



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
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   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: bjh21@bjh21.me.uk


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

   Email: logan@cyberstorm.mu
