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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.

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 .

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 () 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

• .  Introduction
• .  Conventions Used in This Document
  • .  Requirements Language
• .  Public Key Algorithm
• .  Public Key Format
• .  Signature Algorithm
• .  Signature Format
• .  Verification Algorithm
• .  SSHFP DNS Resource Records
• .  IANA Considerations
• . Security Considerations
• . References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Authors' Addresses

Introduction Secure Shell (SSH) 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 is a digital signature system. 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 and the string data type from .

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

string

"ssh-ed448"

string

key

Here, 'key' is the 57-octet public key described in .

Signature Algorithm Signatures are generated according to the procedure in Sections and of .

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 . 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 .

Verification Algorithm Ed25519 signatures are verified according to the procedure in . Ed448 signatures are verified according to the procedure in .

SSHFP DNS Resource Records Usage and generation of the SSHFP DNS resource record is described in . The generation of SSHFP resource records for "ssh-ed25519" keys is described in . 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 . The generation of SSHFP resource records for "ssh-ed448" keys is described as follows. The encoding of Ed448 public keys is described in . 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 .

IANA Considerations This document augments the Public Key Algorithm Names in . IANA has added the following entry to "Public Key Algorithm Names" in the "Secure Shell (SSH) Protocol Parameters" registry :

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 :

Value	Description	Reference
6	Ed448	RFC 8709

Security Considerations The security considerations in apply to all SSH implementations, including those using Ed25519 and Ed448. The security considerations in and apply to all uses of Ed25519 and Ed448, including those in SSH.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document defines the instructions to the IANA and the initial state of the IANA assigned numbers for the Secure Shell (SSH) protocol. It is intended only for the initialization of the IANA registries referenced in the set of SSH documents. [STANDARDS-TRACK] The Secure Shell (SSH) Protocol is a protocol for secure remote login and other secure network services over an insecure network. This document describes the architecture of the SSH protocol, as well as the notation and terminology used in SSH protocol documents. It also discusses the SSH algorithm naming system that allows local extensions. The SSH protocol consists of three major components: The Transport Layer Protocol provides server authentication, confidentiality, and integrity with perfect forward secrecy. The User Authentication Protocol authenticates the client to the server. The Connection Protocol multiplexes the encrypted tunnel into several logical channels. Details of these protocols are described in separate documents. [STANDARDS-TRACK] The Secure Shell (SSH) is a protocol for secure remote login and other secure network services over an insecure network. This document describes the SSH transport layer protocol, which typically runs on top of TCP/IP. The protocol can be used as a basis for a number of secure network services. It provides strong encryption, server authentication, and integrity protection. It may also provide compression. Key exchange method, public key algorithm, symmetric encryption algorithm, message authentication algorithm, and hash algorithm are all negotiated. This document also describes the Diffie-Hellman key exchange method and the minimal set of algorithms that are needed to implement the SSH transport layer protocol. [STANDARDS-TRACK] This document describes a method of verifying Secure Shell (SSH) host keys using Domain Name System Security (DNSSEC). The document defines a new DNS resource record that contains a standard SSH key fingerprint. [STANDARDS-TRACK] This document updates the IANA registries in RFC 4255, which defines SSHFP, a DNS Resource Record (RR) that contains a standard Secure Shell (SSH) key fingerprint used to verify SSH host keys using DNS Security Extensions (DNSSEC). This document defines additional options supporting SSH public keys applying the Elliptic Curve Digital Signature Algorithm (ECDSA) and the implementation of fingerprints computed using the SHA-256 message digest algorithm in SSHFP Resource Records. [STANDARDS-TRACK] This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References IANA IANA The Ed25519 signature algorithm has been implemented in OpenSSH. This document updates the IANA "SSHFP RR Types for public key algorithms" registry by adding an algorithm number for Ed25519.

Acknowledgements The OpenSSH implementation of Ed25519 in SSH was written by . We are also grateful to , , and for their comments.

Authors' Addresses

2A Eachard Road Cambridge CB3 0HY United Kingdom bjh21@bjh21.me.uk

cyberstorm.mu

88, Avenue De Plevitz Roches Brunes Mauritius logan@cyberstorm.mu