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Versions: 00 01 02 draft-ietf-tls-downgrade-scsv

Network Working Group                                         B. Moeller
Internet-Draft                                                A. Langley
Updates: 2246,4346,5246                                           Google
(if approved)                                          November 28, 2013
Intended status: Standards Track
Expires: June 1, 2014


TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol
                           Downgrade Attacks
                  draft-bmoeller-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.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 1, 2014.

Copyright Notice

   Copyright (c) 2013 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



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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.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     6.2.  Informal References  . . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10




































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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 similary 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
   protocol versions.

   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].
















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2.  Protocol values

   [[ NOTE IN DRAFT: The following registry allocations require
   Standards Action, i.e. will only be official with the IESG's
   Standards Track RFC approval. ]]

   This document allocates a new cipher suite value in the TLS Cipher
   Suite Registry [RFC5246]:

        TLS_FALLBACK_SCSV          {0x56, 0x00}


   This is a signaling cipher suite value, i.e., it does not actually
   correspond to a suite of cryptosystems, and it can never be selected
   by the server in the handshake; rather, its presence in the client
   hello message serves as a backwards-compatible signal from the client
   to the server.

   This document also allocates a new alert value in the TLS Alert
   Registry [RFC5246]:

        enum {
          /* ... */
          inappropriate_fallback(86),
          /* ... */
          (255)
        } AlertDescription;


   This alert is only generated by servers, as described in Section 3.
   It is always fatal.




















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3.  Server behavior

   This section specifies server behavior when receiving the
   TLS_FALLBACK_SCSV cipher suite from a client in
   ClientHello.cipher_suites.

   o  If TLS_FALLBACK_SCSV appears in ClientHello.cipher_suites and the
      highest protocol version supported by the server is higher than
      the version indicated in ClientHello.client_version, the server
      MUST respond with a inappropriate_fallback alert.

   Otherwise (either TLS_FALLBACK_SCSV does not appear, or it appears
   and the client's protocol version is at least the highest protocol
   version supported by the server), the server proceeds with the
   handshake as usual.




































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4.  Client behavior

   The TLS_FALLBACK_SCSV cipher suite value is meant for use by clients
   that repeat a connection attempt with a downgraded protocol in order
   to avoid interoperability problems with legacy servers.  This section
   specifies when to send it.

   o  If a client sends a ClientHello.client_version containing a lower
      value than the latest (highest-valued) version supported by the
      client, it SHOULD include the TLS_FALLBACK_SCSV cipher suite value
      in ClientHello.cipher_suites.  This does not apply when the client
      intends to perform an abbreviated handshake to resume a previously
      negotiated session and sets ClientHello.client_version to the
      protocol version negotiated for that session.

   Note that in the above, a protocol version is not considered
   supported by the client if it has been disabled by any applicable
   system or user settings: it is about the highest protocol version
   that the client would attempt using in a handshake, not about the
   highest protocol version implemented if its use is not actually
   enabled.  (For example, if the implementation supports TLS 1.2 but
   the user has disabled this protocol version, a TLS 1.1 handshake is
   expected and does not warrant sending TLS_FALLBACK_SCSV.)




























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5.  Security Considerations

   Section 4 does not require client implementations to send
   TLS_FALLBACK_SCSV in any particular case, it merely recommends it;
   behavior can be adapted according to the client's security needs.
   For example, during the initial deployment of a new protocol version
   (when some interoperability problems may have to be expected),
   smoothly falling back to the previous protocol version in case of
   problems may be preferrable to potentially not being able to connect
   at all: so TLS_FALLBACK_SCSV could be omitted for this particular
   protocol downgrade step.

   However, it is particularly strongly recommended to send
   TLS_FALLBACK_SCSV when downgrading to SSL 3.0 as the CBC cipher
   suites in SSL 3.0 have weaknesses that cannot be addressed by
   implementation workarounds like the remaining weaknesses in later
   (TLS) protocol versions.


































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6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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

6.2.  Informal References

   [RFC6101]  Freier, A., Karlton, P., and P. Kocher, "The Secure
              Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
              August 2011.






























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Appendix A.  Acknowledgements

   This specification was inspired by an earlier proposal by Eric
   Rescorla.















































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Authors' Addresses

   Bodo Moeller
   Google Switzerland GmbH
   Brandschenkestrasse 110
   Zurich  8002
   Switzerland

   Email: bmoeller@acm.org


   Adam Langley
   Google Inc.
   76 9th Ave
   New York, NY  10011
   USA

   Email: agl@google.com

































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