TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks
draft-ietf-tls-downgrade-scsv-00

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Document Type Active Internet-Draft (tls WG)
Last updated 2014-07-04
Replaces draft-bmoeller-tls-downgrade-scsv
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Network Working Group                                         B. Moeller
Internet-Draft                                                A. Langley
Updates: 2246, 4346, 5246                                         Google
(if approved)                                               July 4, 2014
Intended status: Standards Track
Expires: January 5, 2015

TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol
                           Downgrade Attacks
                    draft-ietf-tls-downgrade-scsv-00

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.

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   This Internet-Draft will expire on January 5, 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
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   the Trust Legal Provisions and are provided without warranty as

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Protocol values  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Server behavior  . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Client behavior  . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informal References  . . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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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 protocol downgrades 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 to this document, by having the client indicate that the
   current connection attempt is merely a fallback.

   This specification applies to implementations of TLS 1.0 [RFC2246],
   TLS 1.1 [RFC4346], and TLS 1.2 [RFC5246].  (It is particularly
   relevant if such implementations also include support for predecessor
   protocol SSL 3.0 [RFC6101].)  It can be applied similarly to later
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