Secure Shell (SSH) Key Exchange Method Using Curve25519 and Curve448
RFC 8731
| Document | Type | RFC - Proposed Standard (February 2020; No errata) | |
|---|---|---|---|
| Authors | Aris Adamantiadis , Simon Josefsson , Mark Baushke | ||
| Last updated | 2020-02-28 | ||
| Stream | Internent Engineering Task Force (IETF) | ||
| Formats | plain text html xml pdf htmlized (tools) htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Daniel Migault | ||
| Shepherd write-up | Show (last changed 2017-04-12) | ||
| IESG | IESG state | RFC 8731 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Yes | ||
| Telechat date | |||
| Responsible AD | Benjamin Kaduk | ||
| Send notices to | Daniel Migault <daniel.migault@ericsson.com> | ||
| IANA | IANA review state | IANA OK - Actions Needed | |
| IANA action state | RFC-Ed-Ack | ||
Internet Engineering Task Force (IETF) A. Adamantiadis
Request for Comments: 8731 libssh
Category: Standards Track S. Josefsson
ISSN: 2070-1721 SJD AB
M. Baushke
Juniper Networks, Inc.
February 2020
Secure Shell (SSH) Key Exchange Method Using Curve25519 and Curve448
Abstract
This document describes the specification for using Curve25519 and
Curve448 key exchange methods in the Secure Shell (SSH) protocol.
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/rfc8731.
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. Requirements Language
3. Key Exchange Methods
3.1. Shared Secret Encoding
4. Security Considerations
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Secure Shell (SSH) [RFC4251] is a secure remote login protocol. The
key exchange protocol described in [RFC4253] supports an extensible
set of methods. [RFC5656] defines how elliptic curves are integrated
into this extensible SSH framework, and this document reuses the
Elliptic Curve Diffie-Hellman (ECDH) key exchange protocol messages
defined in Section 7.1 (ECDH Message Numbers) of [RFC5656]. Other
parts of [RFC5656], such as Elliptic Curve Menezes-Qu-Vanstone
(ECMQV) key agreement and Elliptic Curve Digital Signature Algorithm
(ECDSA), are not considered in this document.
This document describes how to implement key exchange based on
Curve25519 and Curve448 [RFC7748] in SSH. For Curve25519 with
SHA-256 [RFC6234][SHS], the algorithm described is equivalent to the
privately defined algorithm "curve25519-sha256@libssh.org", which at
the time of publication was implemented and widely deployed in libssh
[libssh] and OpenSSH [OpenSSH]. The Curve448 key exchange method is
similar but uses SHA-512 [RFC6234][SHS].
2. 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. Key Exchange Methods
The key exchange procedure is similar to the ECDH method described in
Section 4 of [RFC5656], though with a different wire encoding used
for public values and the final shared secret. Public ephemeral keys
are encoded for transmission as standard SSH strings.
The protocol flow, the SSH_MSG_KEX_ECDH_INIT and
SSH_MSG_KEX_ECDH_REPLY messages, and the structure of the exchange
hash are identical to Section 4 of [RFC5656].
The method names registered by this document are "curve25519-sha256"
and "curve448-sha512".
The methods are based on Curve25519 and Curve448 scalar
multiplication, as described in [RFC7748]. Private and public keys
are generated as described therein. Public keys are defined as
strings of 32 bytes for Curve25519 and 56 bytes for Curve448.
The key-agreement schemes "curve25519-sha256" and "curve448-sha512"
perform the Diffie-Hellman protocol using the functions X25519 and
X448, respectively. Implementations SHOULD compute these functions
using the algorithms described in [RFC7748]. When they do so,
implementations MUST check whether the computed Diffie-Hellman shared
secret is the all-zero value and abort if so, as described in
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