Deprecating RC4 in Secure Shell (SSH) cyberstorm.mu
Mauritius logan@cyberstorm.mu
Security curdle This document deprecates RC4 in Secure Shell (SSH). Therefore, this document formally moves RFC 4345 to Historic status.
Status of This Memo This memo documents an Internet Best Current Practice. 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 BCPs is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .
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Table of Contents
  • .  Introduction
    • .  Requirements Language
  • .  Updates to RFC 4253
  • .  IANA Considerations
  • .  Security Considerations
  • .  References
    • .  Normative References
    • .  Informative References
  • Acknowledgements
  • Author's Address
Introduction The usage of RC4 suites (also designated as "arcfour") for SSH is specified in and . specifies the allocation of the "arcfour" cipher for SSH. specifies and allocates the "arcfour128" and "arcfour256" ciphers for SSH. RC4 encryption has known weaknesses ; therefore, this document starts the deprecation process for their use in Secure Shell (SSH) . Accordingly, is updated to note the deprecation of the RC4 ciphers, and is moved to Historic status, as all ciphers it specifies MUST NOT be used.
Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.
Updates to RFC 4253 is updated to prohibit arcfour's use in SSH. allocates the "arcfour" cipher by defining a list of defined ciphers in which the "arcfour" cipher appears as optional, as shown in .
arcfour OPTIONAL the ARCFOUR stream cipher with a 128-bit key
This document updates the status of the "arcfour" ciphers in the list found in by moving it from OPTIONAL to MUST NOT.
arcfour MUST NOT the ARCFOUR stream cipher with a 128-bit key
defines the "arcfour" ciphers with the following text:
The "arcfour" cipher is the Arcfour stream cipher with 128-bit keys. The Arcfour cipher is believed to be compatible with the RC4 cipher . Arcfour (and RC4) has problems with weak keys, and should be used with caution.
This document updates 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 . Arcfour (and RC4) has known weaknesses and MUST NOT be used.
IANA Considerations The IANA has updated the "Encryption Algorithm Names" subregistry in the "Secure Shell (SSH) Protocol Parameters" registry . The registration procedure is IETF review, which is achieved by this document. The registry has been updated as follows:
Encryption Algorithm Name Reference Note
arcfour RFC 8758 HISTORIC
arcfour128 RFC 8758 HISTORIC
arcfour256 RFC 8758 HISTORIC
Security Considerations This document only prohibits the use of RC4 in SSH; it introduces no new security considerations.
References Normative References Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Secure Shell (SSH) Protocol Parameters The Secure Shell (SSH) Transport Layer Protocol The Secure Shell (SSH) is a protocol for secure remote login and other secure network services over an insecure network. This document describes the SSH transport layer protocol, which typically runs on top of TCP/IP. The protocol can be used as a basis for a number of secure network services. It provides strong encryption, server authentication, and integrity protection. It may also provide compression. Key exchange method, public key algorithm, symmetric encryption algorithm, message authentication algorithm, and hash algorithm are all negotiated. This document also describes the Diffie-Hellman key exchange method and the minimal set of algorithms that are needed to implement the SSH transport layer protocol. [STANDARDS-TRACK] Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol This document specifies methods of using the Arcfour cipher in the Secure Shell (SSH) protocol that mitigate the weakness of the cipher's key-scheduling algorithm. [STANDARDS-TRACK] Prohibiting RC4 Cipher Suites 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. Deprecate Triple-DES (3DES) and RC4 in Kerberos The triple-DES (3DES) and RC4 encryption types are steadily weakening in cryptographic strength, and the deprecation process should begin for their use in Kerberos. Accordingly, RFC 4757 has been moved to Historic status, as none of the encryption types it specifies should be used, and RFC 3961 has been updated to note the deprecation of the triple-DES encryption types. RFC 4120 is likewise updated to remove the recommendation to implement triple-DES encryption and checksum types. Applied Cryptography Second Edition: Protocols, Algorithms, and Source in Code in C
Acknowledgements The author would like to thank , , and .
Author's Address cyberstorm.mu
Mauritius logan@cyberstorm.mu