Internet Engineering Task Force L. Camara
Obsoletes: 4345 (if approved) L. Velvindron
Intended status: Best Current Practice
Expires: July 20, 2019 January 16, 2019

Deprecating RC4 in Secure Shell (SSH)


This document deprecates RC4 in Secure Shell (SSH). Therefore, this document formally obsoletes and moves to Historic RFC4345.

Status of This Memo

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This Internet-Draft will expire on July 20, 2019.

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Table of Contents

1. Introduction

The usage of RC4 suites ( also designated as arcfour ) for SSH are specified in [RFC4253] and [RFC4345]. [RFC4253] specifies the allocation of the "arcfour" cipher for SSH. [RFC4345] specifies and allocates the "arcfour-128" and "arcfour-256" ciphers for SSH. RC4 encryption has known weaknesses [RFC7465] [I-D.ietf-curdle-des-des-des-die-die-die], and the deprecation process should be begun for their use in Secure Shell (SSH) [RFC4253]. Accordingly, [RFC4253] is updated to note the deprecation of the RC4 ciphers and [RFC4345] is moved to Historic as all ciphers it specifies MUST NOT be used.

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. Updates to RFC 4253

[RFC4253] is updated to prohibit arcfour's use in SSH. [RFC4253] allocates the "arcfour" cipher in Section 6.3 by defining a list of defined ciphers where the "arcfour" cipher appears as optional as mentioned below:

arcfour OPTIONAL the ARCFOUR stream cipher with a 128-bit key

The current document updates the status of the "arcfour" ciphers in the list of [RFC4253] Section 6.3 by moving it from OPTIONAL to MUST NOT.

arcfour MUST NOT the ARCFOUR stream cipher with a 128-bit key

[RFC4253] defines the "arcfour" ciphers with the text mentioned below:

The "arcfour" cipher is the Arcfour stream cipher with 128-bit keys. The Arcfour cipher is compatible with the RC4 cipher [SCHNEIER]. Arcfour (and RC4) has problems with weak keys, and should be used with caution.

The current document updates [RFC4253] Section 6.3 by replacing the text above with the following text:

The "arcfour" cipher is the Arcfour stream cipher with 128-bit keys. The Arcfour cipher is compatible with the RC4 cipher [SCHNEIER]. Arcfour (and RC4) has known weakenesses [RFC7465] [I-D.ietf-curdle-des-des-des-die-die-die], and MUST NOT be used.

3. IANA Considerations

The IANA is requested to update the Encryption Algorithm Name Registry of the Secure Shell (SSH) Protocol Parameters [IANA]. The Registration procedure is IETF Review which is achieved by this document. The registry should be updated as follows:

Encryption Algorithm Name Reference Note
arcfour [RFC-TBD]
arcfour128 [RFC-TBD]
arcfour256 [RFC-TBD]

Where TBD is the RFC number assigned to the document.

4. Acknowledgements

The authors would like to thank Eric Rescorla, Daniel Migault and Rich Salz.

5. Security Considerations

This document only prohibits the use of RC4 in SSH, and introduces no new security considerations.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.

6.2. Informative References

[I-D.ietf-curdle-des-des-des-die-die-die] Kaduk, B. and M. Short, "Deprecate 3DES and RC4 in Kerberos", Internet-Draft draft-ietf-curdle-des-des-des-die-die-die-05, September 2017.
[IANA] "Secure Shell (SSH) Protocol Parameters: Encryption Algorithm Names"
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006.
[RFC4345] Harris, B., "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol", RFC 4345, DOI 10.17487/RFC4345, January 2006.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, DOI 10.17487/RFC7465, February 2015.
[SCHNEIER] Schneier, B., "Applied Cryptography Second Edition: protocols algorithms and source in code in C", , 1996.

Authors' Addresses

Luis Camara EMail:
Loganaden Velvindron Mauritius EMail: