Prohibiting RC4 Cipher Suites
RFC 7465

Document Type RFC - Proposed Standard (February 2015; No errata)
Last updated 2015-10-14
Replaces draft-popov-tls-prohibiting-rc4
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Document shepherd Joseph Salowey
Shepherd write-up Show (last changed 2014-09-30)
IESG IESG state RFC 7465 (Proposed Standard)
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Responsible AD Stephen Farrell
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Internet Engineering Task Force (IETF)                          A. Popov
Request for Comments: 7465                               Microsoft Corp.
Updates: 5246, 4346, 2246                                  February 2015
Category: Standards Track
ISSN: 2070-1721

                     Prohibiting RC4 Cipher Suites

Abstract

   This document requires that Transport Layer Security (TLS) clients
   and servers never negotiate the use of RC4 cipher suites when they
   establish connections.  This applies to all TLS versions.  This
   document updates RFCs 5246, 4346, and 2246.

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/rfc7465.

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.

Popov                        Standards Track                    [Page 1]
RFC 7465              Prohibiting RC4 Cipher Suites        February 2015

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Changes to TLS  . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   3
     4.2.  Informative References  . . . . . . . . . . . . . . . . .   3
   Appendix A.  RC4 Cipher Suites  . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   RC4 is a stream cipher that is described in [SCH]; it is widely
   supported, and often preferred by TLS servers.  However, RC4 has long
   been known to have a variety of cryptographic weaknesses, e.g., see
   [PAU], [MAN], and [FLU].  Recent cryptanalysis results [ALF] exploit
   biases in the RC4 keystream to recover repeatedly encrypted
   plaintexts.

   These recent results are on the verge of becoming practically
   exploitable; currently, they require 2^26 sessions or 13x2^30
   encryptions.  As a result, RC4 can no longer be seen as providing a
   sufficient level of security for TLS sessions.

   This document requires that TLS ([RFC5246] [RFC4346] [RFC2246])
   clients and servers never negotiate the use of RC4 cipher suites.

1.1.  Requirements Language

   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].

2.  Changes to TLS

   Because of the RC4 deficiencies noted in Section 1, the following
   apply:

   o  TLS clients MUST NOT include RC4 cipher suites in the ClientHello
      message.

   o  TLS servers MUST NOT select an RC4 cipher suite when a TLS client
      sends such a cipher suite in the ClientHello message.

Popov                        Standards Track                    [Page 2]
RFC 7465              Prohibiting RC4 Cipher Suites        February 2015

   o  If the TLS client only offers RC4 cipher suites, the TLS server
      MUST terminate the handshake.  The TLS server MAY send the
      insufficient_security fatal alert in this case.

   Appendix A lists the RC4 cipher suites defined for TLS.

3.  Security Considerations

   This document helps maintain the security guarantees of the TLS
   protocol by prohibiting the use of the RC4-based cipher suites
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