Secure Shell Key Exchange Method Using Hybrid Classic McEliece and X25519 with SHA-512: mceliece6688128x25519-sha512
draft-josefsson-ssh-mceliece-00
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Simon Josefsson | ||
| Last updated | 2023-12-26 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
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draft-josefsson-ssh-mceliece-00
Network Working Group S. Josefsson
Internet-Draft 26 December 2023
Intended status: Standards Track
Expires: 28 June 2024
Secure Shell Key Exchange Method Using Hybrid Classic McEliece and
X25519 with SHA-512: mceliece6688128x25519-sha512
draft-josefsson-ssh-mceliece-00
Abstract
This document specify a hybrid key exchange method in the Secure
Shell (SSH) protocol based on Classic McEliece (mceliece6688128) and
X25519 with SHA-512.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-josefsson-ssh-mceliece/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/jas/ietf-ssh-mceliece.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 28 June 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Key Exchange Method: mceliece6688128x25519-sha512 . . . . . . 3
4. mceliece6688128f . . . . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 4
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 4
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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 [RFC8731] specify
"curve25519-sha256" to support the pre-quantum elliptic-curve Diffie-
Hellman X25519 function [RFC7748]. In [I-D.josefsson-ntruprime-ssh]
it is described how the post-quantum lattice-based Streamlined NTRU
Prime is combined with X25519 for SSH, and we base our protocol and
document on it but replace sntrup761 with mceliece6688128.
Classic McEliece [I-D.josefsson-mceliece] [CM-spec] provides a code-
based Key Encapsulation Method designed to be safe even against
quantum computers. The variant "mceliece6688128" offers a balance
between performance and output sizes.
To hedge against attacks on either of mceliece6688128 or X25519 a
hybrid construction may be used, with the intention that the hybrid
would be secure if either of the involved algorithms are flawed.
This document specify how to implement key exchange based on a hybrid
between Classic McEliece mceliece6688128 and X25519 with SHA-512
[RFC6234] in SSH.
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2. Conventions and Definitions
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 Method: mceliece6688128x25519-sha512
The key-agreement is done by the X25519 Diffie-Hellman protocol as
described in Section 3 (Key Exchange Methods) of [RFC8731], and the
mceliece6688128 key encapsulation method described in
[I-D.josefsson-mceliece] [CM-spec].
The key exchange procedure reuse the Elliptic Curve Diffie-Hellman
(ECDH) key exchange defined in Sections 4 (ECDH Key Exchange) and 7.1
(ECDH Message Numbers) of [RFC5656]. The protocol flow and the
SSH_MSG_KEX_ECDH_INIT and SSH_MSG_KEX_ECDH_REPLY messages are
identical, except that we use different ephemeral public values Q_C
and Q_S and shared secret K as described below.
The SSH_MSG_KEX_ECDH_INIT value Q_C that holds the client's ephemeral
public key MUST be constructed by concatenating the 1044992 byte
public key output from the key generator of mceliece6688128 (or
mceliece6688128f, see Section 4) with the 32 byte K_A = X25519(a, 9)
as described in [I-D.josefsson-mceliece] [CM-spec] and [RFC8731].
The Q_C value is thus 1045024 bytes.
The SSH_MSG_KEX_ECDH_REPLY value Q_S that holds the server's
ephemeral public key MUST be constructed by concatenating the 208
byte ciphertext output from the key encapsulation mechanism of
mceliece6688128 (or mceliece6688128f, see Section 4) with the 32 byte
K_B = X25519(b, 9) as described in [I-D.josefsson-mceliece] [CM-spec]
and [RFC8731]. The Q_S value is thus 240 bytes.
Clients and servers MUST abort if the length of the received public
keys Q_C or Q_S are not the expected lengths. An abort for these
purposes is defined as a disconnect (SSH_MSG_DISCONNECT) of the
session and SHOULD use the SSH_DISCONNECT_KEY_EXCHANGE_FAILED reason
for the message, see Section 11.1 (Disconnection Message) of
[RFC4253]. No further validation is required beyond what is
described in [RFC7748], [RFC8731] and [I-D.josefsson-mceliece]
[CM-spec].
The SSH_MSG_KEX_ECDH_REPLY signature value is computed as described
in [RFC5656] with the following changes. Instead of encoding the
shared secret K as 'mpint', it MUST be encoded as 'string'. The
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shared secret K value MUST be the 64-byte output octet string of the
SHA-512 hash computed with the input as the 32-byte octet string key
output from the key encapsulation mechanism of mceliece6688128 (or
mceliece6688128f, see Section 4) concatenated with the 32-byte octet
string of X25519(a, X25519(b, 9)) = X25519(b, X25519(a, 9)).
4. mceliece6688128f
The f and non-f versions are interoperable. The f versions have
faster key generation, while the non-f versions have simpler key
generation. For example, a key generated with mceliece6688128f can
decapsulate ciphertexts that were encapsulated with mceliece6688128,
and vice versa. The secret-key sizes (and formats) are the same, the
encapsulation functions are the same, and the decapsulation functions
are the same.
Implementations of this protocol can chose between mceliece6688128 or
mceliece6688128f, however the name of this protocol is
"mceliece6688128x25519-sha512" even for implementations that use
mceliece6688128f internally.
Choosing mceliece6688128 generally reduce code size and complexity
(at the expense of performance), and choosing mceliece6688128f
generally improve performance (at the expense of code size and
complexity).
5. Acknowledgments
The protocol and document is based on [I-D.josefsson-ntruprime-ssh].
The authors would like to thank Daniel J. Bernstein for discussion
and suggesting the mceliece6688128 variant.
6. Implementation Status
One implementation of this protocol is available as a patch
[OpenSSH-McEliece-patch] for OpenSSH [OpenSSH], released under a BSD-
style license.
7. Security Considerations
The security considerations of [RFC4251], [RFC5656], [RFC7748],
[RFC8731] and [I-D.josefsson-mceliece] [CM-spec] [CM-security]
[CM-impl] are inherited.
Classic McEliece is a KEM designed for IND-CCA2 security at a very
high security level, even against quantum computers. The algorithm
has been studied by researchers for many years, and there are
implementations in the public domain for a wide range of
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architectures. However new cryptographic primitives should be
introduced and trusted conservatively, and new research findings may
be published at any time that may warrant implementation
reconsiderations. The method described here to combine X25519 with
mceliece6688128, i.e., SHA-512 hashing the concatenated outputs, is
also available for the same kind of cryptographic scrutiny.
The increase in communication size and computational requirements may
be a concern for limited computational devices, which would then not
be able to take advantage of the improved security properties offered
by this work.
As discussed in the security considerations of Curve25519-sha256
[RFC8731], the X25519 shared secret K is used bignum-encoded in that
document, and this raise a potential for a hash-processing time side-
channel that could leak one bit of the secret due to different length
of the bignum sign pad. This document resolve that problem by using
string-encoding instead of bignum-encoding.
8. IANA Considerations
IANA is requested to add a new "Method Name" of
"mceliece6688128x25519-sha512" to the "Key Exchange Method Names"
registry for Secure Shell (SSH) Protocol Parameters [IANA-KEX] with a
"reference" field to this RFC and the "OK to implement" field of
"MAY".
9. References
9.1. 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>.
[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>.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer",
RFC 5656, DOI 10.17487/RFC5656, December 2009,
<https://www.rfc-editor.org/info/rfc5656>.
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[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>.
[RFC8731] Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure
Shell (SSH) Key Exchange Method Using Curve25519 and
Curve448", RFC 8731, DOI 10.17487/RFC8731, February 2020,
<https://www.rfc-editor.org/info/rfc8731>.
9.2. Informative References
[CM-impl] Classic McEliece Team, "Classic McEliece: conservative
code-based cryptography: guide for implementors", October
2022,
<https://classic.mceliece.org/mceliece-impl-20221023.pdf>.
[CM-security]
Classic McEliece Team, "Classic McEliece: conservative
code-based cryptography: guide for security reviewers",
October 2022, <https://classic.mceliece.org/mceliece-
security-20221023.pdf>.
[CM-spec] Classic McEliece Team, "Classic McEliece: conservative
code-based cryptography: cryptosystem specification",
October 2022,
<https://classic.mceliece.org/mceliece-spec-20221023.pdf>.
[I-D.josefsson-mceliece]
Josefsson, S., "Classic McEliece", Work in Progress,
Internet-Draft, draft-josefsson-mceliece-00, 13 October
2023, <https://datatracker.ietf.org/doc/html/draft-
josefsson-mceliece-00>.
[I-D.josefsson-ntruprime-ssh]
Friedl, M., Mojzis, J., and S. Josefsson, "Secure Shell
(SSH) Key Exchange Method Using Hybrid Streamlined NTRU
Prime sntrup761 and X25519 with SHA-512:
sntrup761x25519-sha512", Work in Progress, Internet-Draft,
draft-josefsson-ntruprime-ssh-02, 19 September 2023,
<https://datatracker.ietf.org/doc/html/draft-josefsson-
ntruprime-ssh-02>.
[IANA-KEX] IANA, "Secure Shell (SSH) Protocol Parameters: Key
Exchange Method Names", n.d.,
<https://www.iana.org/assignments/ssh-parameters/>.
[OpenSSH] OpenSSH team, "OpenSSH", n.d., <https://www.openssh.com/>.
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[OpenSSH-McEliece-patch]
OpenSSH team, Simon Josefsson, "GitLab branch of OpenSSH
with McEliece support", n.d., <https://gitlab.com/jas/
openssh-portable/-/tree/jas/mceliece>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
Author's Address
Simon Josefsson
Email: simon@josefsson.org
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