



Limited Additional Mechanisms for PKIX and SMIME                 J. Gray
Internet-Draft                                                   Entrust
Intended status: Informational                                 L. Prabel
Expires: 14 November 2026                                         Huawei
                                                             13 May 2026


    Preventing Key Reuse and Cross-Key Forgeries in Composite ML-DSA
                    draft-gray-lamps-compositepkc-00

Abstract

   This document defines a small, backwards-compatible change to
   composite ML-DSA that *cryptographically binds the signature to the
   specific composite public key*. It does so by defining a *Public-Key
   Context* value (pkc) equal to a hash of the *serialized composite
   public key*, and by setting the composite context field to that
   value.  This prevents *key reuse* and *cross-key forgeries* across
   different composite keys, while preserving the API of Composite
   ML-DSA.  The construction introduces two helper procedures to compute
   pkc from either the composite private key or the composite public
   key.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Limited Additional
   Mechanisms for PKIX and SMIME Working Group mailing list
   (spasm@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/spasm/.

   Source for this draft and an issue tracker can be found at
   https://github.com/johngray-dev/draft-gray-lamps-compositepkc.

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-
   Drafts is at https://datatracker.ietf.org/drafts/current/.







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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Threat Model and Goals  . . . . . . . . . . . . . . . . . . .   3
   4.  Overview of the Construction  . . . . . . . . . . . . . . . .   4
   5.  Public-Key Context (PKC) Routines . . . . . . . . . . . . . .   4
     5.1.  ComputePublicKeyContext from Private Key  . . . . . . . .   4
     5.2.  ComputePublicKeyContext from Public Key . . . . . . . . .   5
   6.  Algorithms  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Serialization and ASN.1 Usage . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  Implementation Considerations . . . . . . . . . . . . . . . .   7
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Composite signature schemes (e.g., Composite ML-DSA) pre-hash the
   application message and prepend a *Prefix*, an algorithm-specific
   *Label*, and an application-provided *ctx* byte string to form the
   message representative M', which is then signed by each component
   primitive.  The current composite signature construction:



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   M' :=  Prefix || Label || len(ctx) || ctx || PH( M )

   The ctx is an application context of up to 255 bytes.

   While the existing design already mitigates several cross-protocol
   issues via Prefix and Label, and explicitly *forbids key reuse*, some
   deployments may still reuse component keys or attempt to combine
   component signatures across keys.  This opens the door to *cross-key
   “mix-and-match” forgeries* (splicing a valid ML-DSA component from
   one composite with a valid traditional component from another).

   This document proposes a *minimal change*: set ctx to a *hash of the
   composite public key*. Because the hash depends on the _exact_ public
   key bytes, both component signatures become bound to the same key
   material, preventing cross-key recombination.

2.  Conventions and Definitions

   This document inherits the notation of the Composite ML-DSA draft
   (e.g., Prefix, Label, PH, SerializePublicKey, etc.) and the
   conventional *KeyGen/Sign/Verify* API of a signature scheme.

   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.  Threat Model and Goals

   _Goal_: prevent an adversary from taking valid component signatures
   produced under *different* composite keys and combining them into a
   valid composite signature for a target key.

   _Approach_: bind both component signatures to the *same exact
   composite public key* by including pkc = H(SerializePublicKey(..))
   inside M'.  Because pkc changes with any bit of the key, component
   signatures extracted from different keys no longer verify together.

      This does *not* change the malleability properties of individual
      primitives (e.g., ECDSA); therefore SUF-CMA is still not claimed.
      It primarily removes *cross-key* splicing and reduces the impact
      of accidental key reuse.








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4.  Overview of the Construction

   This construction proposes len(pkc) as the length of the len(ctx) and
   pkc as the ctx value; all signature computation remains as specified
   in the base document.

   Let Hash_ctx denote the hash function chosen by the algorithm’s OID
   (e.g., SHA-256, SHA-512, SHAKE256/64).  The *Public-Key Context* is:

   pkc = Hash_ctx( SerializePublicKey(mldsaPK, tradPK) )

   The new message representative is:

   M' := Prefix || Label || len(pkc) || pkc || PH(M)

   Prefix and Label are unchanged. len(pkc) is encoded as a single
   unsigned byte, which is sufficient because the mandated hash outputs
   are ≤ 64 bytes.  The PH(M) is the pre-hash of the application message
   as in [I-D.ietf-lamps-pq-composite-sigs].

5.  Public-Key Context (PKC) Routines

   This draft defines two convenience routines to compute pkc from
   either the composite private key or the composite public key.

5.1.  ComputePublicKeyContext from Private Key

Composite-ML-DSA<OID>.ComputePublicKeyContext(sk) -> pkc

Inputs:
sk: composite private key

Implicit inputs (from <OID>):
Hash_ctx: the hash function for PKC (same as the algorithm’s PH unless specified otherwise)

Process:

1.  (mldsaSeed, tradSK) = DeserializePrivateKey(sk)
2.  (mldsaPK, mldsaSK) = ML-DSA.KeyGen_internal(mldsaSeed)     // FIPS 204, seed-based expansion
4.  tradPK  = Trad.PublicKey(tradSK)                        // derive public key from private key
5.  pk = SerializePublicKey(mldsaPK, tradPK)
6.  pkc = Hash_ctx(pk)
7.  return pkc

      Notes: The seed-based ML-DSA private key representation and the
      ability to re-derive mldsaPK from mldsaSeed are already normative
      in [I-D.ietf-lamps-pq-composite-sigs]




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   During the signing operation, access to the public key is required.
   The above method suggests generating the composite public key from
   the composite private key by Deserializing the private key into its
   component keys, deriving the public component key for ML-DSA and the
   public component key for the traditional component, and then using
   the SerializePublicKey() method as defined in section 4.1
   [I-D.ietf-lamps-pq-composite-sigs].  This is only one of several
   options, but is a non-normative, non-exhautive list.

   1.  Derive or extract from private key as suggested above.  Many
       cryptographic modules expose functionality to obtain an RSA or EC
       public key from the corresponding private key.  For applications
       where such functionality does not exist, see section 10.4.1 and
       10.4.2 in [I-D.ietf-lamps-pq-composite-kem] for mechanisms for
       extracting the public keys from private keys for RSA and ECDSA
       respectively.  It is assumed that this is not required for
       Ed25519 or Ed448 since those private keys are seeds from which
       the public key can be obtained.

   2.  Fetch it from an external data source, for example from the
       public-key certificate corresponding to this private key.

   3.  If the composite signature private key is being carried within a
       PKCS#8 OneAsymmetricKey object, place the full composite public
       key within the optional OneAsymmetricKey.publicKey field (and re-
       encode as necessary for correctly using it in the context).

   4.  Use an alternate private key encoding that explicitly carries the
       composite public key.

5.2.  ComputePublicKeyContext from Public Key

   Composite-ML-DSA<OID>.ComputePublicKeyContext(pk) -> pkc

   Inputs:
   pk: composite public key

   Implicit inputs (from <OID>):
   Hash_ctx

   Process:
   1.  pkc = Hash_ctx(pk)
   2.  return pkc








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6.  Algorithms

   Both Composite-ML-DSA<OID>.Sign(sk, M, ctx) and Composite-ML-
   DSA<OID>.Verify(pk, M, s, ctx) remain the same.  The only difference
   is that pkc is passed in as the ctx value.

   The signature interface remains the same.

   Note: The application specific ctx argument is *ignored* with this
   current design.  To keep that property, a future version of this
   specification could set pkc = HASH (ctx || publickey).

7.  Serialization and ASN.1 Usage

   This document *does not* change the composite public/private key or
   signature *serialization formats* from
   [I-D.ietf-lamps-pq-composite-sigs] and signatures remain
   concatenations of the component encodings.  It also does not change
   DER wrapping in SPKI/PKCS#8.

   Because wire compatibility requires peers to know whether ctx is
   application-set or PKC-bound, this document could have registered
   *new algorithm identifiers* for each PKC-bound combination.  However,
   that is not within the scope of this document.  This is meant for
   specific application context use-cases where the preventing key reuse
   is a desired security property.  For example, applications which
   choose to profile a set of composite signatures could choose to also
   adopt the use of this context.

8.  Security Considerations

   *Key Reuse*: [I-D.ietf-lamps-pq-composite-sigs] strictly forbids
   reusing component keys.  Binding ctx to pkc provides a cryptographic
   backstop: even if component keys were (improperly) reused, cross-key
   splicing will fail because pkc differs for each public key instance.

   *Non-separability*: [I-D.ietf-lamps-pq-composite-sigs] achieved Weak
   Non-Separability (WNS) and a limited form of SNS for ML-DSA via the
   mldsa_ctx=Label.  PKC-binding additionally prevents forming
   (mldsaSig1, tradSig2) under different keys, because both signatures
   are now bounded to the same pkc.  This does *not* fix primitive-level
   malleability (e.g., ECDSA) and therefore does not claim SUF-CMA.
   However, for algorithms like EdDSA or Ed448 which are SUF secure,
   this property should remain.

   *Prefix*: Existing *Prefix* and *Label* remain unchanged; deployments
   that implemented the optional Prefix in traditional verifiers can
   keep it as is.



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   *Hash Choices*: Hash_ctx MUST be the algorithm’s registered pre-hash
   function (e.g., SHA-256, SHA-512, SHAKE256/64).  This keeps
   implementation complexity minimal and ensures digest sizes fit within
   the ctx length field.

   *Privacy*: pkc reveals nothing beyond what the public key already
   reveals; it is a hash of public data.

9.  Implementation Considerations

   *Signer Access to pk*: The signer computes pkc either by deriving
   compositePK from compositeSK, or by keeping a cached copy of
   compositepk alongside compositesk.

   *Interoperability*: Because M' changes when this context type is
   used, peers MUST know that this context will be used.  One way to
   achieve this is for application specific use cases to specify this
   context type as part of the usage.  For example, if an application
   using composite signatures desired this security property, it could
   make use of the public key binding in the context mandatory.

10.  IANA Considerations

   None

11.  References

11.1.  Normative References

   [FIPS.204] National Institute of Standards and Technology (NIST),
              "Module-Lattice-Based Digital Signature Standard", FIPS
              PUB 204, August 2024,
              <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.204.pdf>.

   [I-D.ietf-lamps-pq-composite-kem]
              Ounsworth, M., Gray, J., Pala, M., Klaußner, J., and S.
              Fluhrer, "Composite ML-KEM for use in X.509 Public Key
              Infrastructure", Work in Progress, Internet-Draft, draft-
              ietf-lamps-pq-composite-kem-14, 27 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
              pq-composite-kem-14>.

   [I-D.ietf-lamps-pq-composite-sigs]
              Ounsworth, M., Gray, J., Pala, M., Klaußner, J., and S.
              Fluhrer, "Composite Module-Lattice-Based Digital Signature
              Algorithm (ML-DSA) for use in X.509 Public Key
              Infrastructure", Work in Progress, Internet-Draft, draft-



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              ietf-lamps-pq-composite-sigs-19, 21 April 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
              pq-composite-sigs-19>.

   [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

   [I-D.draft-ietf-pquip-hybrid-signature-spectrums-07]
              Bindel, N., Hale, B., Connolly, D., and F. D, "Hybrid
              signature spectrums", Work in Progress, Internet-Draft,
              draft-ietf-pquip-hybrid-signature-spectrums-07, 20 June
              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
              pquip-hybrid-signature-spectrums-07>.

Appendix A.  Acknowledgments

   Thanks to the Composite ML-DSA authors and LAMPS WG for the existing
   design and analyses of pre-hashing, non-separability, and key-reuse
   risks which this document builds upon.  Thanks to Daniel Van Geest
   for his feedback on this document.

Authors' Addresses

   John Gray
   Entrust Limited
   2500 Solandt Road – Suite 100
   Ottawa, Ontario  K2K 3G5
   Canada
   Email: john.gray@entrust.com


   Lucas Prabel
   Huawei
   Email: lucas.prabel@huawei.com









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