Network Working Group S. Josefsson Internet-Draft March 24, 2006 Expires: September 25, 2006 Channel bindings for SASL over TLS draft-josefsson-sasl-tls-cb-00 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 September 25, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document define how to compute data that is bound to a specific Transport Layer Security (TLS) session, for use by Simple Authentication and Security Layer (SASL) mechanisms. Josefsson Expires September 25, 2006 [Page 1] Internet-Draft Channel bindings for SASL over TLS March 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in this Document . . . . . . . . . . . . . . . 3 3. Deriving the Channel Binding . . . . . . . . . . . . . . . . . 3 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 3 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 4 6.1. Normative References . . . . . . . . . . . . . . . . . . . 4 6.2. Informative References . . . . . . . . . . . . . . . . . . 4 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5 Intellectual Property and Copyright Statements . . . . . . . . . . 6 Josefsson Expires September 25, 2006 [Page 2] Internet-Draft Channel bindings for SASL over TLS March 2006 1. Introduction Binding an authentication attempt to a specific encrypted session can protect from certain attacks [4]. This document describe how to generate data that can be used by SASL [2] mechanisms to bind it to a specific TLS [3] session. An application that use SASL under TLS is responsible for deriving channel binding data for use by SASL. This document make it possible to derive channel bindings that can be used by any SASL mechanism. Compared to specifying TLS channel bindings for each and every SASL mechanism, the process described below enable an interface between applications and SASL implementations that is not mechanism specific. 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 [1]. 3. Deriving the Channel Binding The TLS Pseudo-Random Function (PRF) generate, using the constant string "SASL channel binding", and based on the master secret and the random values established during a TLS handshake, a 64 octet string that make up the SASL channel binding data. Using the terminology of TLS [3], the SASL channel binding data is computed as follows: SASL_channel_binding = PRF(SecurityParameters.master_secret, "SASL channel binding", SecurityParameters.server_random + SecurityParameters.client_random) [0..64]; The derived data MUST NOT be used for any other purpose than in combination with an SASL authentication. 4. IANA Considerations None. Josefsson Expires September 25, 2006 [Page 3] Internet-Draft Channel bindings for SASL over TLS March 2006 5. Security Considerations It is critical to protect the TLS master secret. We claim that by appropriately using the channel binding data derived from a TLS session a SASL mechanism can protect itself from the attacks in attacks [4]. To guarantee this property, the derived data is only to be used for the intended purpose. The security considerations in SASL and TLS should also be considered. 6. References 6.1. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997. [3] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. 6.2. Informative References [4] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle in Tunneled Authentication", WWW http://www.saunalahti.fi/~asokan/research/mitm.html. Josefsson Expires September 25, 2006 [Page 4] Internet-Draft Channel bindings for SASL over TLS March 2006 Author's Address Simon Josefsson Email: simon@josefsson.org Josefsson Expires September 25, 2006 [Page 5] Internet-Draft Channel bindings for SASL over TLS March 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and 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. Josefsson Expires September 25, 2006 [Page 6]