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Versions: 00 01 02 03                                                   
Network Working Group                                       S. Josefsson
Internet-Draft                                                    SJD AB
Intended status: Standards Track                         August 20, 2009
Expires: February 21, 2010


               Channel Bindings for TLS based on the PRF
                     draft-josefsson-sasl-tls-cb-02

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Internet-Draft  Channel Bindings for TLS based on the PRF    August 2009


   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

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).


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions Used in this Document . . . . . . . . . . . . . . . 3
   3.  Channel Bindings Syntax . . . . . . . . . . . . . . . . . . . . 3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 5























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Internet-Draft  Channel Bindings for TLS based on the PRF    August 2009


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
   [RFC5246] 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 "EXPORTER Channel Binding".  The key
   will be the master secret in a TLS session.  The seed is the
   concatenation of the client/server random and finished messages as
   described below.  We will use the first 32 octets computed by the
   PRF.

   Using the terminology, conventions and and pseudo-language in TLS
   [RFC5246] and [I-D.ietf-tls-extractor], the channel bindings is
   computed as follows:

    TLS_channel_bindings = PRF(SecurityParameters.master_secret,
                               "EXPORTER Channel Binding",
                               SecurityParameters.client_random +
                               SecurityParameters.server_random +
                               Finished) [0..31]

   The seed will be the concatenation of the current TLS session's
   client/server random with the client's TLS Finished message from the
   first handshake of the connection.

   The derived data MUST NOT be used for any other purpose than channel
   bindings as described in [RFC5056].






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Internet-Draft  Channel Bindings for TLS based on the PRF    August 2009


4.  IANA Considerations

   The IANA is requested to allocate a string "EXPORTER Channel Binding"
   in the TLS Exporter Label registry as per [I-D.ietf-tls-extractor].

   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

   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.






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Internet-Draft  Channel Bindings for TLS based on the PRF    August 2009


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.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [I-D.ietf-tls-extractor]
              Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", draft-ietf-tls-extractor-06 (work
              in progress), July 2009.

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 AB

   Email: simon@josefsson.org













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