COSE Algorithms for Two-Party Signing

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Table of Contents

1. Introduction

CBOR Object Signing and Encryption (COSE) [RFC9052] algorithm identifiers are used to specify the cryptographic operations performed when creating cryptographic data structures, but do not record all the details of how the cryptography was performed, since those details are typically irrelevant for the recipient. The algorithm identifiers defined by this specification facilitate the cooperation of two parties to perform COSE signing operations together. They are used to specify the division of responsibilities between the two parties. Consumers of the cryptographic data structures thus cooperatively produced do not use these algorithm identifiers; rather, consumers use the normal COSE algorithm identifiers that correspond to the cryptographic operation cooperatively performed together by the two parties.¶

A use case for this is performing a signature operation split between two parties, such as a software application and a discrete hardware security module (HSM) holding the private key. In particular, since the data link between them may have limited bandwidth, it may not be practical to send the entire original message to the HSM. Instead, since most signature algorithms begin with digesting the message into a fixed-length intermediate input, this initial digest can be computed by the software application while the HSM computes the rest of the signature algorithm on the digest.¶

Since different signature algorithms digest the message in different ways and at different stages of the algorithm, there is no generally-applicable way to define such a division point for every possible signature algorithm. Therefore, this document defines algorithm identifiers encoding, for a specific set of signature algorithms, which steps of the signature algorithm are performed by the digester (e.g., software application) and which are performed by the signer (e.g., HSM). In general, the signer holds exclusive control of the signing private key.¶

Note that these algorithm identifiers do not define new "pre-hashed" variants of the base signature algorithm, nor an intermediate "hash envelope" data structure, such as that defined in [COSE-Hash-Envelope]. Rather, these identifiers correspond to existing signature algorithms that would typically be executed by a single party, but split into two stages. The resulting signatures are identical to those computed by a single party, and can be verified using the same verification procedures without additional special steps to process the signed data.¶

However some signature algorithms, for example, PureEdDSA [RFC8032] and ML-DSA [FIPS-204], cannot be split in this way and therefore cannot be assigned two-party signing algorithm identifiers. However, if such a signature algorithm defines a "pre-hashed" variant, such as Ed25519ph [RFC8032] or HashML-DSA [FIPS-204], that "pre-hashed" algorithm can also be assigned a two-party signing algorithm identifier, enabling the hashing step to be performed by the digester and the signing step to be executed by the signer.¶

2. Two-Party Signing Algorithms

This section defines divisions of signing algorithm steps between a digester and a signer in a two-party signing protocol, and assigns algorithm identifiers to these algorithm divisions. The digester performs the first part of the divided algorithm and does not have access to the signing private key, while the signer performs the second part of the divided algorithm and has access to the signing private key. For signing algorithms that format the message to insert domain separation tags, as described in Section 2.2.5 of [RFC9380], this message formatting is also performed by the signer.¶

The algorithm identifiers defined in this document MUST NOT appear in COSE structures other than COSE_Key_Ref (see Section 3). They are meant only for coordination between the digester and the signer in a two-party signing protocol. Representations of the keys used and the resulting signatures MUST use the corresponding conventional algorithm identifiers instead. These are listed in the "Base algorithm" column in the tables defining two-party signing algorithm identifiers.¶

3. COSE Key Reference Types

While keys used by many other algorithms can usually be referenced by a single atomic identifier, such as that used in the kid parameter in a COSE_Key object or in the unprotected header of a COSE_Recipient, some signature algorithms use additional parameters to the signature generation beyond the signing private key and message to be signed. For example, ML-DSA [FIPS-204] has the additional parameter ctx and ARKG-Derive-Secret-Key [ARKG] has the parameters kh and info, in addition to the private key.¶

While these additional parameters are simple to provide to the API of the signing procedure in a single-party context, in a two-party context these additional parameters also need to be conveyed from the digester to the signer. For this purpose, we define new COSE key types, collectively called "COSE key reference types". This enables defining a unified, algorithm-agnostic protocol between the digester and the signer, rather than requiring a distinct protocol for each signature algorithm for the sake of conveying algorithm-specific parameters.¶

A COSE key reference is a COSE_Key object whose kty value is defined to represent a reference to a key. The kid parameter MUST be present when kty is a key reference type. These requirements are encoded in the CDDL [RFC8610] type COSE_Key_Ref:¶

COSE_Key_Ref = COSE_Key .within {
  1 ^ => $COSE_kty_ref   ; kty: Any reference type
  2 ^ => any,            ; kid is required
  any => any,            ; Any other entries allowed by COSE_Key
}

The following CDDL example represents a reference to an ML-DSA-65 key, which uses the AKP key type [COSE-ML-DSA], along with the value of the ctx parameter to ML-DSA.Sign [FIPS-204]:¶

{
  1: TBD,      ; kty: Ref-AKP
               ; kid: Opaque identifier of the AKP key
  2: h'92bc2bfa738f5bb07803fb9c0c58020791acd29fbe253baa7a03ac84f4b26d44',

  3: TBD,      ; alg: ML-DSA-65

               ; ctx argument to ML-DSA.Sign
  -1: 'Example application info',
}

The following CDDL example represents a reference to a key derived by ARKG-P256ADD-ECDH [ARKG] and restricted for use with the ESP256 [fully-spec-algs] signature algorithm:¶

{
  1: -65538,   ; kty: Ref-ARKG-derived
               ; kid: Opaque identifier of ARKG-pub
  2: h'60b6dfddd31659598ae5de49acb220d8
       704949e84d484b68344340e2565337d2',
  3: -9,       ; alg: ESP256

               ; ARKG-P256ADD-ECDH key handle
               ; (HMAC-SHA-256-128 followed by
                  SEC1 uncompressed ECDH public key)
  -1: h'ae079e9c52212860678a7cee25b6a6d4
        048219d973768f8e1adb8eb84b220b0ee3
          a2532828b9aa65254fe3717a29499e9b
          aee70cea75b5c8a2ec2eb737834f7467
          e37b3254776f65f4cfc81e2bc4747a84',

               ; info argument to ARKG-Derive-Private-Key
  -2: 'Example application info',
}

4. IANA Considerations

$COSE_kty_ref /= -1       ; Value TBD
$COSE_kty_ref /= -2       ; Value TBD
$COSE_kty_ref /= TBD      ; Value TBD

5. References

Document History

-00¶

• Initial individual draft¶

Authors' Addresses