One document matched: draft-ietf-cat-sasl-gssapi-00.txt
Network Working Group J. Myers
Internet Draft March, 1999
Document: draft-ietf-cat-sasl-gssapi-00.txt
SASL GSSAPI mechanisms
Status of this Memo
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This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
A revised version of this draft document will be submitted to the RFC
editor as a Proposed Standard for the Internet Community. Discussion
and suggestions for improvement are requested.
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1. Abstract
The Simple Authentication and Security Layer [SASL] is a method for
adding authentication support to connection-based protocols. This
document describes the method for using the Generic Security Service
Application Program Interface [GSSAPI] in the Simple Authentication
and Security Layer [SASL].
This document amends section 7.2 of RFC 2222 [SASL], the definition
of the "GSSAPI" SASL mechanism.
2. Organization of this Document
2.1. How to Read This Document
[TODO: is this section needed?]
2.2. Conventions Used in this Document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in "Key words for
use in RFCs to Indicate Requirement Levels" [KEYWORDS].
2.3. Examples
[TODO: No examples included. Needed?]
Examples in this document are for the IMAP profile [IMAP4] of this
specification. The base64 encoding of challenges and responses, as
well as the "+ " preceding the responses are part of the IMAP4
profile, not part of the SASL specification itself.
3. Introduction and Overview
Each GSSAPI mechanism used within SASL MUST have a separate SASL
mechanism registered. The registration procedure for SASL mechanisms
is defined in the base SASL specification [SASL].
For backwards compatibility with existing implementations of Kerberos
V5 under SASL, the SASL mechanism name for the Kerberos V5 GSSAPI
mechanism is "GSSAPI". The SASL mechanism name for any other GSSAPI
mechanism is the concatenation of "GSS-" and the [TODO] of the GSSAPI
mechanism.
SASL mechanism names starting with "GSS-" are reserved for SASL
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mechanisms which conform to this document.
The specification of all SASL mechanisms conforming to this document
is in the "Specification common to all GSSAPI mechanisms" section of
this document.
The IESG is considered to be the owner of all SASL mechanisms which
conform to this document. This does NOT imply that the IESG is
considered to be the owner of the underlying GSSAPI mechanism.
4. SPNEGO
Implementations SHOULD NOT use the Simple and Protected GSS-API
Negotiation Mechanism [SPNEGO] underneath SASL.
A client which supports, for example, the Kerberos V5 GSSAPI
mechanism only underneath SPNEGO underneath the "GSS-SPNEGO" SASL
mechanism will not interoperate with a server which supports the
Kerberos V5 GSSAPI mechanism only underneath the "GSSAPI" SASL
mechanism.
If a client's policy is to first prefer GSSAPI mechanism X, then
non-GSSAPI mechanism Y, then GSSAPI mechanism Z, and if a server
supports mechanisms Y and Z but not X, then if the client attempts to
negotiate mechanism X by using the "GSS-SPNEGO" SASL mechanism, it
may end up using mechanism Z when it should have used mechanism Y.
One reason a server or client might want to violate the above SHOULD
directive is if it has a policy of only using mechanisms below a
certain strength if their negotiation is protected. In such a case,
it would only want to negotiate those weaker mechnisms through
SPNEGO. In any case, there is no down-negotiation security
consideration with using the strongest mechanism and set of options
the implementation supports, so for interoperability that mechanism
and set of options MUST be negotiable without using the "GSS-SPNEGO"
mechanism.
6. Specification common to all GSSAPI mechanisms
Each SASL mechanism which uses a GSSAPI mechanism uses the following
specification.
6.1. Client side of authentication protocol exchange
The client calls GSS_Init_sec_context, passing in
input_context_handle of 0 (initially), mech_type of the GSSAPI
mechanism for which this SASL mechanism is registered, and targ_name
equal to output_name from GSS_Import_Name called with input_name_type
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of GSS_C_NT_HOSTBASED_SERVICE and input_name_string of
"service@hostname" where "service" is the service name specified in
the protocol's profile, and "hostname" is the fully qualified host
name of the server. The client then responds with the resulting
output_token. If GSS_Init_sec_context returns GSS_S_CONTINUE_NEEDED,
then the client should expect the server to issue a token in a
subsequent challenge. The client must pass the token to another call
to GSS_Init_sec_context, repeating the actions in this paragraph.
When GSS_Init_sec_context returns GSS_S_COMPLETE, the client takes
the following actions: If the last call to GSS_Init_sec_context
returned an output_token, then the client responds with the
output_token, otherwise the client responds with no data. The client
should then expect the server to issue a token in a subsequent
challenge. The client passes this token to GSS_Unwrap and interprets
the first octet of resulting cleartext as a bit-mask specifying the
security layers supported by the server and the second through fourth
octets as the maximum size output_message to send to the server. The
client then constructs data, with the first octet containing the
bit-mask specifying the selected security layer, the second through
fourth octets containing in network byte order the maximum size
output_message the client is able to receive, and the remaining
octets containing the authorization identity. The client passes the
data to GSS_Wrap with conf_flag set to FALSE, and responds with the
generated output_message. The client can then consider the server
authenticated.
6.2. Server side of authentication protocol exchange
The server passes the initial client response to
GSS_Accept_sec_context as input_token, setting input_context_handle
to 0 (initially). If GSS_Accept_sec_context returns
GSS_S_CONTINUE_NEEDED, the server returns the generated output_token
to the client in challenge and passes the resulting response to
another call to GSS_Accept_sec_context, repeating the actions in this
paragraph.
When GSS_Accept_sec_context returns GSS_S_COMPLETE, the client takes
the following actions: If the last call to GSS_Accept_sec_context
returned an output_token, the server returns it to the client in a
challenge and expects a reply from the client with no data. Whether
or not an output_token was returned (and after receipt of any
response from the client to such an output_token), the server then
constructs 4 octets of data, with the first octet containing a bit-
mask specifying the security layers supported by the server and the
second through fourth octets containing in network byte order the
maximum size output_token the server is able to receive. The server
must then pass the plaintext to GSS_Wrap with conf_flag set to FALSE
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and issue the generated output_message to the client in a challenge.
The server must then pass the resulting response to GSS_Unwrap and
interpret the first octet of resulting cleartext as the bit-mask for
the selected security layer, the second through fourth octets as the
maximum size output_message to send to the client, and the remaining
octets as the authorization identity. The server must verify that
the src_name is authorized to authenticate as the authorization
identity. After these verifications, the authentication process is
complete.
6.3. Security layer
The security layers and their corresponding bit-masks are as follows:
1 No security layer
2 Integrity protection.
Sender calls GSS_Wrap with conf_flag set to FALSE
4 Privacy protection.
Sender calls GSS_Wrap with conf_flag set to TRUE
Other bit-masks may be defined in the future; bits which are not
understood must be negotiated off.
7. IANA Considerations
The IANA is directed to modify the existing registration for "GSSAPI"
in the "sasl-mechanisms" so that this document is listed as the
published specification. Add the descriptive text "This mechanism is
for the Kerberos V5 mechanism of GSSAPI. Other GSSAPI mechanisms use
other SASL mechanism names, as described in this mechanism's
published specification."
The IANA is advised that SASL mechanism names starting with "GSS-"
are reserved for SASL mechanisms which conform to this document.
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8. References
[IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4",
RFC 1730, University of Washington, December 1994.
[GSSAPI] Linn, J., "Generic Security Service Application Program
Interface, Version 2", RFC 2078, January 1997
[GSSAPI-KERBEROS] Linn, J., "The Kerberos Version 5 GSS-API
Mechanism", RFC 1964, June 1996
[KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997
[SASL] Myers, J., "Simple Authentication and Security Layer (SASL)",
RFC 2222, October 1997
[SPNEGO] Baize, E., Pinkas., D., "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998
9. Security Considerations
Security issues are discussed throughout this memo.
When a server or client supports multiple authentication mechanisms,
each of which has a different security strength, it is possible for
an active attacker to cause a party to use the least secure mechanism
supported. To protect against this sort of attack, a client or
server which supports mechanisms of different strengths should have a
configurable minimum strength that it will use. It is not sufficient
for this minimum strength check to only be on the server, since an
active attacker can change which mechanisms the client sees as being
supported, causing the client to send authentication credentials for
its weakest supported mechanism.
The client's selection of a SASL mechanism is done in the clear and
may be modified by an active attacker. It is important for any new
SASL mechanisms to be designed such that an active attacker cannot
obtain an authentication with weaker security properties by modifying
the SASL mechanism name and/or the challenges and responses.
SPNEGO [SPNEGO] has protection against many of these down-negotiation
attacks, SASL does not itself have such protection. The section
titled "SPNEGO" mentions considerations of choosing negotiation
through SASL versus SPNEGO.
Additional security considerations are in the SASL [SASL] and GSSAPI
[GSSAPI] specifications.
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10. Author's Address
John G. Myers
Netscape Communications
501 E. Middlefield Road
Mail Stop MV-029
Mountain View, CA 94043-4042
Email: jgmyers@netscape.com
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Table of Contents
Status of this Memo ............................................... i
1. Abstract .................................................... 2
2. Organization of this Document ............................... 2
2.1. How to Read This Document ................................... 2
2.2. Conventions Used in this Document ........................... 2
2.3. Examples .................................................... 2
3. Introduction and Overview ................................... 2
4. SPNEGO ...................................................... 3
6. Specification common to all GSSAPI mechanisms ............... 3
6.1. Client side of authentication protocol exchange ............. 3
6.2. Server side of authentication protocol exchange ............. 4
6.3. Security layer .............................................. 5
7. IANA Considerations ......................................... 5
8. References .................................................. 6
9. Security Considerations ..................................... 6
10. Author's Address ............................................ 7
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