SSH Support of ML-DSA
draft-sfluhrer-ssh-mldsa-07

sshm                                                          S. Fluhrer
Internet-Draft                                             Cisco Systems
Intended status: Informational                                 R. Petrov
Expires: 15 February 2027                                               
                                                                R. Appel
                                                         Bank of America
                                                          14 August 2026

                         SSH Support of ML-DSA
                      draft-sfluhrer-ssh-mldsa-07

Abstract

   This document describes the use of the ML-DSA digital signature
   algorithms in the Secure Shell (SSH) protocol.  Accordingly, this RFC
   updates RFC 4253.

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.

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
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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 . . . . . . . . . . . . . . . . . . . .   4
   4.  Public Key Format . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Signature Algorithm . . . . . . . . . . . . . . . . . . . . .   5
   6.  Signature Format  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Verification Algorithm  . . . . . . . . . . . . . . . . . . .   6
   8.  SSHFP DNS Resource Records  . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   A Cryptographically Relevant Quantum Computer (CRQC) is a quantum
   computer with sufficient compute capability and stability that it is
   able to break traditional asymmetric cryptographic algorithms: e.g
   RSA, ECDSA; which are currently the only authentication algorithms
   available for SSH.  NIST has recently published the post-quantum
   cryptography (PQC) algorithm known as ML-DSA [FIPS204] which is a

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   digital signature algorithm.

   This document describes how to use this algorithm for authentication
   within SSH [RFC4251], as a replacement for the traditional signature
   algorithms (RSA, ECDSA).

   This document standardizes the use of the names *ssh-mldsa-44*, *ssh-
   mldsa-65*, and *ssh-mldsa-87*. These algorithms correspond to
   Table 1.  "ML-DSA parameter sets" defined in [FIPS204] Section 4
   "Parameter Sets".

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 CRQC available to
   them.  There are three parameter sets defined for it which belong to
   a respective NIST Security Category of 2, 3, and 5 (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 computes the
   hash of the message internally and then signs that hash), 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.  This protocol always uses an empty (zero length)
   context.

   FIPS 204 also allows ML-DSA to be run in either deterministic or
   'hedged' mode (where randomness is applied during the signature
   generation operation).  We recommend that implementations use hedged
   mode, as it blocks certain side channel attacks.  However, as both
   are interoperable, we do not place any requirement on which is used
   within this protocol.

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.

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   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 algorithms are "ssh-mldsa-44",
   "ssh-mldsa-65" and "ssh-mldsa-87", to match the level 2, 3 and 5
   parameter sets [FIPS204].  These algorithm support only signing; they
   do not support encryption.

   The below table lists the public key sizes and the signature size (in
   bytes) for the three parameter sets.

     +===========================+=================+================+
     | Public Key Algorithm Name | Public Key Size | Signature Size |
     +===========================+=================+================+
     | ssh-mldsa-44              | 1312            | 2420           |
     +---------------------------+-----------------+----------------+
     | ssh-mldsa-65              | 1952            | 3309           |
     +---------------------------+-----------------+----------------+
     | ssh-mldsa-87              | 2592            | 4627           |
     +---------------------------+-----------------+----------------+

                                 Table 1

   Standard implementations of SSH SHOULD implement *mldsa-65* public
   Key algorithm.  It MAY implement *mldsa-44* and *mldsa-87* public key
   algorithms.

4.  Public Key Format

   *  The *mldsa-44* key format has the following encoding:

      string  ssh-mldsa-44
      string  key

      Here, 'key' is the 1312-octet public key encoded as is described
      in [FIPS204] Algorithm 22.

   *  The *mldsa-65* key format has the following encoding:

      string  ssh-mldsa-65
      string  key

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      Here, 'key' is the 1952-octet public key encoded as is described
      in [FIPS204] Algorithm 22.

   *  The *mldsa-87* key format has the following encoding:

      string  ssh-mldsa-87
      string  key

      Here, 'key' is the 2592-octet public key encoded as is described
      in [FIPS204] Algorithm 22.

5.  Signature Algorithm

   Signatures are generated according to the procedure described in
   Section 5.2 of [FIPS204], and verified according to the procedure
   described in Section 5.3, using an empty string as the context
   parameter.

6.  Signature Format

   *  The *mldsa-44* public key algorithm has the following format for
      encoding the signature:

      string  ssh-mldsa-44
      string  signature

      Here, 'signature' is the 2420-octet signature produced in
      accordance with [FIPS204] Algorithm 2.

   *  The *mldsa-65* public key algorithm has the following format for
      encoding the signature:

      string  ssh-mldsa-65
      string  signature

      Here, 'signature' is the 3309-octet signature produced in
      accordance with [FIPS204] Algorithm 2.

   *  The *mldsa-87* public key algorithm has the following format for
      encoding the signature:

      string  ssh-mldsa-87
      string  signature

      Here, 'signature' is the 4627-octet signature produced in
      accordance with [FIPS204] Algorithm 2.

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7.  Verification Algorithm

   Signatures are verified in two steps.  For the first step, the length
   of the signature must be checked against the parameter set, if the
   signature length does not match the expected signature length for the
   parameter set, it must be rejected.  Then the signature is verified
   according to the procedure in [FIPS204], algorithm 3, using the
   "pure" version of ML-DSA, with an empty context string.

8.  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 as above in section
   4.

   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.

9.  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]:

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               +===========================+===============+
               | Public Key Algorithm Name | Reference     |
               +===========================+===============+
               | ssh-mldsa-44              | This document |
               +---------------------------+---------------+
               | ssh-mldsa-65              | This document |
               +---------------------------+---------------+
               | ssh-mldsa-87              | This document |
               +---------------------------+---------------+

                                  Table 2

   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 document |
                  +-------+-------------+---------------+
                  | TBD2  | ML-DSA-65   | This document |
                  +-------+-------------+---------------+
                  | TBD3  | ML-DSA-87   | This document |
                  +-------+-------------+---------------+

                                  Table 3

10.  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.

11.  References

11.1.  Normative References

   [FIPS204]  "Module-Lattice-Based Digital Signature Standard",
              NIST FIPS 204, August 2024,
              <https://doi.org/10.6028/NIST.FIPS.204>.

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   [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>.

11.2.  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>.

   [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 the original
   template for this document.

Authors' Addresses

   Scott Fluhrer
   Cisco Systems
   Email: sfluhrer@cisco.com

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   Roumen Petrov
   1750 Sofia
   Bulgaria
   Email: pkixssh@roumenpetrov.info
   URI:   https://roumenpetrov.info/secsh/

   Ryan Appel
   Bank of America
   Email: ryan.appel@bofa.com

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