itself or any
mechanism for which the client does not have appropriate
credentials.)
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)
being returned in the reply message: accept-completed, accept-
incomplete, reject, or request-mic. A reject state will terminate
the negotiation; an accept-completed state indicates that the
initiator-selected mechanism was acceptable to the target, and that
the security mechanism token embedded in the first negotiation
message 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 that the MIC token exchange is
REQUIRED if per-message integrity services are available.
Unless the preference order is specified by the application, 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 that
was chosen from the list offered by the initiator.
In case of an unsuccessful negotiation, the reject state is returned,
and the generation of a context-level negotiation token is OPTIONAL.
Once a mechanism has been selected, context establishment tokens
specific to the 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 MUST NOT be used for per-message calls. To
guarantee this, the prot_ready_state [RFC2743] MUST be set to false
on return from GSS_Init_sec_context() and GSS_Accept_sec_context(),
even if the underlying mechanism returned true.
Note that in order to avoid an extra round trip, the first context
establishment 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.) In addition, using the optimistic
mechanism token allows the initiator to recover from non-fatal errors
encountered when trying to produce the first mechanism token before a
mechanism can be selected. In cases where the initiator’s preferred
mechanism is not likely to be selected by the acceptor because of the
significant cost of its generation, implementations MAY omit the
optimistic mechanism token.
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 explicitly
requested or accepted as the default mechanism.
b) The initiator GSS-API implementation generates 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 on 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 passes 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 is not the
acceptor’s most preferred mechanism (i.e., the MIC token
exchange as described in Section 5 is required),
GSS_Accept_sec_context() indicates GSS_S_CONTINUE_NEEDED.
The acceptor MUST output a negotiation token containing a
request-mic state.
III) Otherwise, if at least one additional negotiation token from
the initiator is needed to establish this context,
GSS_Accept_sec_context() indicates GSS_S_CONTINUE_NEEDED and
outputs a negotiation token containing an accept-incomplete
state.
IV) Otherwise, no additional negotiation token from the initiator
is needed to establish this context, GSS_Accept_sec_context()
indicates GSS_S_COMPLETE and outputs a negotiation token
containing an accept_complete state.
If the initiator’s preferred mechanism is accepted, and an
optimistic mechanism token was included, this mechanism token MUST
be passed to the selected mechanism by invoking
GSS_Accept_sec_context(). If a response mechanism token is
returned, it MUST be included in the response negotiation token.
Otherwise, the target will not generate 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
passes 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 in negotiation messages
(see Section 4) until 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. That is, 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 though a particular basic security mechanism
had been requested and was not supported.
When a GSS-API credential is acquired for the SPNEGO mechanism, the
implementation SHOULD produce a credential element for the SPNEGO
mechanism that internally contains GSS-API credential elements for
all mechanisms for which the principal has credentials available,
except for any mechanisms that are not to be negotiated, per
implementation-, site-, or application-specific policy. See Appendix
B for interfaces for expressing application policy.
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 the 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
iso.org.dod.internet.security.mechanism.snego (1.3.6.1.5.5.2).
Subsequent tokens MUST NOT be 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,
negTokenResp [1] NegTokenResp
}
4.2.1. negTokenInit
NegTokenInit ::= SEQUENCE {
mechTypes [0] MechTypeList,
reqFlags [1] ContextFlags OPTIONAL,
-- inherited from RFC 2478 for backward compatibility,
-- RECOMMENDED to be left out
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)
} (SIZE (32))
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 req_flags parameter of
GSS_Init_sec_context()). This field is inherited from RFC 2478
and is not integrity protected. For implementations of this
specification, the initiator SHOULD omit this reqFlags field and
the acceptor MUST ignore this reqFlags field.
The size constraint on the ContextFlags ASN.1 type only applies to
the abstract type. The ASN.1 DER requires that all trailing zero
bits be truncated from the encoding of a bit string type whose
abstract definition includes named bits. Implementations should
not expect to receive exactly 32 bits in an encoding of
ContextFlags.
mechToken
This field, if present, contains the optimistic mechanism token.
mechlistMIC
This field, if present, contains an 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 {
negState [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.
negState
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 additional 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.
This field is REQUIRED in the first reply from the target, and is
OPTIONAL thereafter. When negState is absent, the actual state
should be inferred from the state of the negotiated mechanism
context.
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 an MIC token for the mechanism
list in the initial negotiation message. This MIC token is
computed according to Section 5.
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
that the acceptor would have preferred over the accepted mechanism
had it been present in the mechanism list.
In all other cases, MIC tokens MUST be exchanged after the mechanism
context is fully established.
a) The mechlistMIC token (or simply the MIC token) is computed over
the mechanism list in the initial negotiation message by invoking
GSS_GetMIC() as follows: the input context_handle is the
established mechanism context, the input qop_req is 0, and the
input message is the DER encoding of the value of type
MechTypeList, which is contained in the "mechTypes" field of the
NegTokenInit. The input message is NOT the DER encoding of the
type "[0] MechTypeList".
b) If the selected mechanism exchanges an even number of mechanism
tokens (i.e., the acceptor sends the last mechanism token), the
acceptor does the following when generating the negotiation
message containing the last mechanism token: if the MIC token
exchange is optional, 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 C, should 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.
II) If a mechlistMIC token was included but is incorrect, the
negotiation SHALL be terminated. GSS_Init_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.