KITTEN WG N. Williams Internet-Draft Sun Expires: December 30, 2004 July 2004 Clarifications and Extensions to the GSS-API for the Use of Channel Bindings draft-ietf-kitten-gssapi-channel-bindings-00.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 December 30, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. 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. Williams Expires December 30, 2004 [Page 1] Internet-Draft GSS-API Channel Bindings July 2004 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 . . . . . . . . . . . . . . . . . . . . . . 6 5. Channel Bindings for TLS . . . . . . . . . . . . . . . . . . . 7 5.1 GSS_Make_tls_channel_bindings() . . . . . . . . . . . . . 7 5.1.1 C-Bindings . . . . . . . . . . . . . . . . . . . . . . 7 6. Channel Bindings for IPsec . . . . . . . . . . . . . . . . . . 8 6.1 GSS_Make_ipsec_channel_bindings() . . . . . . . . . . . . 8 6.1.1 C-Bindings . . . . . . . . . . . . . . . . . . . . . . 9 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.1 Normative . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.2 Informative . . . . . . . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12 A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13 Intellectual Property and Copyright Statements . . . . . . . . 14 Williams Expires December 30, 2004 [Page 2] Internet-Draft GSS-API Channel Bindings July 2004 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]. Williams Expires December 30, 2004 [Page 3] Internet-Draft GSS-API Channel Bindings July 2004 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. Williams Expires December 30, 2004 [Page 4] Internet-Draft GSS-API Channel Bindings July 2004 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. Williams Expires December 30, 2004 [Page 5] Internet-Draft GSS-API Channel Bindings July 2004 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. 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 ); Williams Expires December 30, 2004 [Page 6] Internet-Draft GSS-API Channel Bindings July 2004 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. 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 ); Williams Expires December 30, 2004 [Page 7] Internet-Draft GSS-API Channel Bindings July 2004 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. The initiator ID payload as used in the key exchange protocol used for setting up the channel's SAs 5. The responder ID payload as used in the key exchange protocol used for setting up the channel's SAs 6. 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: 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. Williams Expires December 30, 2004 [Page 8] Internet-Draft GSS-API Channel Bindings July 2004 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 ); Williams Expires December 30, 2004 [Page 9] Internet-Draft GSS-API Channel Bindings July 2004 7. Security Considerations For general security considerations relating to channel bindings see [CHANNEL-BINDINGS] Williams Expires December 30, 2004 [Page 10] Internet-Draft GSS-API Channel Bindings July 2004 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. Williams Expires December 30, 2004 [Page 11] Internet-Draft GSS-API Channel Bindings July 2004 Author's Address Nicolas Williams Sun Microsystems 5300 Riata Trace Ct Austin, TX 78727 US EMail: Nicolas.Williams@sun.com Williams Expires December 30, 2004 [Page 12] Internet-Draft GSS-API Channel Bindings July 2004 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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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Williams Expires December 30, 2004 [Page 14]