Internet Engineering Task Force A. Popov Internet-Draft Microsoft Corp. Updates: 5246,4346,2246 (if approved) October 05, 2013 Intended status: Standards Track Expires: April 08, 2014 Prohibiting RC4 Cipher Suites draft-popov-tls-prohibiting-rc4-01 Abstract This document requires that Transport Layer Security (TLS) clients and servers never negotiate the use of RC4 cipher suites when they establish connections. 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 April 08, 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 described in the Simplified BSD License. Popov Expires April 08, 2014 [Page 1]
Internet-Draft Prohibiting RC4 Cipher Suites October 2013 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 2. Changes to TLS . . . . . . . . . . . . . . . . . . . . . . . 2 3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 3 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 5. Security Considerations . . . . . . . . . . . . . . . . . . . 3 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 3 6.1. Normative References . . . . . . . . . . . . . . . . . . 3 6.2. Informative References . . . . . . . . . . . . . . . . . 3 Appendix A. RC4 Cipher Suites . . . . . . . . . . . . . . . . . 4 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction RC4 is a stream cipher described in [SCH], which is widely supported, and often preferred, by TLS servers. However, RC4 has long been known to have a variety of cryptographic weaknesses, e.g. [PAU], [MAN], [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 deficiencies noted in Section 1: 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 Expires April 08, 2014 [Page 2]
Internet-Draft Prohibiting RC4 Cipher Suites October 2013 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. 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. 4. IANA Considerations This memo includes no request to IANA. 5. 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. 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. 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.", 2013, <https://www.usenix.org /conference/usenixsecurity13/security-rc4-tls>. Popov Expires April 08, 2014 [Page 3]
Internet-Draft Prohibiting RC4 Cipher Suites October 2013 [FLU] Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the Key Scheduling Algorithm of RC4. Selected Areas in Cryptography, pp. 1-24", 2001. [MAN] Mantin, I. and A. Shamir, "A Practical Attack on Broadcast RC4. FSE, pp. 152-164.", 2001. [PAU] Paul, G. and S. Maitra, "Permutation after RC4 Key Scheduling Reveals the Secret Key. In Proceedings of the 14th Workshop on Selected Areas in Cryptography (SAC), pp. 360-377, vol. 4876, LNCS, Springer.", 2007. [SCH] Schneier, B., "Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd ed.", 1996. Appendix A. RC4 Cipher Suites The following cipher suites defined for TLS use RC4: o TLS_RSA_WITH_RC4_128_MD5 o TLS_RSA_WITH_RC4_128_SHA o TLS_DH_anon_WITH_RC4_128_MD5 o TLS_RSA_EXPORT_WITH_RC4_40_MD5 o TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 Author's Address Andrei Popov Microsoft Corp. One Microsoft Way Redmond, WA 98052 USA Email: andreipo@microsoft.com Popov Expires April 08, 2014 [Page 4]