RFC 7465

Prohibiting RC4 Cipher Suites

📅 published February 2015, from ietf

RFC 7465: Prohibiting RC4 Cipher Suites 🎀

Okay bestie, so this one is by A. Popov from Microsoft Corp., published February 2015. It's a Standards Track doc, which means it's an official Internet Standard and not just a suggestion 💅. It updates RFCs 5246, 4346, and 2246, and it doesn't obsolete anything. The ISSN is 2070-1721. ✨

Abstract 🌸

Here's the tea: TLS clients and servers must never negotiate RC4 cipher suites when they set up connections, and that applies to every version of TLS 🔒. This doc also updates RFCs 5246, 4346, and 2246 💖.

Status of This Memo 📜

This is an Internet Standards Track document 🦋.

It's a product of the Internet Engineering Task Force (IETF), so it represents what the IETF community agreed on. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG) 🫶. If you want more on Internet Standards, check Section 2 of RFC 5741 🧚.

For the current status, any errata, and how to send feedback, go to http://www.rfc-editor.org/info/rfc7465 💜.

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. 🙈

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 🧚

Okay so RC4 is a stream cipher described in [SCH] 🌈. It's widely supported and TLS servers really like it. The problem is that RC4 has been known for a long time to have a bunch of cryptographic weaknesses, see [PAU], [MAN], and [FLU] 😭. Then the recent cryptanalysis in [ALF] found biases in the RC4 keystream that let attackers recover plaintexts that get encrypted over and over 💀.

These newer results are basically about to become practically exploitable. Right now they need 2^26 sessions or 13x2^30 encryptions 👀. So RC4 can no longer be seen as giving TLS sessions a sufficient level of security. It's over for RC4 🍓.

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 problems we just covered in Section 1, the following apply 💅:

  • TLS clients MUST NOT include RC4 cipher suites in the ClientHello message. 🙅‍♀️
  • TLS servers MUST NOT select an RC4 cipher suite when a TLS client sends such a cipher suite in the ClientHello message. 🙅‍♀️
  • 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 🔐

Real talk, this is the important part 😤. This document keeps the TLS protocol's security guarantees intact by banning the RC4-based cipher suites (listed in Appendix A). Those suites just don't give a high enough level of security, so they're out 💔.

4. References 📚

4.1. 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.

[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.

Appendix A. RC4 Cipher Suites 🧁

The following cipher suites defined for TLS use RC4 🍬:

  • TLS_RSA_EXPORT_WITH_RC4_40_MD5
  • TLS_RSA_WITH_RC4_128_MD5
  • TLS_RSA_WITH_RC4_128_SHA
  • TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
  • TLS_DH_anon_WITH_RC4_128_MD5
  • TLS_KRB5_WITH_RC4_128_SHA
  • TLS_KRB5_WITH_RC4_128_MD5
  • TLS_KRB5_EXPORT_WITH_RC4_40_SHA
  • TLS_KRB5_EXPORT_WITH_RC4_40_MD5
  • TLS_PSK_WITH_RC4_128_SHA
  • TLS_DHE_PSK_WITH_RC4_128_SHA
  • TLS_RSA_PSK_WITH_RC4_128_SHA
  • TLS_ECDH_ECDSA_WITH_RC4_128_SHA
  • TLS_ECDHE_ECDSA_WITH_RC4_128_SHA
  • TLS_ECDH_RSA_WITH_RC4_128_SHA
  • TLS_ECDHE_RSA_WITH_RC4_128_SHA
  • TLS_ECDH_anon_WITH_RC4_128_SHA
  • TLS_ECDHE_PSK_WITH_RC4_128_SHA

Acknowledgements 🫶

This doc was inspired by chats 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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