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KITTEN WG N. Williams
Internet-Draft Sun
Expires: April 19, 2006 October 16, 2005
Clarifications and Extensions to the GSS-API for the Use of Channel
Bindings
draft-ietf-kitten-gssapi-channel-bindings-01.txt
Status of this Memo
By submitting this Internet-Draft, each 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 becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 19, 2006.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document clarifies and generalizes the GSS-API "channel
bindings" facility. This document also specifies the format of the
various types of channel bindings.
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Table of Contents
1. Conventions used in this document . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Generic Structure for GSS-API Channel Bindings . . . . . . . . 5
3.1. Proper Mechanism Use of Channel Bindings . . . . . . . . . 5
4. Channel Bindings for SSHv2 . . . . . . . . . . . . . . . . . . 6
4.1. GSS_Make_sshv2_channel_bindings() . . . . . . . . . . . . 6
4.1.1. C-Bindings . . . . . . . . . . . . . . . . . . . . . . 7
5. Channel Bindings for TLS . . . . . . . . . . . . . . . . . . . 8
5.1. GSS_Make_tls_channel_bindings() . . . . . . . . . . . . . 8
5.1.1. C-Bindings . . . . . . . . . . . . . . . . . . . . . . 9
6. Channel Bindings for IPsec . . . . . . . . . . . . . . . . . . 10
6.1. GSS_Make_ipsec_channel_bindings() . . . . . . . . . . . . 10
6.1.1. C-Bindings . . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 13
8.2. Informative . . . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. 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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2. Introduction
The concept of "channel bindings" and the abstract construction of
channel bindings for several types of channels are described in
[CHANNEL-BINDINGS]
To actually use channel bindings in GSS-API aplications additional
details are required that are given below.
First the structure given to channel bindings data in [RFC2744] is
generalized to all of the GSS-API, not just its C-Bindings.
Then the actual construction of channel bindings to SSHv2, TLS and
IPsec channels is given.
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3. Generic Structure for GSS-API Channel Bindings
The base GSS-API v2, update 1 specification [RFC2743]describes
channel bindings as an OCTET STRING and leaves it to the GSS-API v2,
update 1 C-Bindings specification to specify the structure of the
contents of the channel bindings OCTET STRINGs. The C-Bindings
specification [RFC2744]then defines, in terms of C, what should be
generic structure for channel bindings. The Kerberos V GSS mechanism
[RFC1964]then defines a method for encoding GSS channel bindings in a
way that is independent of the C-Bindings!
In other words, the structure of GSS channel bindings given in
[RFC2744] is actually generic, rather than specific to the C
programming language.
Here, then, is a generic re-statement of this structure, in pseudo-
ASN.1:
GSS-CHANNEL-BINDINGS := SEQUENCE {
initiator-address-type INTEGER,
initiator-address OCTET STRING,
acceptor-address-type INTEGER,
acceptor-address OCTET STRING,
application-data OCTET STRING,
}
The values for the address fields are described in [RFC2744].
Language-specific bindings of the GSS-API should specify a language-
specific formulation of this structure.
3.1. Proper Mechanism Use of Channel Bindings
As described in [CHANNEL-BINDINGS], GSS mechanisms should exchange
integrity protected proofs of channel bindings, where the proof is
obtained by running a strong hash of the channel bindings data
(encoded as per some mechanism-specific, such as in [RFC1964]) and a
binary value to represent the initiator->acceptor, and opposite,
direction.
The encoding of channel bindings used in [RFC1964], with the addition
of a binary value as described above, and the substitution of SHA-1
for MD5 is a reasonable, generic encoding of GSS-CHANNEL-BINDINGS
that any future GSS mechanisms can use.
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4. Channel Bindings for SSHv2
The SSHv2 channel bindings are constructed as an octet string for the
'application-data' field of the channel bindings by concatenating the
following values and in this order:
1. The ASCII string "GSS SSHv2 CB:"
2. The SSHv2 session ID
3. Any additional application-provided data, encoded as the DER
encoding of an ASN.1 OCTET STRING
4.1. GSS_Make_sshv2_channel_bindings()
Inputs:
o session_id OCTET STRING,
o additional_app_data OCTET STRING
Outputs:
o major_status INTEGER,
o minor_status INTEGER,
o channel_bindings_app_data OCTET STRING
Return major_status codes:
o GSS_S_COMPLETE indicates no error.
o GSS_S_FAILURE indicates failure to construct the channel bindings
as a result, perhaps, of a memory management, or similar failure.
This function constructs an OCTET STRING for use as the value of the
application-data field of the GSS-CHANNEL-BINDINGS structure
described above.
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4.1.1. C-Bindings
OM_uint32 gss_make_sshv2_channel_bindings(
OM_uint32 *minor_status,
const gss_buffer_t session_id,
const gss_buffer_t additional_app_data,
gss_buffer_t channel_bindings_app_data
);
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5. Channel Bindings for TLS
The TLS channel bindings are constructed as an octet string for the
'application-data' field of the channel bindings by concatenating the
following values and in this order:
1. The ASCII string "GSS TLSv1.0 CB:"
2. The TLS finished message sent by the client
3. The TLS finished message sent by the server
4. Any additional application-provided data, encoded as the DER
encoding of an ASN.1 OCTET STRING
5.1. GSS_Make_tls_channel_bindings()
Inputs:
o client_finished_msg OCTET STRING,
o server_finished_msg OCTET STRING,
o additional_app_data OCTET STRING
Outputs:
o major_status INTEGER,
o minor_status INTEGER,
o channel_bindings_app_data OCTET STRING
Return major_status codes:
o GSS_S_COMPLETE indicates no error.
o GSS_S_FAILURE indicates failure to construct the channel bindings
as a result, perhaps, of a memory management, or similar failure.
This function constructs an OCTET STRING for use as the value of the
application-data field of the GSS-CHANNEL-BINDINGS structure
described above.
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5.1.1. C-Bindings
OM_uint32 gss_make_tls_channel_bindings(
OM_uint32 *minor_status,
const gss_buffer_t client_finished_msg,
const gss_buffer_t server_finished_msg,
const gss_buffer_t additional_app_data,
gss_buffer_t channel_bindings_app_data
);
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6. Channel Bindings for IPsec
The IPsec channel bindings are constructed as an octet string for the
'application-data' field of the channel bindings by concatenating the
following values and in this order:
1. The ASCII string "GSS IPsec CB:"
2. The transform ID for encryption, as a 16-bit big-endian word
3. The transform ID for integrity protection, as 16-bit in big-
endian word
4. NOTE: The following needs to be updated to take into account
progress of BTNS.
5. The initiator ID payload as used in the key exchange protocol
used for setting up the channel's SAs
6. The responder ID payload as used in the key exchange protocol
used for setting up the channel's SAs
7. Any additional application-provided data, encoded as the DER
encoding of an ASN.1 OCTET STRING
Note that traffic selectors are not included. Inclusion of
confidentiality/integrity algorithms protects against MITMs that can
compromise weaker algorithms that policy might permit, for the same
peers, for other traffic.
6.1. GSS_Make_ipsec_channel_bindings()
Inputs:
o encr_alg INTEGER,
o integ_alg INTEGER,
o initiator_id OCTET_STRING,
o acceptor_id OCTET_STRING,
o additional_app_data OCTET STRING
Outputs:
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o major_status INTEGER,
o minor_status INTEGER,
o channel_bindings_app_data OCTET STRING
Return major_status codes:
o GSS_S_COMPLETE indicates no error.
o GSS_S_FAILURE indicates failure to construct the channel bindings
as a result, perhaps, of a memory management, or similar failure.
This function constructs an OCTET STRING for use as the value of the
application-data field of the GSS-CHANNEL-BINDINGS structure
described above.
6.1.1. C-Bindings
OM_uint32 gss_make_ipsec_channel_bindings(
OM_uint32 *minor_status,
OM_uint32 encr_alg,
OM_uint32 integ_alg,
const gss_buffer_t initiator_id,
const gss_buffer_t acceptor_id,
const gss_buffer_t additional_app_data,
gss_buffer_t channel_bindings_app_data
);
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7. Security Considerations
For general security considerations relating to channel bindings see
[CHANNEL-BINDINGS]
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8. References
8.1. Normative
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, January 2000.
8.2. Informative
[RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2025] Adams, C., "The Simple Public-Key GSS-API Mechanism
(SPKM)", RFC 2025, October 1996.
[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, September 1997.
[RFC2478] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998.
[RFC2623] Eisler, M., "NFS Version 2 and Version 3 Security Issues
and the NFS Protocol's Use of RPCSEC_GSS and Kerberos V5",
RFC 2623, June 1999.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File System
(NFS) version 4 Protocol", RFC 3530, April 2003.
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Appendix A. Acknowledgments
The author would like to thank Mike Eisler for his work on the
Channel Conjunction Mechanism I-D and for bringing the problem to a
head, Sam Hartman for pointing out that channel bindings provide a
general solution to the channel binding problem, Jeff Altman for his
suggestion of using the TLS finished messages as the TLS channel
bindings, Bill Sommerfeld, for his help in developing channel
bindings for IPsec, and Radia Perlman for her most helpful comments.
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Author's Address
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
Email: Nicolas.Williams@sun.com
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Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
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except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Williams Expires April 19, 2006 [Page 16]
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