sshm S. Fluhrer
Internet-Draft Cisco Systems
Intended status: Informational 16 March 2025
Expires: 17 September 2025
SSH Support of ML-DSA
draft-sfluhrer-ssh-mldsa-00
Abstract
This document describes the use of ML-DSA digital signatures in the
Secure Shell (SSH) protocol.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://sfluhrer.github.io/ssh-mldsa/draft-sfluhrer-ssh-mldsa.html.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-sfluhrer-ssh-mldsa/.
Discussion of this document takes place on the Secure Shell
Maintenance Security Area mailing list (mailto:ssh@ietf.org), which
is archived at https://mailarchive.ietf.org/arch/browse/ssh/.
Subscribe at https://www.ietf.org/mailman/listinfo/ssh/.
Source for this draft and an issue tracker can be found at
https://github.com/sfluhrer/ssh-mldsa.
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This Internet-Draft will expire on 17 September 2025.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Background on ML-DSA . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Public Key Algorithms . . . . . . . . . . . . . . . . . . . . 3
4. Public Key Format . . . . . . . . . . . . . . . . . . . . . . 4
5. Signature Format . . . . . . . . . . . . . . . . . . . . . . 4
6. Verification Algorithm . . . . . . . . . . . . . . . . . . . 4
7. SSHFP DNS Resource Records . . . . . . . . . . . . . . . . . 4
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. Security Considerations . . . . . . . . . . . . . . . . . . . 6
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 6
10.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
A Cryptographically Relevant Quantum Computer (CRQC) could break
traditional asymmetric cryptograph algorithms: e.g RSA, ECDSA; which
are widely deployed authentication options of SSH. NIST has recently
published the postquantum digitial signature algorithm ML-DSA
[FIPS204].
This document describes how to use this algorithm for authentication
within SSH [RFC4251], as a replacement for the traditional signature
algorithms (RSA, ECDSA).
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1.1. Background on ML-DSA
ML-DSA (as specified in FIPS 204) is a signature algorithm that is
believed to be secure against attackers who have a Quantum Computer
available to them. There are three strengths defined for it (with
the parameter sets being known as ML-DSA-44, ML-DSA-65 and ML-DSA-
87). In addition, for each defined parameter set, there are two
versions, the 'pure' version (where ML-DSA directly signs the
message) and a 'prehashed' version (where ML-DSA signs a hash that
was computed outside of ML-DSA). For this protocol, we will always
use the pure version.
In addition, ML-DSA also has a 'context' input, which is a short
string that is common to the sender and the recceiver. It is
intended to allow for domain separation between separate uses of the
same public key.
FIPS 204 also allows ML-DSA to be run in either determanistic or
'hedged' mode (where randomness is applied to the signature
operation). We place no requirement on which is used; the
implementation should select based on the quality of their random
number source.
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.
The descriptions of key and signature formats use the notation
introduced in [RFC4251], Section 3, and the string data type from
[RFC4251], Section 5. Identifiers and terminology from ML-DSA
[FIPS204] are used throughout the document.
3. Public Key Algorithms
This document describes three public key algorithms for use with SSH,
as per [RFC4253], Section 6.6, corresponding to the three parameter
sets of ML-DSA. The names of the algorithm are "ssh-ml-dsa-44",
"ssh-ml-dsa-65" and "ssh-ml-dsa-87", to match the level 2, 3 and 5
parameter sets [FIPS204]. These algorithm only support signing and
not encryption.
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4. Public Key Format
The key format for all three parameter sets have the following
encoding:
string "ssh-mldsa44" (or "ssh-mldsa65" or "ssh-mldsa87")
string key
Here, 'key' is the public key described in [FIPS204].
# Signature Algorithm
Signatures are generated according to the procedure in Section 5.2
[FIPS204], using the "pure" version of ML-DSA, with an empty context
string.
5. Signature Format
The "ssh-mldsa" key format has the following encoding:
string "ssh-mldsa44" (or "ssh-mldsa65" or "ssh-mldsa87")
string signature
Here, 'signature' is the signature produced in accordance with the
previous section.
6. Verification Algorithm
Signatures are verified according to the procedure in [FIPS204],
Section 5.3, using the "pure" version of ML-DSA, with an empty
context strong.
7. SSHFP DNS Resource Records
Usage and generation of the SSHFP DNS resource record is described in
[RFC4255]. This section illustrates the generation of SSHFP resource
records for ML-DSA keys, and this document also specifies the
corresponding 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 keys for ML-DSA is described
as follows.
The encoding of ML-DSA public keys is described in [FIPS204].
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The SSHFP Resource Record for an ML-DSA key fingerprint (with a
SHA-256 fingerprint) would, for example, be:
pqserver.example.com. IN SSHFP TBD 2 (
a87f1b687ac0e57d2a081a2f28267237 34d90ed316d2b818ca9580ea384d9240 )
Replace TBD with the value eventually allocated by IANA.
8. IANA Considerations
This document augments the Public Key Algorithm Names in [RFC4250],
Section 4.11.3.
IANA is requested to add the following entries to "Public Key
Algorithm Names" in the "Secure Shell (SSH) Protocol Parameters"
registry [IANA-SSH]:
+===========================+==========+================+===========+
| Public Key Algorithm Name | Key Size | Signature | Reference |
| | | Size | |
+===========================+==========+================+===========+
| ssh-mldsa44 | 1312 | 2420 | THIS-RFC |
+---------------------------+----------+----------------+-----------+
| ssh-mldsa65 | 1952 | 3309 | THIS-RFC |
+---------------------------+----------+----------------+-----------+
| ssh-mldsa87 | 2592 | 4627 | THIS-RFC |
+---------------------------+----------+----------------+-----------+
Table 1
IANA is requested to add the following entries to "SSHFP RR Types for
public key algorithms" in the "DNS SSHFP Resource Record Parameters"
registry [IANA-SSHFP]:
+=======+=============+===========+
| Value | Description | Reference |
+=======+=============+===========+
| TBD1 | ML-DSA-44 | THIS RFC |
+-------+-------------+-----------+
| TBD2 | ML-DSA-65 | THIS RFC |
+-------+-------------+-----------+
| TBD3 | ML-DSA-87 | THIS RFC |
+-------+-------------+-----------+
Table 2
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9. Security Considerations
The security considerations in [RFC4251], Section 9 apply to all SSH
implementations, including those using ML-DSA.
The security considerations in ML-DSA [FIPS204] apply to all uses of
ML-DSA, including those in SSH.
Cryptographic algorithms and parameters are usually broken or
weakened over time. Implementers and users need to continously re-
evaluate that cryptographic algorithms continue to provide the
expected level of security.
10. References
10.1. Normative References
[FIPS204] "Module-Lattice-Based Digital Signature Standard",
NIST FIPS 204, August 2024,
<https://doi.org/10.6028/NIST.FIPS.204>.
[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/rfc/rfc2119>.
[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/rfc/rfc8174>.
10.2. Informative References
[IANA-SSH] "Secure Shell (SSH) Protocol Parameters", n.d.,
<https://www.iana.org/assignments/ssh-parameters>.
[IANA-SSHFP]
"DNS SSHFP Resource Record Parameters", n.d.,
<https://www.iana.org/assignments/dns-sshfp-rr-
parameters)>.
[RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250,
DOI 10.17487/RFC4250, January 2006,
<https://www.rfc-editor.org/rfc/rfc4250>.
[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/rfc/rfc4251>.
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[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/rfc/rfc4253>.
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
DOI 10.17487/RFC4255, January 2006,
<https://www.rfc-editor.org/rfc/rfc4255>.
Acknowledgments
The text of draft-josefsson-ssh-sphincs was used as a template for
this document.
Author's Address
Scott Fluhrer
Cisco Systems
Email: sfluhrer@cisco.com
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