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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

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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.









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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







































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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.






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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.




















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Author's Address

   Simon Josefsson

   Email: simon@josefsson.org














































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