Network Working Group S. Josefsson Internet-Draft SJD Intended status: Standards Track August 12, 2008 Expires: February 13, 2009 Channel Bindings for TLS based on PRF draft-josefsson-sasl-tls-cb-01 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 February 13, 2009. Abstract This document specify how to compute data, "channel bindings", that is cryptographically bound to a specific Transport Layer Security (TLS) session. The intention is to use this data as a name of the secure channel for the purpose of a channel binding. The channel bindings can be used by authentication protocols to avoid tunneling attacks and security layer re-use. The data is derived using the TLS Pseudo-Random Function (PRF). Josefsson Expires February 13, 2009 [Page 1] Internet-Draft Channel Bindings for TLS based on PRF August 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in this Document . . . . . . . . . . . . . . . 3 3. Channel Bindings Syntax . . . . . . . . . . . . . . . . . . . . 3 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 3 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 4 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 4 7.1. Normative References . . . . . . . . . . . . . . . . . . . 4 7.2. Informative References . . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5 Intellectual Property and Copyright Statements . . . . . . . . . . 6 Josefsson Expires February 13, 2009 [Page 2] Internet-Draft Channel Bindings for TLS based on PRF August 2008 1. Introduction Binding authentication to a specific encrypted session can protect from certain attacks [mitm]. It can also help to improve performance by having peers agree to re-use a secure channel rather than to set up a new. This document describe how to generate data that can be used by application protocols to bind authentication to a specific TLS [RFC4346] session. 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]. 3. Channel Bindings Syntax The channel bindings is computed using the TLS Pseudo-Random Function (PRF). The PRF takes three inputs, a secret, a fixed label, and a seed. Here the label will be "channel binding". The key will be the master secret in a TLS session. The seed will be the hash of the handshake messages, computed the same way as for the TLS Finished message. We will use the first 32 octets computed by the PRF. Using the terminology and pseudo-language in TLS [RFC4346], the channel bindings is computed as follows: TLS_channel_bindings = PRF(SecurityParameters.master_secret, "channel binding", MD5(handshake_messages) + SHA-1(handshake_messages)) [0..31] The derived data MUST NOT be used for any other purpose than channel bindings as desribed in [RFC5056]. 4. IANA Considerations The IANA is requested to register this channel binding using the following templates and the process described in [RFC5056]. Subject: Registration of channel binding TLS Channel binding unique prefix (name): tls-unique-prf Josefsson Expires February 13, 2009 [Page 3] Internet-Draft Channel Bindings for TLS based on PRF August 2008 Channel binding type: unique Channel type: TLS Published specification (recommended, optional): This document Channel binding is secret (requires confidentiality protection): no Description (optional if a specification is given; required if no Published specification is specified): See earlier in this document. Intended usage: COMMON Person and email address to contact for further information: simon@josefsson.org Owner/Change controller name and email address: simon@josefsson.org Expert reviewer name and contact information: 5. Security Considerations For the intended use and other important considerations, see [RFC5056]. We claim that by appropriately using a channel binding an application can protect itself from the attacks in [mitm]. To guarantee this property, the derived data is only to be used for the intended purpose. The security considerations in TLS should be considered. In particular, the TLS master secret must be protected. 6. Acknowledgements Thanks to Eric Rescorla and Sam Hartman who pointed out a problem with the construct used in earlier versions of this document when TLS server authentication is not used or checked. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Josefsson Expires February 13, 2009 [Page 4] Internet-Draft Channel Bindings for TLS based on PRF August 2008 [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006. [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, November 2007. 7.2. Informative References [mitm] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle in Tunneled Authentication", WWW http://www.saunalahti.fi/~asokan/research/mitm.html. Author's Address Simon Josefsson SJD Email: simon@josefsson.org Josefsson Expires February 13, 2009 [Page 5] Internet-Draft Channel Bindings for TLS based on PRF August 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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. 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, THE IETF TRUST 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. 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Josefsson Expires February 13, 2009 [Page 6]