draft-zhu-spnego-2478bis-00.txt [plain text]
NETWORK WORKING GROUP L. Zhu
Internet-Draft K. Jaganathan
Obsoletes: 2478 (if approved) R. Ward
Expires: April 18, 2005 Microsoft Corporation
October 18, 2004
The Simple and Protected GSS-API Negotiation Mechanism
draft-zhu-spnego-2478bis-00
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document specifies a security negotiation mechanism for the
Generic Security Service Application Program Interface (GSS-API)
which is described in RFC 2743.
This mechanism allows negotiating and choosing one security mechanism
from a common set of security mechanisms shared by GSS-API peers.
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Once the common security mechanism is identified, the security
mechanism MAY also negotiate mechanism-specific options during its
context establishment, but that will be inside the mechanism tokens,
and invisible to this protocol.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Negotiation Model . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Negotiation Description . . . . . . . . . . . . . . . . . 5
3.2 Negotiation Procedure . . . . . . . . . . . . . . . . . . 6
4. Data Elements . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1 Mechanism Type . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Negotiation Tokens . . . . . . . . . . . . . . . . . . . . 11
4.2.1 negTokenInit . . . . . . . . . . . . . . . . . . . . . 12
4.2.2 negTokenResp . . . . . . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
A. Changes since RFC2478 . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . 18
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1. Introduction
The GSS-API [RFC2743] provides a generic interface which can be
layered atop different security mechanisms such that if communicating
peers acquire GSS-API credentials for the same security mechanism,
then a security context MAY be established between them (subject to
policy). However, GSS-API doesn't prescribe the method by which
GSS-API peers can establish whether they have a common security
mechanism.
The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism
defined here is a pseudo-security mechanism, represented by the
object identifier iso.org.dod.internet.security.mechanism.snego
(1.3.6.1.5.5.2) which enables GSS-API peers to determine in-band
whether their credentials share common GSS-API security mechanism(s),
and if so, to invoke normal security context establishment for a
selected common security mechanism. This is most useful for
applications that are based on GSS-API implementations which support
multiple security mechanisms.
The simple and protected GSS-API mechanism negotiation is based on
the following negotiation model: the initiator proposes one security
mechanism or a list of security mechanisms in its preference order
(favorite choice first), the acceptor (the target) either accepts the
proposed security mechanism, or chooses one from the offered list, or
rejects the proposed value(s). The target then informs the initiator
of its choice.
In order to avoid an extra round trip, the initial security token of
the preferred mechanism for the initiator SHOULD be embedded in the
initial negotiation token (as defined in Section 4.2). If the target
preferred mechanism matches the initiator's preferred mechanism, no
additional round trips may be incurred by using the negotiation
protocol.
The negotiation is protected and all the underlying mechanisms
offered by the initiator MUST be capable of integrity protection.
The Simple and Protected GSS-API Negotiation Mechanism uses the
concepts developed in the GSS-API specification [RFC2743]. The
negotiation data is encapsulated in context-level tokens. Therefore,
callers of the GSS-API do not need to be aware of the existence of
the negotiation tokens but only of the new pseudo-security mechanism.
A failure in the negotiation phase causes a major status code to be
returned: GSS_S_BAD_MECH.
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2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Negotiation Model
3.1 Negotiation Description
Each OID represents one GSS-API mechanism or one variant of it.
The first negotiation token sent by the initiator contains an ordered
list of mechanisms (in preference order, favorite choice first), and
OPTIONALLY the initial security token for the preferred mechanism of
the initiator (i.e. the first of the list).
The target then processes the token from the initiator. This will
result in one of three possible states (as defined in Section 4.2.2):
accept_completed, accept_incomplete, or reject. A reject state will
terminate the negotiation. An accept_completed state indicates that
not only was the initiator-selected mechanism acceptable to the
target, but that the initial token was sufficient to complete the
authentication. An accept_incomplete state indicates that the target
has selected a different mechanism or the preferred mechanism is
acceptable, but this mechanism requires at least one additional
message to complete the authentication. The target MAY produce a
context level token for a reject state.
The first negotiation token sent by the acceptor contains the result
of the negotiation (accept_completed, accept_incomplete or reject)
and, in case of accept, the agreed security mechanism. It MUST also
include the response mechanism token to the initial mechanism token
from the initiator, when the first proposed mechanism of the
initiator has been selected and an initial mechanism token was
provided by the initiator. However, if the initiator's preferred
mechanism is not possible, the target will not emit a response
mechanism token in the first reply.
The policy by which the target chooses a mechanism is an
implementation-specific local matter. In the absence of other
policy, the target MUST choose the first mechanism in the list for
which valid credentials are available.
The first negotiation token is the negTokenInit message and all
subsequent negotiation tokens are the negTokenResp message, as
defined in Section 4.2.
The use of partially-established contexts (as indicated by the
prot_ready_state in [RFC2743]), either for this mechanism or
mechanisms negotiated using this mechanism, is prohibited.
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3.2 Negotiation Procedure
The negotiation procedure is summarized as follows:
(a) The GSS-API initiator invokes GSS_Init_sec_context() as normal,
but requests (either explicitly, with the negotiation mechanism,
or through accepting a default, when the default is the
negotiation mechanism) that the Simple and Protected GSS-API
Negotiation Mechanism is used;
(b) The initiator GSS-API implementation emits a negotiation token
containing a list of supported security mechanisms for the
credentials used for this context establishment, and OPTIONALLY an
initial security token for the first mechanism from that list
(i.e. the preferred mechanism), and indicates
GSS_S_CONTINUE_NEEDED status;
(c) The GSS-API initiator application sends the token to the target
application;
(d) The GSS-API target application deposits the token through
invoking GSS_Accept_sec_context. The target GSS-API application
will do one of the following:
(I) If the initiator's preferred mechanism is accepted by the
target, an initial token is included in the first token from
the initiator, no further mechanism token from the initiator is
needed for the chosen mechanism to establish the security
context, (e.g. when the authentication mechanism is unilateral
or mutual authentication has been performed and involves a
single token in either direction), and the initiator has not
sent a MIC token (the output token of the GSS_GetMIC() call
[RFC2743], the input to GSS_GetMIC() is the OTCET STRING field
representing the MechTypes in the initial NegTokenInit token),
of the mechanism list, the acceptor will do one of the
following:
1) If the initiator's preferred mechanism is accepted and there
is no policy on the target such that a different mechanism
other than the initiator's preferred mechanism could have
been selected given a different list of mechanisms,
GSS_Accept_sec_context() MUST indicate GSS_S_COMPLETE and it
MUST produce a negotiation token with the accept_completed
state, and with no MIC of the mechanism list. This is
referred in this document as the Safe to Omit MIC (SOMIC)
rule number 1. The resulting negotiation token MUST include
the security token if one is returned by the selected
mechanism;
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2) If the initiator's preferred mechanism is accepted and there
is policy exists on the target such that a different
mechanism other than the initiator's preferred mechanism
could have been selected given a different list of
mechanisms, GSS_Accept_sec_context() MUST indicate
GSS_S_CONTINUE_NEEDED with the accept_incomplete state, and
a MIC MUST be generated by the target. This MIC is to be
verified by the initiator and the result will be sent back
to the acceptor. This is referred in this document as the
Safe to Omit MIC (SOMIC) rule number 2. The resulting
negotiation token MUST include the security token if one is
returned by the selected mechanism.
3) If there is a MIC token and it is correct,
GSS_Accept_sec_context() MUST indicate GSS_S_COMPLETE with
no output token; If there is an incorrect MIC token,
GSS_Accept_sec_context() must indicate GSS_S_BAD_MIC status,
OPTIONALLY returning a negotiation token with the reject
state.
(II) If the initiator's preferred mechanism is accepted, and an
initial token from this mechanism is sent by the initiator, but
a failure is returned by the chosen mechanism,
GSS_Accept_sec_context() MUST report the failure and the
mech_type output parameter indicates the selected mechanism.
The target MUST produce a negotiation token with the reject
state if the selected mechanism returns a response token (e.g.
a KRB_ERROR when the Kerberos Version 5 GSS-API mechanism is
chosen [GSSAPICFX]);
(III) If the initiator's preferred mechanism is accepted, and an
initial token from this mechanism is sent by the initiator, but
at last one more initiator token need to be transferred to
establish the context, GSS_Accept_sec_context() MUST indicate
GSS_S_CONTINUE_NEEDED status, returning a negotiation token
with the accept_incomplete state, the response mechanism token,
and no MIC token.
(IV) If the initiator's preferred mechanism is accepted, but no
initial token from this mechanism is sent by the initiator,
GSS_Accept_sec_context() MUST indicate GSS_S_CONTINUE_NEEDED
status, returning a negotiation token with the
accept_incomplete state, the selected mechanism, no response
mechanism token or MIC token.
(V) If a proposed mechanism is accepted, and it is not the
initiator's preferred mechanism, GSS_Accept_sec_context() MUST
indicate GSS_S_CONTINUE_NEEDED status, returning a negotiation
token with the accept_incomplete state, the selected mechanism,
no response mechanism token or MIC token. The negotiation will
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be the agreed security mechanism if the negotiation is
successful.
(e) The GSS-API target application returns the negotiation token to
the initiator application;
(f) The GSS-API initiator application deposits the token through
invoking GSS_Init_sec_context(). The initiator will do one of the
following:
(I) When the negotiation token carries an accept_completed result,
the initiator MUST do one of the following:
1) If the selected mechanism is the initiator's preferred
mechanism, the initiator SHALL NOT reject the negotiation if
no MIC token is present. This is referred in this document
as the Safe to Omit MIC ("SOMIC") rule number 3. The
initiator MUST deposit the security token if one is
included, GSS_Init_sec_context() MUST indicate
GSS_S_BAD_MECH status if the context is not established
after this GSS_Init_sec_context() call. If a MIC token is
present, the initiator MUST verify it and a GSS_S_BAD_MIC
must be returned if the MIC is incorrect;
2) If the selected mechanism is not the initiator's preferred
mechanism, and there is no or an incorrect MIC token,
GSS_Init_sec_context() MUST indicate GSS_S_BAD_MIC status.
This is referred in this document as the Safe to Omit MIC
("SOMIC") rule number 4.
(II) When the negotiation token carries a reject result without a
response security token, GSS_Init_sec_context() MUST indicate
GSS_S_BAD_MECH status;
(III) When the negotiation token carries a reject result with a
response security token, the initiator MUST deposit the
security token, and GSS_Init_sec_context() MUST indicate a
failure status reported by the underlying mechanism, and the
output mech_type indicates the selected mechanism;
(IV) When the negotiation token carries an accept_incomplete
result and further mechanism tokens from the acceptor must be
transferred in order to complete context establishment,
GSS_Init_sec_context() MUST indicate GSS_S_CONTINUE_NEEDED
status, returning an output token with the accept_incomplete,
and the selected mechanism's context level token;
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(V) When the negotiation token carries an accept_incomplete
result, no further mechanism token need to be transferred from
the acceptor to complete the context establishment, the
initiator MUST do one of the following:
1) If a MIC token was included, the initiator MUST verify it
and GSS_Init_sec_context() MUST indicate GSS_S_BAD_MIC if
the MIC is incorrect; GSS_Init_sec_context() MUST indicate
GSS_S_COMPLETE and produce a negotiation with the
accept_completed state if the MIC is correct. This is
referred in this document as the Safe to Omit MIC ("SOMIC")
rule number 5;
2) If no MIC token was present, GSS_Init_sec_context() MUST
indicate GSS_S_BAD_MIC statue, This is referred in this
document as the Safe to Omit MIC ("SOMIC") rule number 6.
(g) The initiator application then sends the output_token to the
target if one is returned. 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. When no further mechanism token is
needed to be transferred and the context for the chosen mechanism
is established, the initiator and the acceptor will need to either
apply the "SOMIC" rules above and skip MIC generation and
verification, or generate and verify the MIC token to protect the
negotiation.
(h) When GSS_S_CONTINUE_NEEDED is returned, the mech_type output
parameter is not yet valid. When GSS_S_COMPLETE is returned, the
mech_type output parameter indicates the selected mechanism.
Note that the *_req_flag input parameters for context establishment
are relative to the selected mechanism, as are the *_state output
parameters. i.e., these parameters are not applicable to the
negotiation process per se.
On receipt of a negotiation token on the target side, a GSS-API
implementation that does not support negotiation would indicate the
GSS_S_BAD_MECH status as if a particular basic security mechanism had
been requested but was not supported.
When GSS_Acquire_cred is invoked with the negotiation mechanism as
desired_mechs, an implementation-specific default credential is used
to carry on the negotiation. A set of mechanisms as specified
locally by the system administrator is then available for
negotiation. If there is a desire for the caller to make its own
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choice, then an additional API has to be used as defined in [PRTSTK].
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4. Data Elements
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)
} 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 Type
MechType ::= OBJECT IDENTIFIER
-- OID represents each security mechanism as suggested by
-- [RFC2743]
4.2 Negotiation Tokens
The syntax of the initial negotiation tokens follows the
InitialContextToken syntax defined in [RFC2743]. The security
mechanism of the initial negotiation token is identified by the
Object Identifier in Section 1. All subsequent tokens are not
encapsulated in the above generic token framing.
This section specifies the syntax of initial and subsequent context
level tokens.
NegotiationToken ::= CHOICE {
negTokenInit [0] NegTokenInit,
negTokenResp [1] negTokenResp
}
MechTypeList ::= SEQUENCE OF MechType
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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 message for the initial negotiation token.
mechTypes
This field contains one or more security mechanisms in the
preference order (favorite choice first) supported by the
initiator (as indicated in the field mechTypes).
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 influence the
outcome of the negotiation.
mechToken
This field, is present, contains an optimistic negotiation
response.
mechListMIC
This field, if present, contains the result of a GSS_GetMIC()
[RFC2743] of the MechTypes field in the initial NegTokenInit
token. It allows verifying that the list initially sent by the
initiator has been received unmodified by the target.
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4.2.2 negTokenResp
NegTokenResp ::= SEQUENCE {
negResult [0] ENUMERATED {
accept_completed (0),
accept_incomplete (1),
reject (2)
},
supportedMech [1] MechType OPTIONAL,
responseToken [2] OCTET STRING OPTIONAL,
mechListMIC [3] OCTET STRING OPTIONAL,
-- used only by the acceptor
...
}
This is the message for all the subsequent tokens.
negResult
Result of the negotiation exchange, specified by the target.
This can be:
accept_completed
A security mechanism is selected, and the context is
established for the sender;
accept_incomplete
Further exchanges are necessary;
reject
The sender reject the proposed security mechanism(s).
accept_completed indicates that a context has been successfully
established, while the result accept_incomplete indicates that
additional token exchanges are needed.
For those targets that support piggybacking the initial
mechToken, an optimistic negotiation response is possible and
includes in that case a responseToken which MAY continue the
authentication exchange (e.g. when mutual authentication has
been requested or when unilateral authentication requires
several round trips). Otherwise the responseToken is used to
carry the tokens specific to the mechanism selected.
The mechListMIC, when present, is a MIC computed over the
MechTypes using the mechanism list field in the initial token
(encoded in DER).
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supportedMech
This field is present and only present in the first
negTokenResp token. It is a choice from the mechanisms offered
by the initiator.
responseToken
This field, if present, contains the security token of the
selected mechanism.
mechListMIC
This field, if present, contains the result of a GSS_GetMIC()
[RFC2743] of the MechTypes field in the initial NegTokenInit
token. It allows verifying that the list initially sent by the
initiator has been received unmodified by the target.
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5. Security Considerations
In order to produce the MIC for the mechanism list, the mechanism
MUST provide integirty protection. When one of the mechanisms
proposed by the initiator does not support integrity protection, then
the negotiation is exposed to all threats a non secured service is
exposed. In particular, an active attacker can force to use a
security mechanism which is not the common preferred one (when
multiple security mechanisms are shared between peers) but which is
acceptable anyway to the target, thus this mechanism does not support
selecting a mechanism that does not support integrity protection.
In any case, the communicating peers MAY be exposed to the denial of
service threat.
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6. Acknowledgments
The authors wish to thank Paul Leach and Todd Stecher for theirs
comments and suggestions on earlier versions of this document.
Eric Baize and Denis Pinkas wrote the original SPNEGO specification
[RFC2478], of which some of the text has been retained in this
document.
7 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.
[PRTSTK] RFC-Editor: To be replaced by RFC number for draft-williams
-gssapi-stackable-pseudo-mechs. Work in progress.
[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
Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: karthikj@microsoft.com
Richard B. Ward
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: richardw@microsoft.com
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Appendix A. Changes since RFC2478
The following changes are designed to be compatible with an
incorrect implementation of RFC 2478 shipped in Windows 2000. A
correct implementation of this protocol that negotiates the 2 leg
Kerberos GSS-API mechanism as the only available security
mechanism should be ale to interoperate with the implementation of
Windows 2000 when the mangled OID (1.2.840.48018.1.2.2) can be
used to identify Kerberos.
* The negTokenTarg is changed to negTokenResp and it is now the
format for all subsequent negotiation messages.
* negTokenInit is the message for the initial token and that
token only.
* mechTypes in negTokenInit is no longer optional.
* negResult is no longer optional in the negTokenResp token.
* The initiator does not send the MIC token.
* Add rules when it is safe to omit the MIC token. Search for
SOMIC.
The following changes are to address the problems in RFC 2478.
* reqFlags is not protected therefore it should not impact the
negotiation.
* BER encoding is required.
* GSS_GetMIC() input is clarified.
* Nico's stackable pseudo mechanism draft is used to replace the
support APIs.
* We no longer support negotiating mechanisms that do not provide
for integrity. That support does not add security values but
blows up the interoperability test matrix.
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