Use of the SLH-DSA Signature Algorithm in the Cryptographic Message Syntax (CMS)
draft-ietf-lamps-cms-sphincs-plus-17

Network Working Group                                         R. Housley
Internet-Draft                                            Vigil Security
Intended status: Standards Track                              S. Fluhrer
Expires: 3 June 2025                                       Cisco Systems
                                                           P. Kampanakis
                                                     Amazon Web Services
                                                           B. Westerbaan
                                                              Cloudflare
                                                        30 November 2024

  Use of the SLH-DSA Signature Algorithm in the Cryptographic Message
                              Syntax (CMS)
                  draft-ietf-lamps-cms-sphincs-plus-17

Abstract

   SLH-DSA is a stateless hash-based signature scheme.  This document
   specifies the conventions for using the SLH-DSA signature algorithm
   with the Cryptographic Message Syntax (CMS).  In addition, the
   algorithm identifier and public key syntax are provided.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 3 June 2025.

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   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  ASN.1 . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  SLH-DSA Hash-based Signature Algorithm Overview . . . . . . .   3
   3.  SLH-DSA Public Key Identifier . . . . . . . . . . . . . . . .   4
   4.  Signed-data Conventions . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Appendix: ASN.1 Module . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   This document specifies the conventions for using the SLH-DSA hash-
   based signature algorithm [FIPS205] with the Cryptographic Message
   Syntax (CMS) [RFC5652] signed-data content type.

   SLH-DSA offers two signature modes: pure mode and pre-hash mode.
   SLH-DSA signature operations include a context string as input.  The
   context string has a maximum length of 255 bytes.  By default, the
   context string is the empty string.  This document only specifies the
   use of pure mode with an empty context string for the CMS signed-data
   content type.

   SLH-DSA offers three security levels.  The parameters for each of the
   security levels were chosen to provide 128 bits of security, 192 bits
   of security, and 256 bits of security.  Separate algorithm
   identifiers have been assigned for SLH-DSA at each of these security
   levels.

   SLH-DSA is a stateless hash-based signature algorithm.  Other hash-
   based signature algorithms are stateful, including HSS/LMS [RFC8554]
   and XMSS [RFC8391].  Without the need for state kept by the signer,
   SLH-DSA is much less fragile.

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1.1.  ASN.1

   CMS values are generated using ASN.1 [X680], using the Basic Encoding
   Rules (BER) and the Distinguished Encoding Rules (DER) [X690].

1.2.  Motivation

   There have been recent advances in cryptanalysis and advances in the
   development of quantum computers.  Each of these advances pose a
   threat to widely deployed digital signature algorithms.

   If cryptographically relevant quantum computers (CRQC) are ever
   built, they will be able to break many of the public-key
   cryptosystems currently in use, including RSA, DSA, ECDSA, and EdDSA.
   A post-quantum cryptosystem (PQC) is secure against quantum computers
   that have more than a trivial number of quantum bits (qu-bits).  It
   is open to conjecture when it will be feasible to build such quantum
   computers; however, it is prudent to use cryptographic algorithms
   that remain secure if a CRQC is invented.  SLH-DSA is a PQC signature
   algorithm.

   One use of a PQC signature algoritm is the protection of software
   updates, perhaps using the format described in [RFC4108], to enable
   deployment of software that implements other new PQC algorithms for
   key management and confidentiality.

1.3.  Terminology

   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.

2.  SLH-DSA Hash-based Signature Algorithm Overview

   SLH-DSA is a hash-based signature scheme which consists of a few time
   signature construction, namely Forest of Random Subsets (FORS) and a
   hypertree.  FORS signs a message with a private key.  The
   corresponding FORS public keys are the leaves in k binary trees.  The
   roots of these trees are hashed together to form a FORS root.  SLH-
   DSA uses a one-time signature scheme called WOTS+. The FORS tree
   roots are signed by a WOTS+ one-time signature private key.  The
   corresponding WOTS+ public keys form the leaves in d-layers of Merkle
   subtrees in the SLH-DSA hypertree.  The bottom layer of that
   hypertree signs the FORS roots with WOTS+. The root of the bottom
   Merkle subtrees are then signed with WOTS+ and the corresponding
   WOTS+ public keys form the leaves of the next level up subtree.

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   Subtree roots are consequently signed by their corresponding subtree
   layers until we reach the top subtree.  The top layer subtree forms
   the hypertree root which is trusted at the verifier.

   A SLH-DSA signature consists of the randomization string, the FORS
   signature, the WOTS+ signature in each layer, and the path to the
   root of each subtree until the root of the hypertree is reached.

   A SLH-DSA signature is verified by verifying the FORS signature, the
   WOTS+ signatures and the path to the root of each subtree.  When
   reaching the root of the hypertree, the signature verifies only if it
   hashes to the pre-trusted root of the SLH-DSA hypertree.

   SLH-DSA is a stateless hash-based signature algorithm.  Stateful
   hash-based signature schemes require that the WOTS+ private key
   (generated by using a state index) is never reused or the scheme
   loses it security.  Although its security decreases, FORS which is
   used at the bottom of the SLH-DSA hypertree does not collapse if the
   same private key used to sign two or more different messages like in
   stateful hash-based signature schemes.  Without the need for state
   kept by the signer to ensure it is not reused, SLH-DSA is much less
   fragile.

   SLH-DSA was designed to sign up to 2^64 messages and offers three
   security levels.  The parameters of the SLH-DSA hypertree include the
   security parameter, the hash function, the tree height, the number of
   layers of subtrees, the Winternitz parameter of WOTS+, the number of
   FORS trees and leaves in each.  The parameters for each of the
   security levels were chosen to at least as secure as a generic block
   cipher of 128, 192, or 256 bits.

3.  SLH-DSA Public Key Identifier

   The AlgorithmIdentifier for a SLH-DSA public key MUST use one of the
   twelve id-slh-dsa object identifiers listed below, based on the
   security level used to generate the SLH-DSA hypertree, the small or
   fast version of the algorithm, and the use of SHA2 [FIPS180] or SHAKE
   [FIPS202].  For example, id-slh-dsa-shake-256s represents the 256-bit
   security level, the small version of the algorithm, and the use of
   SHAKE256.  The parameters field of the AlgorithmIdentifier for the
   SLH-DSA public key MUST be absent.

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      nistAlgorithms OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
        country(16) us(840) organization(1) gov(101) csor(3) 4 }

      sigAlgs OBJECT IDENTIFIER ::= { nistAlgorithms 3 }

      id-slh-dsa-sha2-128s OBJECT IDENTIFIER ::= { sigAlgs 20 }

      id-slh-dsa-sha2-128f OBJECT IDENTIFIER ::= { sigAlgs 21 }

      id-slh-dsa-sha2-192s OBJECT IDENTIFIER ::= { sigAlgs 22 }

      id-slh-dsa-sha2-192f OBJECT IDENTIFIER ::= { sigAlgs 23 }

      id-slh-dsa-sha2-256s OBJECT IDENTIFIER ::= { sigAlgs 24 }

      id-slh-dsa-sha2-256f OBJECT IDENTIFIER ::= { sigAlgs 25 }

      id-slh-dsa-shake-128s OBJECT IDENTIFIER ::= { sigAlgs 26 }

      id-slh-dsa-shake-128f OBJECT IDENTIFIER ::= { sigAlgs 27 }

      id-slh-dsa-shake-192s OBJECT IDENTIFIER ::= { sigAlgs 28 }

      id-slh-dsa-shake-192f OBJECT IDENTIFIER ::= { sigAlgs 29 }

      id-slh-dsa-shake-256s OBJECT IDENTIFIER ::= { sigAlgs 30 }

      id-slh-dsa-shake-256f OBJECT IDENTIFIER ::= { sigAlgs 31 }

   When this AlgorithmIdentifier appears in the SubjectPublicKeyInfo
   field of an X.509 certificate [RFC5280], the certificate key usage
   extension MAY contain digitalSignature, nonRepudiation, keyCertSign,
   and cRLSign; the certificate key usage extension MUST NOT contain
   other values.

      pk-slh-dsa-sha2-128s PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-sha2-128s
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      pk-slh-dsa-sha2-128f PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-sha2-128f
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

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      pk-slh-dsa-sha2-192s PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-sha2-192s
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      pk-slh-dsa-sha2-192f PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-sha2-192f
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      pk-slh-dsa-sha2-256s PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-sha2-256s
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      pk-slh-dsa-sha2-256f PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-sha2-256f
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      pk-slh-dsa-shake-128s PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-shake-128s
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      pk-slh-dsa-shake-128f PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-shake-128f
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      pk-slh-dsa-shake-192s PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-shake-192s
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

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      pk-slh-dsa-shake-192f PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-shake-192f
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      pk-slh-dsa-shake-256s PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-shake-256s
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      pk-slh-dsa-shake-256f PUBLIC-KEY ::= {
          IDENTIFIER id-slh-dsa-shake-256f
          -- KEY no ASN.1 wrapping --
          CERT-KEY-USAGE
            { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
          -- PRIVATE-KEY no ASN.1 wrapping -- }

      SLH-DSA-PublicKey ::= OCTET STRING (SIZE (32 | 48 | 64))

      SLH-DSA-PrivateKey ::= OCTET STRING (SIZE (64 | 96 | 128))

   No additional encoding of the SLH-DSA public key is applied in the
   SubjectPublicKeyInfo field of an X.509 certificate [RFC5280].

   No additional encoding of the SLH-DSA private key is applied in the
   PrivateKeyInfo field of the privateKey field of the OneAsymmetricKey
   type of an Asymmetric Key Package [RFC5958].

   When a SLH-DSA public key appears outside of a SubjectPublicKeyInfo
   type in an environment that uses ASN.1 encoding, the SLH-DSA public
   key can be encoded as an OCTET STRING by using the SLH-DSA-PublicKey
   type.

   When a SLH-DSA private key appears outside of an Asymmetric Key
   Package in an environment that uses ASN.1 encoding, the SLH-DSA
   private key can be encoded as an OCTET STRING by using the SLH-DSA-
   PrivateKey type.

4.  Signed-data Conventions

   As specified in CMS [RFC5652], the digital signature is produced from
   the message digest and the signer's private key.  The signature is
   computed over different values depending on whether signed attributes
   are absent or present.

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   When signed attributes are absent, the SLH-DSA (pure mode) signature
   is computed over the content.  When signed attributes are present, a
   hash MUST be computed over the content using the same hash function
   that is used in the SLH-DSA tree.  The signed attributes MUST include
   a content-type attribute and a message-digest attribute.  The
   message-digest attribute contains the hash value of the content.  The
   SLH-DSA signature is computed over the DER encoding of the set of
   signed attributes.  The SLH-DSA signature generation operation is
   called slh_sign; see Section 10.2.1 of [FIPS205].  In summary:

      IF (signed attributes are absent)
      THEN slh_sign(content)
      ELSE message-digest attribute = Hash(content);
           slh_sign(DER(SignedAttributes))

   In some implementations, performance may be significantly improved by
   signing and verifying DER(SignedAttributes) when the content is
   large.  That is, passing an entire large message content to the
   signing function or the signature validation function can have an
   impact on performance.  When the signed attributes are present,
   Section 5.3 of [RFC5652] requires the inclusion of the content-type
   attribute and the message-digest attribute.  Other attributes can
   also be included.

   When using SLH-DSA and signed attributes are present in the
   SignerInfo, the digestAlgorithms field in the SignedData MUST include
   the identifier for the one-way hash function used to compute the
   message digest.

   When using SLH-DSA, the fields in the SignerInfo are used as follows:

   digestAlgorithm:
      The digestAlgorithm MUST identify a one-way hash function.  When
      signed attributes are absent, the digestAlgorithm identifier MUST
      match the hash function used in the SLH-DSA tree (as shown in the
      list below).  When signed attributes are present, to ensure
      collision resistance, the identified hash function SHOULD produce
      a hash value that is at least twice the size of the hash function
      used in the SLH-DSA tree.  The hash functions defined in [FIPS180]
      and [FIPS202] MUST be supported for use with the variants of SLH-
      DSA as shown below:

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         id-slh-dsa-sha2-128s:  SHA-256
         id-slh-dsa-sha2-128f:  SHA-256
         id-slh-dsa-sha2-192s:  SHA-512
         id-slh-dsa-sha2-192f:  SHA-512
         id-slh-dsa-sha2-256s:  SHA-512
         id-slh-dsa-sha2-256f:  SHA-512
         id-slh-dsa-shake-128s: SHAKE128 with 256 bit output
         id-slh-dsa-shake-128f: SHAKE128 with 256 bit output
         id-slh-dsa-shake-192s: SHAKE256 with 512 bit output
         id-slh-dsa-shake-192f: SHAKE256 with 512 bit output
         id-slh-dsa-shake-256s: SHAKE256 with 512 bit output
         id-slh-dsa-shake-256f: SHAKE256 with 512 bit output

         The object identifiers for SHA-256 and SHA-512 are included
         in [RFC5754].  The object identifiers for SHAKE128 and
         SHAKE256 are included in [RFC8702].  In all four cases, the
         AlgorithmIdentifier SHOULD NOT include parameters.

   signatureAlgorithm:
      The signatureAlgorithm MUST contain one of the the SLH-DSA
      algorithm identifiers, and the algorithm parameters field MUST be
      absent.  The algorithm identifier MUST be one of the following:

         id-slh-dsa-sha2-128s,  id-slh-dsa-sha2-128f,
         id-slh-dsa-sha2-192s,  id-slh-dsa-sha2-192f,
         id-slh-dsa-sha2-256s,  id-slh-dsa-sha2-256f,
         id-slh-dsa-shake-128s, id-slh-dsa-shake-128f,
         id-slh-dsa-shake-192s, id-slh-dsa-shake-192f,
         id-slh-dsa-shake-256s, id-slh-dsa-shake-256f.

   signature:
      The signature contains the signature value resulting from the SLH-
      DSA signing operation with the parameters associated with the
      selected signatureAlgorithm.  The SLH-DSA signature generation
      operation is specified in Section 10.2.1 of [FIPS205], and the
      SLH-DSA signature verification operation is specified in
      Section 10.3 of [FIPS205].  Signature verification MUST include
      checking that the signatureAlgorithm field identifies SLH-DSA
      parameters that are consistent with public key used to validate
      the signature.

5.  Security Considerations

   Implementations MUST protect the private keys.  Compromise of the
   private keys may result in the ability to forge signatures.

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   When generating an SLH-DSA key pair, an implementation MUST generate
   each key pair independently of all other key pairs in the SLH-DSA
   hypertree.

   A SLH-DSA tree MUST NOT be used for more than 2^64 signing
   operations.

   The generation of private keys relies on random numbers.  The use of
   inadequate pseudo-random number generators (PRNGs) to generate these
   values can result in little or no security.  An attacker may find it
   much easier to reproduce the PRNG environment that produced the keys,
   searching the resulting small set of possibilities, rather than brute
   force searching the whole key space.  The generation of quality
   random numbers is difficult, and [RFC4086] offers important guidance
   in this area.

   To avoid algorithm substitution attacks, the CMSAlgorithmProtection
   attribute defined in [RFC6211] SHOULD be included in signed
   attributes.

   Implementers SHOULD consider their particular use cases and may
   choose to implement OPTIONAL fault attack countermeasures [CMP2018]
   [Ge2023].  Verifying a signature before releasing the signature value
   is a typical fault attack countermeasure; however, this
   countermeasure is not effective for SLH-DSA [Ge2023].  Redundancy by
   replicating the signature generation process MAY be used as an
   effective fault attack countermeasure for SLH-DSA [Ge2023]; however,
   the SLH-DSA signature generation is already considered slow.

   Likewise, Implementers SHOULD consider their particular use cases and
   may choose to implement protections against passive power and
   emissions side-channel attacks [SLotH].

6.  Operational Considerations

   If slh_sign is implemented in a hardware device such as hardware
   security module (HSM) or portable cryptographic token,
   implementations might want to avoid sending the full content to the
   device.  By including signed attributes, which necessarily include
   the message-digest attribute and the content-type attribute as
   described in Section 5.3 of [RFC5652], the much smaller set of signed
   attributes are sent to the device for signing.

   Following the approach in the previous paragraph is essentially the
   same as using SLH-DSA in pre-hash mode, which means that a hash of
   the content is passed to the SLH-DSA signature operation instead of
   the full message content.  For this reason, this document only
   specifies the use of SLH-DSA pure mode.

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   When using SLH-DSA in pure mode, it is not possible to single-pass
   process the content to verify a SignedData message that does not
   contain signed attributes.  To assist recipients that might make use
   of stream-based APIs, implementers SHOULD include signed attributes
   within any SignerInfo that uses SLH-DSA as signature algorithm.
   Doing so allows the recipient implementation to avoid keeping the
   signed content in memory.  Recall that when signed attributes are
   present, they MUST contain a content-type attribute and a message-
   digest attribute, and they SHOULD contain a CMSAlgorithmProtection
   attribute.

7.  IANA Considerations

   For the ASN.1 Module in the Appendix of this document, IANA is
   requested to assign an object identifier (OID) for the module
   identifier (TBD1) with a Description of "id-mod-slh-dsa-2024".  The
   OID for the module should be allocated in the "SMI Security for S/
   MIME Module Identifier" (1.2.840.113549.1.9.16.0).

8.  Acknowledgements

   Thanks to Mike Ounsworth, Tomas Gustavsson, Daniel Van Geest, Carl
   Wallace, Phillip Hallam-Baker, and Dieter Bratko for their careful
   review and constructive comments.

9.  References

9.1.  Normative References

   [FIPS180]  National Institute of Standards and Technology (NIST),
              "Secure Hash Standard (SHS)", FIPS PUB 180-4, August 2015.

   [FIPS202]  National Institute of Standards and Technology (NIST),
              "SHA-3 Standard: Permutation-Based Hash and Extendable-
              Output Functions", FIPS PUB 202, August 2015.

   [FIPS205]  National Institute of Standards and Technology (NIST),
              "Stateless Hash-Based Digital Signature Standard", FIPS
              PUB 205, 13 August 2024,
              <https://doi.org/10.6028/NIST.FIPS.205>.

   [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>.

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   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/rfc/rfc5652>.

   [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
              Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
              2010, <https://www.rfc-editor.org/rfc/rfc5754>.

   [RFC5958]  Turner, S., "Asymmetric Key Packages", RFC 5958,
              DOI 10.17487/RFC5958, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5958>.

   [RFC6211]  Schaad, J., "Cryptographic Message Syntax (CMS) Algorithm
              Identifier Protection Attribute", RFC 6211,
              DOI 10.17487/RFC6211, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6211>.

   [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>.

   [RFC8702]  Kampanakis, P. and Q. Dang, "Use of the SHAKE One-Way Hash
              Functions in the Cryptographic Message Syntax (CMS)",
              RFC 8702, DOI 10.17487/RFC8702, January 2020,
              <https://www.rfc-editor.org/rfc/rfc8702>.

   [X680]     ITU-T, "Information technology -- Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, ISO/IEC 8824-1:2021, February 2021,
              <https://www.itu.int/rec/T-REC-X.680>.

   [X690]     ITU-T, "Information technology -- ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1-2021,
              February 2021, <https://www.itu.int/rec/T-REC-X.690>.

9.2.  Informative References

   [CMP2018]  Castelnovi, L., Martinelli, A., and T. Prest, "Grafting
              Trees: A Fault Attack Against the SPHINCS Framework",
              Post-Quantum Cryptography pp. 165-184, PQCrypto 2018,

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              Lecture Notes in Computer Science vol 10786, 2018,
              <https://link.springer.com/
              chapter/10.1007/978-3-319-79063-3_8>.

   [Ge2023]   Genêt, A., "On Protecting SPHINCS+ Against Fault Attacks",
              TCHES 2023/02, DOI 10.46586/tches.v2023.i2.80-114, 2023,
              <https://tches.iacr.org/index.php/TCHES/article/
              view/10278/9726>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/rfc/rfc4086>.

   [RFC4108]  Housley, R., "Using Cryptographic Message Syntax (CMS) to
              Protect Firmware Packages", RFC 4108,
              DOI 10.17487/RFC4108, August 2005,
              <https://www.rfc-editor.org/rfc/rfc4108>.

   [RFC5911]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for
              Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911,
              DOI 10.17487/RFC5911, June 2010,
              <https://www.rfc-editor.org/rfc/rfc5911>.

   [RFC8391]  Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
              Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
              RFC 8391, DOI 10.17487/RFC8391, May 2018,
              <https://www.rfc-editor.org/rfc/rfc8391>.

   [RFC8554]  McGrew, D., Curcio, M., and S. Fluhrer, "Leighton-Micali
              Hash-Based Signatures", RFC 8554, DOI 10.17487/RFC8554,
              April 2019, <https://www.rfc-editor.org/rfc/rfc8554>.

   [SLotH]    Saarinen, M.-J., "Accelerating SLH-DSA by Two Orders of
              Magnitude with a Single Hash Unit", 2024,
              <https://eprint.iacr.org/2024/367.pdf>.

Appendix A.  Appendix: ASN.1 Module

   This ASN.1 Module builds upon the conventions established in
   [RFC5911].

   <CODE BEGINS>
   SLH-DSA-Module-2024
     { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
       id-smime(16) id-mod(0) id-mod-slh-dsa-2024(TBD1) }

   DEFINITIONS IMPLICIT TAGS ::= BEGIN

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   EXPORTS ALL;

   IMPORTS
     PUBLIC-KEY, SIGNATURE-ALGORITHM, SMIME-CAPS
       FROM AlgorithmInformation-2009  -- in [RFC5911]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-algorithmInformation-02(58) } ;

   --
   -- Object Identifiers
   --

   nistAlgorithms OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
     country(16) us(840) organization(1) gov(101) csor(3) 4 }

   sigAlgs OBJECT IDENTIFIER ::= { nistAlgorithms 3 }

   id-slh-dsa-sha2-128s OBJECT IDENTIFIER ::= { sigAlgs 20 }

   id-slh-dsa-sha2-128f OBJECT IDENTIFIER ::= { sigAlgs 21 }

   id-slh-dsa-sha2-192s OBJECT IDENTIFIER ::= { sigAlgs 22 }

   id-slh-dsa-sha2-192f OBJECT IDENTIFIER ::= { sigAlgs 23 }

   id-slh-dsa-sha2-256s OBJECT IDENTIFIER ::= { sigAlgs 24 }

   id-slh-dsa-sha2-256f OBJECT IDENTIFIER ::= { sigAlgs 25 }

   id-slh-dsa-shake-128s OBJECT IDENTIFIER ::= { sigAlgs 26 }

   id-slh-dsa-shake-128f OBJECT IDENTIFIER ::= { sigAlgs 27 }

   id-slh-dsa-shake-192s OBJECT IDENTIFIER ::= { sigAlgs 28 }

   id-slh-dsa-shake-192f OBJECT IDENTIFIER ::= { sigAlgs 29 }

   id-slh-dsa-shake-256s OBJECT IDENTIFIER ::= { sigAlgs 30 }

   id-slh-dsa-shake-256f OBJECT IDENTIFIER ::= { sigAlgs 31 }

   --
   -- Signature Algorithm, Public Key, and Private Key
   --

   sa-slh-dsa-sha2-128s SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-sha2-128s

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       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-sha2-128s }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-sha2-128s } }

   sa-slh-dsa-sha2-128f SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-sha2-128f
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-sha2-128f }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-sha2-128f } }

   sa-slh-dsa-sha2-192s SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-sha2-192s
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-sha2-192s }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-sha2-192s } }

   sa-slh-dsa-sha2-192f SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-sha2-192f
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-sha2-192f }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-sha2-192f } }

   sa-slh-dsa-sha2-256s SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-sha2-256s
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-sha2-256s }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-sha2-256s } }

   sa-slh-dsa-sha2-256f SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-sha2-256f
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-sha2-256f }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-sha2-256f } }

   sa-slh-dsa-shake-128s SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-shake-128s
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-shake-128s }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-shake-128s } }

   sa-slh-dsa-shake-128f SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-shake-128f
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-shake-128f }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-shake-128f } }

   sa-slh-dsa-shake-192s SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-shake-192s

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       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-shake-192s }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-shake-192s } }

   sa-slh-dsa-shake-192f SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-shake-192f
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-shake-192f }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-shake-192f } }

   sa-slh-dsa-shake-256s SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-shake-256s
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-shake-256s }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-shake-256s } }

   sa-slh-dsa-shake-256f SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-slh-dsa-shake-256f
       PARAMS ARE absent
       PUBLIC-KEYS { pk-slh-dsa-shake-256f }
       SMIME-CAPS { IDENTIFIED BY id-slh-dsa-shake-256f } }

   pk-slh-dsa-sha2-128s PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-sha2-128s
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-sha2-128f PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-sha2-128f
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-sha2-192s PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-sha2-192s
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-sha2-192f PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-sha2-192f
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }

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       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-sha2-256s PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-sha2-256s
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-sha2-256f PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-sha2-256f
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-shake-128s PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-shake-128s
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-shake-128f PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-shake-128f
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-shake-192s PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-shake-192s
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-shake-192f PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-shake-192f
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-shake-256s PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-shake-256s
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE

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         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   pk-slh-dsa-shake-256f PUBLIC-KEY ::= {
       IDENTIFIER id-slh-dsa-shake-256f
       -- KEY no ASN.1 wrapping --
       CERT-KEY-USAGE
         { digitalSignature, nonRepudiation, keyCertSign, cRLSign }
       -- PRIVATE-KEY no ASN.1 wrapping -- }

   SLH-DSA-PublicKey ::= OCTET STRING (SIZE (32 | 48 | 64))

   SLH-DSA-PrivateKey ::= OCTET STRING (SIZE (64 | 96 | 128))

   --
   -- Expand the signature algorithm set used by CMS [RFC5911]
   --

   SignatureAlgorithmSet SIGNATURE-ALGORITHM ::=
       { sa-slh-dsa-sha2-128s |
         sa-slh-dsa-sha2-128f |
         sa-slh-dsa-sha2-192s |
         sa-slh-dsa-sha2-192f |
         sa-slh-dsa-sha2-256s |
         sa-slh-dsa-sha2-256f |
         sa-slh-dsa-shake-128s |
         sa-slh-dsa-shake-128f |
         sa-slh-dsa-shake-192s |
         sa-slh-dsa-shake-192f |
         sa-slh-dsa-shake-256s |
         sa-slh-dsa-shake-256f,
         ... }

   --
   -- Expand the S/MIME capabilities set used by CMS [RFC5911]
   --

   SMimeCaps SMIME-CAPS ::=
       { sa-slh-dsa-sha2-128s.&smimeCaps |
         sa-slh-dsa-sha2-128f.&smimeCaps |
         sa-slh-dsa-sha2-192s.&smimeCaps |
         sa-slh-dsa-sha2-192f.&smimeCaps |
         sa-slh-dsa-sha2-256s.&smimeCaps |
         sa-slh-dsa-sha2-256f.&smimeCaps |
         sa-slh-dsa-shake-128s.&smimeCaps |
         sa-slh-dsa-shake-128f.&smimeCaps |
         sa-slh-dsa-shake-192s.&smimeCaps |
         sa-slh-dsa-shake-192f.&smimeCaps |

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         sa-slh-dsa-shake-256s.&smimeCaps |
         sa-slh-dsa-shake-256f.&smimeCaps,
         ... }

   END
   <CODE ENDS>

Authors' Addresses

   Russ Housley
   Vigil Security, LLC
   Email: housley@vigilsec.com

   Scott Fluhrer
   Cisco Systems
   Email: sfluhrer@cisco.com

   Panos Kampanakis
   Amazon Web Services
   Email: kpanos@amazon.com

   Bas Westerbaan
   Cloudflare
   Email: bas@westerbaan.name

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