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Internet Engineering Task Force (IETF)                          A. PopovRequest for Comments: 7465                               Microsoft Corp.Updates:5246,4346,2246                                  February 2015Category: Standards TrackISSN: 2070-1721Prohibiting RC4 Cipher SuitesAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 2015Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Requirements Language . . . . . . . . . . . . . . . . . .22.  Changes to TLS  . . . . . . . . . . . . . . . . . . . . . . .23.  Security Considerations . . . . . . . . . . . . . . . . . . .34.  References  . . . . . . . . . . . . . . . . . . . . . . . . .34.1.  Normative References  . . . . . . . . . . . . . . . . . .34.2.  Informative References  . . . . . . . . . . . . . . . . .3Appendix A.  RC4 Cipher Suites  . . . . . . . . . . . . . . . . .5   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .6   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .61.  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 inSection 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   (listed inAppendix A), which do not provide a sufficiently high   level of security.4.  References4.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",RFC 2246, January 1999,              <http://www.rfc-editor.org/info/rfc2246>.   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security              (TLS) Protocol Version 1.1",RFC 4346, April 2006,              <http://www.rfc-editor.org/info/rfc4346>.   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security              (TLS) Protocol Version 1.2",RFC 5246, August 2008,              <http://www.rfc-editor.org/info/rfc5246>.4.2.  Informative References   [ALF]      AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,              and J. Schuldt, "On the Security of RC4 in TLS and WPA",              USENIX Security Symposium, July 2013,              <https://www.usenix.org/conference/usenixsecurity13/security-rc4-tls>.   [FLU]      Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the              Key Scheduling Algorithm of RC4", Selected Areas of              Cryptography: SAC 2001, Lecture Notes in Computer Science              Vol. 2259, pp 1-24, 2001.Popov                        Standards Track                    [Page 3]

RFC 7465              Prohibiting RC4 Cipher Suites        February 2015   [MAN]      Mantin, I. and A. Shamir, "A Practical Attack on Broadcast              RC4", Fast Software Encryption: FSE 2001, Lecture Notes in              Computer Science Vol. 2355, pp 152-164, 2002.   [PAU]      Paul, G. and S. Maitra, "Permutation after RC4 Key              Scheduling Reveals the Secret Key", Selected Areas of              Cryptography: SAC 2007, Lecture Notes on Computer Science,              Vol. 4876, pp 360-337, 2007.   [SCH]      Schneier, B., "Applied Cryptography: Protocols,              Algorithms, and Source Code in C", 2nd Edition, 1996.Popov                        Standards Track                    [Page 4]

RFC 7465              Prohibiting RC4 Cipher Suites        February 2015Appendix A.  RC4 Cipher Suites   The following cipher suites defined for TLS use RC4:   o  TLS_RSA_EXPORT_WITH_RC4_40_MD5   o  TLS_RSA_WITH_RC4_128_MD5   o  TLS_RSA_WITH_RC4_128_SHA   o  TLS_DH_anon_EXPORT_WITH_RC4_40_MD5   o  TLS_DH_anon_WITH_RC4_128_MD5   o  TLS_KRB5_WITH_RC4_128_SHA   o  TLS_KRB5_WITH_RC4_128_MD5   o  TLS_KRB5_EXPORT_WITH_RC4_40_SHA   o  TLS_KRB5_EXPORT_WITH_RC4_40_MD5   o  TLS_PSK_WITH_RC4_128_SHA   o  TLS_DHE_PSK_WITH_RC4_128_SHA   o  TLS_RSA_PSK_WITH_RC4_128_SHA   o  TLS_ECDH_ECDSA_WITH_RC4_128_SHA   o  TLS_ECDHE_ECDSA_WITH_RC4_128_SHA   o  TLS_ECDH_RSA_WITH_RC4_128_SHA   o  TLS_ECDHE_RSA_WITH_RC4_128_SHA   o  TLS_ECDH_anon_WITH_RC4_128_SHA   o  TLS_ECDHE_PSK_WITH_RC4_128_SHAPopov                        Standards Track                    [Page 5]

RFC 7465              Prohibiting RC4 Cipher Suites        February 2015Acknowledgements   This document was inspired by discussions with Magnus Nystrom, Eric   Rescorla, Joseph Salowey, Yaron Sheffer, Nagendra Modadugu, and   others on the TLS mailing list.Author's Address   Andrei Popov   Microsoft Corp.   One Microsoft Way   Redmond, WA  98052   USA   EMail: andreipo@microsoft.comPopov                        Standards Track                    [Page 6]