Internet-Draft COSE Algs for Two-Party Signing March 2025
Lundberg & Jones Expires 4 September 2025 [Page]
Workgroup:
COSE
Internet-Draft:
draft-lundberg-cose-two-party-signing-algs-01
Published:
Intended Status:
Standards Track
Expires:
Authors:
E. Lundberg, Ed.
Yubico
M. B. Jones
Self-Issued Consulting

COSE Algorithms for Two-Party Signing

Abstract

This specification defines COSE algorithm identifiers used when the signing operation is performed cooperatively between two parties. When performing two-party signing, the first party typically hashes the data to be signed and the second party signs the hashed data computed by the first party. This can be useful when communication with the party holding the signing private key occurs over a limited-bandwidth channel, such as NFC or Bluetooth Low Energy (BLE), in which it is infeasible to send the complete set of data to be signed. The resulting signatures are identical in structure to those computed by a single party, and can be verified using the same verification procedure without additional steps to preprocess the signed data.

About This Document

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

Status information for this document may be found at https://datatracker.ietf.org/doc/draft-lundberg-cose-two-party-signing-algs/.

Discussion of this document takes place on the COSE Working Group mailing list (mailto:cose@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/cose/. Subscribe at https://www.ietf.org/mailman/listinfo/cose/.

Source for this draft and an issue tracker can be found at https://github.com/YubicoLabs/cose-two-party-signing-algs-rfc.

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/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 4 September 2025.

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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 one generally-applicable way to define such a division point for every possible signature algorithm. Therefore, this specification 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 [I-D.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.

1.1. Requirements Notation and Conventions

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. 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 specification 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.

2.1. ECDSA

Two-party ECDSA [FIPS-186-5] uses the following division between the digester and the signer of the steps of the ECDSA signature generation algorithm [FIPS-186-5]:

  • The signing procedure is defined in Section 6.4.1 of [FIPS-186-5].

  • The digester performs Step 1 of the signing procedure - hashing the message, producing the value H.

  • The message input to the signer is the value H defined in the signing procedure.

  • The signer resumes the signing procedure from Step 2.

The following algorithm identifiers are defined:

Table 1
Name COSE Value Base algorithm Description
ESP256-2p TBD ESP256 ESP256 [I-D.jose-fully-spec-algs] divided for two-party signing as defined here
ESP384-2p TBD ESP384 ESP384 [I-D.jose-fully-spec-algs] divided for two-party signing as defined here
ESP512-2p TBD ESP512 ESP512 [I-D.jose-fully-spec-algs] divided for two-party signing as defined here

2.2. HashEdDSA

Two-party HashEdDSA [RFC8032] uses the following division between the digester and the signer of the steps of the HashEdDSA signing algorithm [RFC8032]:

  • HashEdDSA is a combination of the EdDSA signing procedure and the PureEdDSA signing procedure. The EdDSA signing procedure is defined in the first paragraph of Section 3.3 of [RFC8032]. The PureEdDSA signing procedure is defined in the second paragraph of Section 3.3 of [RFC8032].

  • The digester computes the value PH(M) defined in the EdDSA signing procedure.

  • The message input to the signer is the value PH(M) defined in the EdDSA signing procedure. This value is represented as M in the PureEdDSA signing procedure.

  • The signer executes the PureEdDSA signing procedure, where the value denoted M in the PureEdDSA signing procedure takes the value denoted PH(M) in the EdDSA signing procedure.

PureEdDSA [RFC8032] cannot be divided in this way since such a division would require that the digester has access to the private key.

The following algorithm identifiers are defined:

Table 2
Name COSE Value Base algorithm Description
Ed25519ph-2p TBD Ed25519ph Ed25519ph [I-D.jose-fully-spec-algs] divided for two-party signing as defined here (NOTE: Ed25519ph not yet registered)
Ed448ph-2p TBD Ed448ph Ed448ph [I-D.jose-fully-spec-algs] divided for two-party signing as defined here (NOTE: Ed448ph not yet registered)

2.3. HashML-DSA

Two-party HashML-DSA [FIPS-204] uses the following division between the digester and the signer of the steps of the HashML-DSA.Sign algorithm:

  • The signing procedure is defined in Section 5.4.1 of [FIPS-204].

  • The digester computes the value PHM defined in Steps 10 to 22 of the signing procedure.

  • The message input to the signer is the value PHM defined in the signing procedure. The additional ctx input must also be transmitted to the signer. This may, for example, be done using the ctx (-1) parameter of a COSE_Key_Ref with kty (1): Ref-ML-DSA (TBD) (see Section 4.2 and Section 4.3).

  • The signer executes all steps of the signing procedure except the Steps 13, 16, 19 or similar that compute the value PHM. Note in particular, that the signer generates the value rnd in Steps 5-8 and constructs the value M' in Step 23.

The "pure" ML-DSA version [FIPS-204] cannot be divided in this way because of how the embedding of the ctx and tr values is constructed in ML-DSA.Sign and ML-DSA.Sign_Internal. A division like the one above for HashML-DSA would move control of this embedding from the signer to the digester. This would break the domain separation enforced by the embedding and possibly enable signature malleability attacks or protocol confusion attacks.

The following algorithm identifiers are defined:

Table 3
Name COSE Value Base algorithm Description
HashML-DSA-44-2p TBD HashML-DSA-44 HashML-DSA-44 divided for two-party signing as defined here (NOTE: HashML-DSA-44 not yet registered)
HashML-DSA-65-2p TBD HashML-DSA-65 HashML-DSA-65 divided for two-party signing as defined here (NOTE: HashML-DSA-65 not yet registered)
HashML-DSA-87-2p TBD HashML-DSA-87 HashML-DSA-87 divided for two-party signing as defined here (NOTE: HashML-DSA-87 not yet registered)

3. COSE Key Reference Types

While keys used by many 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-Private-Key [I-D.bradleylundberg-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 [I-D.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 [I-D.bradleylundberg-ARKG] and restricted for use with the ESP256 [I-D.jose-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

4.1. COSE Algorithms Registrations

This section registers the following values in the IANA "COSE Algorithms" registry [IANA.COSE]:

  • Name: ESP256-2p

    • Value: TBD (Requested Assignment -300)

    • Description: ESP256 [I-D.jose-fully-spec-algs] divided for two-party signing

    • Capabilities: [kty]

    • Change Controller: IETF

    • Reference: Section 2.1 of this specification

    • Recommended: Yes

  • Name: ESP384-2p

    • Value: TBD (Requested Assignment -301)

    • Description: ESP384 [I-D.jose-fully-spec-algs] divided for two-party signing

    • Capabilities: [kty]

    • Change Controller: IETF

    • Reference: Section 2.1 of this specification

    • Recommended: Yes

  • Name: ESP512-2p

    • Value: TBD (Requested Assignment -302)

    • Description: ESP512 [I-D.jose-fully-spec-algs] divided for two-party signing

    • Capabilities: [kty]

    • Change Controller: IETF

    • Reference: Section 2.1 of this specification

    • Recommended: Yes

  • Name: Ed25519ph-2p

    • Value: TBD (Requested Assignment -303)

    • Description: Ed25519ph [I-D.jose-fully-spec-algs] divided for two-party signing

    • Capabilities: [kty]

    • Change Controller: IETF

    • Reference: Section 2.2 of this specification

    • Recommended: Yes

  • Name: Ed448ph-2p

    • Value: TBD (Requested Assignment -304)

    • Description: Ed448ph [I-D.jose-fully-spec-algs] divided for two-party signing

    • Capabilities: [kty]

    • Change Controller: IETF

    • Reference: Section 2.2 of this specification

    • Recommended: Yes

  • Name: HashML-DSA-44-2p

    • Value: TBD (Requested Assignment -305)

    • Description: HashML-DSA-44 divided for two-party signing

    • Capabilities: [kty]

    • Change Controller: IETF

    • Reference: Section 2.3 of this specification

    • Recommended: Yes

  • Name: HashML-DSA-65-2p

    • Value: TBD (Requested Assignment -306)

    • Description: HashML-DSA-65 divided for two-party signing

    • Capabilities: [kty]

    • Change Controller: IETF

    • Reference: Section 2.3 of this specification

    • Recommended: Yes

  • Name: HashML-DSA-87-2p

    • Value: TBD (Requested Assignment -307)

    • Description: HashML-DSA-87 divided for two-party signing

    • Capabilities: [kty]

    • Change Controller: IETF

    • Reference: Section 2.3 of this specification

    • Recommended: Yes

4.2. COSE Key Types Registrations

This section registers the following values in the IANA "COSE Key Types" registry [IANA.COSE]:

  • Name: Ref-OKP

    • Value: TBD (Requested assignment -1)

    • Description: Reference to a key pair of key type "OKP"

    • Capabilities: [kty(-1), crv]

    • Reference: Section 3 of this specification

  • Name: Ref-EC2

    • Value: TBD (Requested assignment -2)

    • Description: Reference to a key pair of key type "EC2"

    • Capabilities: [kty(-2), crv]

    • Reference: Section 3 of this specification

  • Name: Ref-AKP

    • Value: TBD (Requested assignment -7)

    • Description: Reference to a key pair of key type "AKP"

    • Capabilities: [kty(TBD), ctx]

    • Reference: Section 3 of this specification

These registrations add the following choices to the CDDL [RFC8610] type socket $COSE_kty_ref:

$COSE_kty_ref /= -1       ; Value TBD
$COSE_kty_ref /= -2       ; Value TBD
$COSE_kty_ref /= -7       ; Value TBD

4.3. COSE Key Type Parameters Registrations

This section registers the following values in the IANA "COSE Key Type Parameters" registry [IANA.COSE]:

  • Key Type: TBD (Ref-AKP)

    • Name: ctx

    • Label: -1

    • CBOR Type: bstr

    • Description: ctx argument to ML-DSA.Sign or HashML-DSA.Sign

    • Reference: Section 3 of this specification

5. References

5.1. Normative References

[I-D.bradleylundberg-ARKG]
Lundberg, E. and J. Bradley, "The Asynchronous Remote Key Generation (ARKG) algorithm", Work in Progress, Internet-Draft, draft-bradleylundberg-cfrg-arkg-03, , <https://datatracker.ietf.org/doc/html/draft-bradleylundberg-cfrg-arkg-03>.
[I-D.COSE-ML-DSA]
Prorock, M., Steele, O., Misoczki, R., Osborne, M., and C. Cloostermans, "ML-DSA for JOSE and COSE", Work in Progress, Internet-Draft, draft-ietf-cose-dilithium-05, , <https://datatracker.ietf.org/doc/html/draft-ietf-cose-dilithium-05>.
[I-D.jose-fully-spec-algs]
Jones, M. B. and O. Steele, "Fully-Specified Algorithms for JOSE and COSE", Work in Progress, Internet-Draft, draft-ietf-jose-fully-specified-algorithms-07, , <https://datatracker.ietf.org/doc/html/draft-ietf-jose-fully-specified-algorithms-07>.
[IANA.COSE]
IANA, "CBOR Object Signing and Encryption (COSE)", n.d., <https://www.iana.org/assignments/cose/>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8032]
Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, , <https://www.rfc-editor.org/rfc/rfc8032>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8610]
Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, , <https://www.rfc-editor.org/rfc/rfc8610>.
[RFC9052]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <https://www.rfc-editor.org/rfc/rfc9052>.
[SEC1]
Certicom Research, "SEC 1: Elliptic Curve Cryptography", , <https://www.secg.org/sec1-v2.pdf>.

5.2. Informative References

[FIPS-186-5]
National Institute of Standards and Technology, "Digital Signature Standard (DSS)", , <https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-5.pdf>.
[FIPS-204]
National Institute of Standards and Technology, "Module-Lattice-Based Digital Signature Standard", , <https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.204.pdf>.
[I-D.COSE-Hash-Envelope]
Steele, O., Lasker, S., and H. Birkholz, "COSE Hash Envelope", Work in Progress, Internet-Draft, draft-ietf-cose-hash-envelope-02, , <https://datatracker.ietf.org/doc/html/draft-ietf-cose-hash-envelope-02>.
[RFC9380]
Faz-Hernandez, A., Scott, S., Sullivan, N., Wahby, R. S., and C. A. Wood, "Hashing to Elliptic Curves", RFC 9380, DOI 10.17487/RFC9380, , <https://www.rfc-editor.org/rfc/rfc9380>.

Document History

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Authors' Addresses

Emil Lundberg (editor)
Yubico
Gävlegatan 22
Stockholm
Sweden
Michael B. Jones
Self-Issued Consulting
United States
URI: https://self-issued.info/