SSH Ed25519/Ed448: Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH) Protocol
Names and wire formats for using Ed25519 and Ed448 signatures as SSH public key algorithms.
Harris, B. and L. Velvindron, "Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH) Protocol", RFC 8709, DOI 10.17487/RFC8709, February 2020, <https://www.rfc-editor.org/info/rfc8709>.
@misc{rfc8709,
series = {Request for Comments},
number = 8709,
howpublished = {RFC 8709},
publisher = {RFC Editor},
doi = {10.17487/RFC8709},
url = {https://www.rfc-editor.org/info/rfc8709},
author = {B. Harris and L. Velvindron},
title = {{Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH) Protocol}},
pagetotal = 7,
year = 2020,
month = feb,
} RFC text
Contents
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: bjh21@bjh21.me.uk
Loganaden Velvindron
cyberstorm.mu
88, Avenue De Plevitz
Roches Brunes
Mauritius
Email: logan@cyberstorm.mu
AI ELI5
On this page
§1Why Name Ed25519 and Ed448 in SSH
Secure Shell (SSH) is a secure remote-login protocol, and it lets implementations choose among an extensible variety of public key algorithms for identifying servers and users to one another. Ed25519 is a digital signature system. OpenSSH 6.5 introduced support for using it for server and user authentication, and other SSH implementations followed.
This RFC describes the method that OpenSSH and others implemented and formalizes the name ssh-ed25519. It also describes Ed448 and formalizes the name ssh-ed448. The RFC updates RFC 4253, the SSH transport layer specification.
People were already using one kind of SSH signature, but nobody had written the rules down. This RFC writes them down and adds a second kind.
§3The Two Public Key Algorithms
Both algorithms plug into SSH as public key algorithms in the sense of RFC 4253, Section 6.6. They are named ssh-ed25519 and ssh-ed448.
Each algorithm supports signing only, not encryption. The RFC says standard implementations of SSH SHOULD implement these signature algorithms, so support is encouraged but not strictly required.
These keys can only sign things, like a stamp. They cannot be used to lock a secret message. Programs are encouraged to know how to use both.
§4Key and Signature Encodings
A public key is encoded as two SSH strings: the algorithm name, then the key itself. For ssh-ed25519 the key is the 32-octet public key from RFC 8032, Section 5.1.5. For ssh-ed448 it is the 57-octet public key from Section 5.2.5.
A signature uses the same two-string layout: the algorithm name, then the signature. The ssh-ed25519 signature is 64 octets, produced as in RFC 8032 Section 5.1.6. The ssh-ed448 signature is 114 octets, produced as in Section 5.2.6.
The RFC does not define new signing math. Signatures are generated by the procedures in Sections 5.1.6 and 5.2.6 of RFC 8032, and verified by Sections 5.1.7 (Ed25519) and 5.2.7 (Ed448).
Each key or signature is a little package with a label saying which kind it is, followed by the real bytes. The math for making and checking them is borrowed from another RFC.
- ssh-ed25519 key32 octets — Public key from RFC 8032, Section 5.1.5.
- ssh-ed25519 signature64 octets — Signature from RFC 8032, Section 5.1.6.
- ssh-ed448 key57 octets — Public key from RFC 8032, Section 5.2.5.
- ssh-ed448 signature114 octets — Signature from RFC 8032, Section 5.2.6.
§8SSHFP DNS Records for Ed448
The SSHFP DNS resource record publishes SSH key fingerprints and is described in RFC 4255. Generating SSHFP records for ssh-ed25519 keys is already covered by RFC 7479, so this RFC covers Ed448.
An Ed448 public key is a 57-octet value representing a 455-bit y-coordinate of an elliptic curve point, plus a sign bit for the corresponding x-coordinate. The RFC shows an example record for an Ed448 key with a SHA-256 fingerprint: example.com. IN SSHFP 6 2 ( ... ). The 6 is the Ed448 algorithm code point and the 2 indicates SHA-256.
A website can list the fingerprint of its key in a public phone book, so you can check you are talking to the right machine. This RFC says how to list the fingerprint for the new Ed448 kind of key.
§9IANA Registrations
IANA added ssh-ed25519 and ssh-ed448 to the Public Key Algorithm Names in the "Secure Shell (SSH) Protocol Parameters" registry, both referencing RFC 8709. This augments the names defined in RFC 4250, Section 4.11.3.
IANA also added value 6, described as Ed448, to the "SSHFP RR Types for public key algorithms" registry.
The official list of SSH signature names got two new entries, and the list of fingerprint types got one.
§10Security Considerations
The RFC adds no new security text of its own. The security considerations in RFC 4251, Section 9 apply to all SSH implementations, including those using Ed25519 and Ed448.
The considerations in RFC 8032, Section 8 and RFC 7479, Section 3 apply to all uses of Ed25519 and Ed448, including their use in SSH.
The safety advice already written for SSH and for these signatures still applies. Nothing new is added.