7465
PROPOSED STANDARD
Prohibiting RC4 Cipher Suites
Authors: A. Popov
Date: February 2015
Area: sec
Working Group: tls
Stream: IETF
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.
RFC 7465
PROPOSED STANDARD
Updated by: 8996
Internet Engineering Task Force (IETF) A. Popov
Request for Comments: 7465 Microsoft Corp.
Updates: <a href="./rfc5246">5246</a>, <a href="./rfc4346">4346</a>, <a href="./rfc2246">2246</a> February 2015
Category: Standards Track
ISSN: 2070-1721
<span class="h1">Prohibiting RC4 Cipher Suites</span>
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 <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="https://www.rfc-editor.org/info/rfc7465">http://www.rfc-editor.org/info/rfc7465</a>.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-2">2</a>
<a href="#section-1.1">1.1</a>. Requirements Language . . . . . . . . . . . . . . . . . . <a href="#page-2">2</a>
<a href="#section-2">2</a>. Changes to TLS . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-2">2</a>
<a href="#section-3">3</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-4">4</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-4.1">4.1</a>. Normative References . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-4.2">4.2</a>. Informative References . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#appendix-A">Appendix A</a>. RC4 Cipher Suites . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
RC4 is a stream cipher that is described in [<a href="#ref-SCH" title=""Applied Cryptography: Protocols, Algorithms, and Source Code in C"">SCH</a>]; 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
[<a href="#ref-PAU" title=""Permutation after RC4 Key Scheduling Reveals the Secret Key"">PAU</a>], [<a href="#ref-MAN" title=""A Practical Attack on Broadcast RC4"">MAN</a>], and [<a href="#ref-FLU" title=""Weaknesses in the Key Scheduling Algorithm of RC4"">FLU</a>]. Recent cryptanalysis results [<a href="#ref-ALF" title=""On the Security of RC4 in TLS and WPA"">ALF</a>] 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 ([<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>] [<a href="./rfc4346" title=""The Transport Layer Security (TLS) Protocol Version 1.1"">RFC4346</a>] [<a href="./rfc2246" title=""The TLS Protocol Version 1.0"">RFC2246</a>])
clients and servers never negotiate the use of RC4 cipher suites.
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Requirements Language</span>
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 [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Changes to TLS</span>
Because of the RC4 deficiencies noted in <a href="#section-1">Section 1</a>, 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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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.
<a href="#appendix-A">Appendix A</a> lists the RC4 cipher suites defined for TLS.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Security Considerations</span>
This document helps maintain the security guarantees of the TLS
protocol by prohibiting the use of the RC4-based cipher suites
(listed in <a href="#appendix-A">Appendix A</a>), which do not provide a sufficiently high
level of security.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. References</span>
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997,
<<a href="https://www.rfc-editor.org/info/rfc2119">http://www.rfc-editor.org/info/rfc2119</a>>.
[<a id="ref-RFC2246">RFC2246</a>] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
<a href="./rfc2246">RFC 2246</a>, January 1999,
<<a href="https://www.rfc-editor.org/info/rfc2246">http://www.rfc-editor.org/info/rfc2246</a>>.
[<a id="ref-RFC4346">RFC4346</a>] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", <a href="./rfc4346">RFC 4346</a>, April 2006,
<<a href="https://www.rfc-editor.org/info/rfc4346">http://www.rfc-editor.org/info/rfc4346</a>>.
[<a id="ref-RFC5246">RFC5246</a>] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", <a href="./rfc5246">RFC 5246</a>, August 2008,
<<a href="https://www.rfc-editor.org/info/rfc5246">http://www.rfc-editor.org/info/rfc5246</a>>.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Informative References</span>
[<a id="ref-ALF">ALF</a>] 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,
<<a href="https://www.usenix.org/conference/usenixsecurity13/security-rc4-tls">https://www.usenix.org/conference/usenixsecurity13/</a>
<a href="https://www.usenix.org/conference/usenixsecurity13/security-rc4-tls">security-rc4-tls</a>>.
[<a id="ref-FLU">FLU</a>] 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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[<a id="ref-MAN">MAN</a>] 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.
[<a id="ref-PAU">PAU</a>] 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.
[<a id="ref-SCH">SCH</a>] Schneier, B., "Applied Cryptography: Protocols,
Algorithms, and Source Code in C", 2nd Edition, 1996.
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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. RC4 Cipher Suites</span>
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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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: [email protected]
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