Internet Engineering Task Force (IETF)                         B. Harris
Request for Comments: 8709
Updates: 4253                                              L. Velvindron
Category: Standards Track                                  cyberstorm.mu
ISSN: 2070-1721                                            February 2020


  Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH)
                               Protocol

Abstract

  This document describes the use of the Ed25519 and Ed448 digital
  signature algorithms in the Secure Shell (SSH) protocol.
  Accordingly, this RFC updates RFC 4253.

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 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
  https://www.rfc-editor.org/info/rfc8709.

Copyright Notice

  Copyright (c) 2020 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
  (https://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.  Conventions Used in This Document
    2.1.  Requirements Language
  3.  Public Key Algorithm
  4.  Public Key Format
  5.  Signature Algorithm
  6.  Signature Format
  7.  Verification Algorithm
  8.  SSHFP DNS Resource Records
  9.  IANA Considerations
  10. Security Considerations
  11. References
    11.1.  Normative References
    11.2.  Informative References
  Acknowledgements
  Authors' Addresses

1.  Introduction

  Secure Shell (SSH) [RFC4251] is a secure remote-login protocol.  It
  provides for an extensible variety of public key algorithms for
  identifying servers and users to one another.  Ed25519 [RFC8032] is a
  digital signature system.  OpenSSH 6.5 [OpenSSH-6.5] introduced
  support for using Ed25519 for server and user authentication and was
  then followed by other SSH implementations.

  This document describes the method implemented by OpenSSH and others
  and formalizes the use of the name "ssh-ed25519".  Additionally, this
  document describes the use of Ed448 and formalizes the use of the
  name "ssh-ed448".

2.  Conventions Used in This Document

  The descriptions of key and signature formats use the notation
  introduced in [RFC4251], Section 3 and the string data type from
  [RFC4251], Section 5.

2.1.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
  capitals, as shown here.

3.  Public Key Algorithm

  This document describes a public key algorithm for use with SSH, as
  per [RFC4253], Section 6.6.  The name of the algorithm is "ssh-
  ed25519".  This algorithm only supports signing and not encryption.

  Additionally, this document describes another public key algorithm.
  The name of the algorithm is "ssh-ed448".  This algorithm only
  supports signing and not encryption.

  Standard implementations of SSH SHOULD implement these signature
  algorithms.

4.  Public Key Format

  The "ssh-ed25519" key format has the following encoding:

  string  "ssh-ed25519"

  string  key

  Here, 'key' is the 32-octet public key described in [RFC8032],
  Section 5.1.5.

  The "ssh-ed448" key format has the following encoding:

  string  "ssh-ed448"

  string  key

  Here, 'key' is the 57-octet public key described in [RFC8032],
  Section 5.2.5.

5.  Signature Algorithm

  Signatures are generated according to the procedure in Sections 5.1.6
  and 5.2.6 of [RFC8032].

6.  Signature Format

  The "ssh-ed25519" key format has the following encoding:

  string  "ssh-ed25519"

  string  signature

  Here, 'signature' is the 64-octet signature produced in accordance
  with [RFC8032], Section 5.1.6.

  The "ssh-ed448" key format has the following encoding:

  string  "ssh-ed448"

  string  signature

  Here, 'signature' is the 114-octet signature produced in accordance
  with [RFC8032], Section 5.2.6.

7.  Verification Algorithm

  Ed25519 signatures are verified according to the procedure in
  [RFC8032], Section 5.1.7.

  Ed448 signatures are verified according to the procedure in
  [RFC8032], Section 5.2.7.

8.  SSHFP DNS Resource Records

  Usage and generation of the SSHFP DNS resource record is described in
  [RFC4255].  The generation of SSHFP resource records for "ssh-
  ed25519" keys is described in [RFC7479].  This section illustrates
  the generation of SSHFP resource records for "ssh-ed448" keys, and
  this document also specifies the corresponding Ed448 code point to
  "SSHFP RR Types for public key algorithms" in the "DNS SSHFP Resource
  Record Parameters" IANA registry [IANA-SSHFP].

  The generation of SSHFP resource records for "ssh-ed448" keys is
  described as follows.

  The encoding of Ed448 public keys is described in [ED448].  In brief,
  an Ed448 public key is a 57-octet value representing a 455-bit
  y-coordinate of an elliptic curve point, and a sign bit indicating
  the corresponding x-coordinate.

  The SSHFP Resource Record for the Ed448 public key with SHA-256
  fingerprint would, for example, be:

  example.com. IN SSHFP 6 2 ( a87f1b687ac0e57d2a081a2f2826723
                              34d90ed316d2b818ca9580ea384d924
                              01 )

  The '2' here indicates SHA-256 [RFC6594].

9.  IANA Considerations

  This document augments the Public Key Algorithm Names in [RFC4250],
  Section 4.11.3.

  IANA has added the following entry to "Public Key Algorithm Names" in
  the "Secure Shell (SSH) Protocol Parameters" registry [IANA-SSH]:

                +---------------------------+-----------+
                | Public Key Algorithm Name | Reference |
                +===========================+===========+
                | ssh-ed25519               | RFC 8709  |
                +---------------------------+-----------+
                | ssh-ed448                 | RFC 8709  |
                +---------------------------+-----------+

                                 Table 1

  IANA has added the following entry to "SSHFP RR Types for public key
  algorithms" in the "DNS SSHFP Resource Record Parameters" registry
  [IANA-SSHFP]:

                   +-------+-------------+-----------+
                   | Value | Description | Reference |
                   +=======+=============+===========+
                   | 6     | Ed448       | RFC 8709  |
                   +-------+-------------+-----------+

                                 Table 2

10.  Security Considerations

  The security considerations in [RFC4251], Section 9 apply to all SSH
  implementations, including those using Ed25519 and Ed448.

  The security considerations in [RFC8032], Section 8 and [RFC7479],
  Section 3 apply to all uses of Ed25519 and Ed448, including those in
  SSH.

11.  References

11.1.  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,
             <https://www.rfc-editor.org/info/rfc2119>.

  [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
             Protocol Assigned Numbers", RFC 4250,
             DOI 10.17487/RFC4250, January 2006,
             <https://www.rfc-editor.org/info/rfc4250>.

  [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
             Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
             January 2006, <https://www.rfc-editor.org/info/rfc4251>.

  [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
             Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
             January 2006, <https://www.rfc-editor.org/info/rfc4253>.

  [RFC4255]  Schlyter, J. and W. Griffin, "Using DNS to Securely
             Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
             DOI 10.17487/RFC4255, January 2006,
             <https://www.rfc-editor.org/info/rfc4255>.

  [RFC6594]  Sury, O., "Use of the SHA-256 Algorithm with RSA, Digital
             Signature Algorithm (DSA), and Elliptic Curve DSA (ECDSA)
             in SSHFP Resource Records", RFC 6594,
             DOI 10.17487/RFC6594, April 2012,
             <https://www.rfc-editor.org/info/rfc6594>.

  [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
             Signature Algorithm (EdDSA)", RFC 8032,
             DOI 10.17487/RFC8032, January 2017,
             <https://www.rfc-editor.org/info/rfc8032>.

  [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

  [ED448]    Hamburg, M., "Ed448-Goldilocks, a new elliptic curve",
             January 2015, <https://eprint.iacr.org/2015/625.pdf>.

  [IANA-SSH] IANA, "Secure Shell (SSH) Protocol Parameters",
             <https://www.iana.org/assignments/ssh-parameters>.

  [IANA-SSHFP]
             IANA, "DNS SSHFP Resource Record Parameters",
             <https://www.iana.org/assignments/dns-sshfp-rr-
             parameters>.

  [OpenSSH-6.5]
             Friedl, M., Provos, N., de Raadt, T., Steves, K., Miller,
             D., Tucker, D., McIntyre, J., Rice, T., and B. Lindstrom,
             "OpenSSH 6.5 release notes", January 2014,
             <http://www.openssh.com/txt/release-6.5>.

  [RFC7479]  Moonesamy, S., "Using Ed25519 in SSHFP Resource Records",
             RFC 7479, DOI 10.17487/RFC7479, March 2015,
             <https://www.rfc-editor.org/info/rfc7479>.

Acknowledgements

  The OpenSSH implementation of Ed25519 in SSH was written by Markus
  Friedl.  We are also grateful to Mark Baushke, Benjamin Kaduk, and
  Daniel Migault for their comments.

Authors' Addresses

  Ben Harris
  2A Eachard Road
  Cambridge
  CB3 0HY
  United Kingdom

  Email: [email protected]


  Loganaden Velvindron
  cyberstorm.mu
  88, Avenue De Plevitz
  Roches Brunes
  Mauritius

  Email: [email protected]

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