TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks
draft-ietf-tls-downgrade-scsv-01

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Document Type Active Internet-Draft (tls WG)
Last updated 2014-11-10
Replaces draft-bmoeller-tls-downgrade-scsv
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Network Working Group                                         B. Moeller
Internet-Draft                                                A. Langley
Updates: 2246, 4346, 4347, 5246, 6347                             Google
         (if approved)                                 November 10, 2014
Intended status: Standards Track
Expires: May 14, 2015

TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol
                           Downgrade Attacks
                    draft-ietf-tls-downgrade-scsv-01

Abstract

   This document defines a Signaling Cipher Suite Value (SCSV) that
   prevents protocol downgrade attacks on the Transport Layer Security
   (TLS) protocol.  It updates RFC 2246, RFC 4346, and RFC 5246.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 14, 2015.

Copyright Notice

   Copyright (c) 2014 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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Protocol values . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Server behavior . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Client behavior . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   To work around interoperability problems with legacy servers, many
   TLS client implementations do not rely on the TLS protocol version
   negotiation mechanism alone, but will intentionally reconnect using a
   downgraded protocol if initial handshake attempts fail.  Such clients
   may fall back to connections in which they announce a version as low
   as TLS 1.0 (or even its predecessor, SSL 3.0) as the highest
   supported version.

   While such fallback retries can be a useful last resort for
   connections to actual legacy servers, there's a risk that active
   attackers could exploit the downgrade strategy to weaken the
   cryptographic security of connections.  Also, handshake errors due to
   network glitches could similarly be misinterpreted as interaction
   with a legacy server and result in a protocol downgrade.

   All unnecessary protocol downgrades are undesirable (e.g., from TLS
   1.2 to TLS 1.1 if both the client and the server actually do support
   TLS 1.2); they can be particularly critical if they mean losing the
   TLS extension feature (when downgrading to SSL 3.0).  This document
   defines a Signaling Cipher Suite Value (SCSV) that can be employed to
   prevent unintended protocol downgrades between clients and servers
   that comply with this document, by having the client indicate that
   the current connection attempt is merely a fallback, and by having
   the server return a fatal alert if it detects an inappropriate
   fallback.  (The alert does not necessarily indicate an intentional
   downgrade attack, since network glitches too could result in
   inappropriate fallback retries.)

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