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IETF RFC 7465
Last modified on Wednesday, February 18th, 2015
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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/RFC 7465.
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.
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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 ([RFC 5246] [RFC 4346] [RFC 2246])
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 [RFC 2119].
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.
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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 in Appendix A), which do not provide a sufficiently high
level of security.
4. References
4.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999,
<http://www.rfc-editor.org/info/RFC 2246>.
[RFC 4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006,
<http://www.rfc-editor.org/info/RFC 4346>.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/RFC 5246>.
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.
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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.
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RFC 7465 Prohibiting RC4 Cipher Suites February 2015
Appendix 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_SHA
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RFC 7465 Prohibiting RC4 Cipher Suites February 2015
Acknowledgements
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.com
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RFC TOTAL SIZE: 8397 bytes
PUBLICATION DATE: Wednesday, February 18th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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