Ed25519 and Ed448 public key algorithms for the Secure Shell (SSH) protocol
draft-ietf-curdle-ssh-ed25519-ed448-09

Abstract

This document describes the use of the Ed25519 and Ed448 digital signature algorithm in the Secure Shell (SSH) protocol.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on February 1, 2020.

Copyright Notice

Copyright (c) 2019 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.

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 its use of the name "ssh-ed25519". Additionally, it also describes the use of Ed448 and formalizes its use of the name "ssh-ed448".

[TO BE REMOVED: Please send comments on this draft to curdle@ietf.org.]

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", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

3. Public Key Algorithm

This document describes a public key algorithm for use with SSH in accordance with [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 by [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 by [RFC8032], Section 5.2.5.

5. Signature Algorithm

Signatures are generated according to the procedure in [RFC8032], Section 5.2.6.

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 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 the document specifies the corresponding Ed448 code point to the "SSHFP RR Types for public key algorithms" IANA registry.

The generation of SSHFP resource records for "ssh-ed25519" keys is described in [RFC7479].

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 the corresponding x-coordinate.

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

example.com. IN SSHFP TBD 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 4.6.2.

IANA is requested to add to the Public Key Algorithm Names registry [IANA-PKA] with the following entry:

IANA is requested to add the following entry to the "SSHFP RR Types for public key algorithms" registry [IANA-SSHFP]:

+--------+-------------+------------+

| Value | Description | Reference |

+--------+-------------+------------+

| TBD | Ed448 | [this-draft] |

+--------+-------------+------------+

We suggest 6 as value.

[TO BE REMOVED: This registration should take place at the following location: <http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xhtml#ssh-parameters-19>]

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] apply to all uses of Ed25519 and Ed448 including those in SSH.

11. 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.

12. References

12.1. Normative References

12.2. Informative References

Authors' Addresses

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. Acknowledgements
• 12. References
• 12.1. Normative References
• 12.2. Informative References
• Authors' Addresses