Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH) Protocol
RFC 8709
Ben Harris
2A Eachard Road
Cambridge
CB3 0HY
United Kingdom
bjh21@bjh21.me.uk
Loganaden Velvindron
cyberstorm.mu
88, Avenue De Plevitz
Roches Brunes
Mauritius
logan@cyberstorm.mu
February 2020
curdle
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.
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".
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.
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.
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.
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.
Signature Algorithm
Signatures are generated according to the procedure in Sections 5.1.6 and 5.2.6 of [RFC8032].
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.
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.
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].
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
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
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.
References
Normative References
Informative References
[IANA-SSH]

Secure Shell (SSH) Protocol Parameters
IANA
[IANA-SSHFP]

DNS SSHFP Resource Record Parameters
IANA
[OpenSSH-6.5]

OpenSSH 6.5 release notes
Marcus Friedl
Niels Provos
Theo de Raadt
Kevin Steves
Damien Miller
Darren Tucker
Jason McIntyre
Tim Rice
Ben Lindstrom
January 2014
[ED448]

Ed448-Goldilocks, a new elliptic curve
Mike Hamburg
January 2015
[RFC4251]
[RFC8032]
[RFC2119]
[RFC8174]
[RFC4253]
[RFC4255]
[RFC7479]
[RFC6594]
[RFC4250]
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.