Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension
RFC 7627

Document Type RFC - Proposed Standard (September 2015; No errata)
Updates RFC 5246
Last updated 2015-10-14
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IESG IESG state RFC 7627 (Proposed Standard)
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IANA IANA review state Version Changed - Review Needed
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Internet Engineering Task Force (IETF)                 K. Bhargavan, Ed.
Request for Comments: 7627                            A. Delignat-Lavaud
Updates: 5246                                                 A. Pironti
Category: Standards Track                       Inria Paris-Rocquencourt
ISSN: 2070-1721                                               A. Langley
                                                             Google Inc.
                                                                  M. Ray
                                                         Microsoft Corp.
                                                          September 2015

            Transport Layer Security (TLS) Session Hash and
                    Extended Master Secret Extension

Abstract

   The Transport Layer Security (TLS) master secret is not
   cryptographically bound to important session parameters such as the
   server certificate.  Consequently, it is possible for an active
   attacker to set up two sessions, one with a client and another with a
   server, such that the master secrets on the two sessions are the
   same.  Thereafter, any mechanism that relies on the master secret for
   authentication, including session resumption, becomes vulnerable to a
   man-in-the-middle attack, where the attacker can simply forward
   messages back and forth between the client and server.  This
   specification defines a TLS extension that contextually binds the
   master secret to a log of the full handshake that computes it, thus
   preventing such attacks.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7627.

Bhargavan, et al.            Standards Track                    [Page 1]
RFC 7627               TLS Session Hash Extension         September 2015

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Requirements Notation ...........................................5
   3. The TLS Session Hash ............................................5
   4. The Extended Master Secret ......................................6
   5. Extension Negotiation ...........................................6
      5.1. Extension Definition .......................................6
      5.2. Client and Server Behavior: Full Handshake .................7
      5.3. Client and Server Behavior: Abbreviated Handshake ..........7
      5.4. Interoperability Considerations ............................9
   6. Security Considerations .........................................9
      6.1. Triple Handshake Preconditions and Impact ..................9
      6.2. Cryptographic Properties of the Hash Function .............11
      6.3. Handshake Messages Included in the Session Hash ...........11
      6.4. No SSL 3.0 Support ........................................12
   7. IANA Considerations ............................................12
   8. References .....................................................12
      8.1. Normative References ......................................12
      8.2. Informative References ....................................13
   Acknowledgments ...................................................14
   Authors' Addresses ................................................15

Bhargavan, et al.            Standards Track                    [Page 2]
RFC 7627               TLS Session Hash Extension         September 2015

1.  Introduction

   In TLS [RFC5246], every session has a "master_secret" computed as:

   master_secret = PRF(pre_master_secret, "master secret",
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