Designated Verifier Signatures for JOSE

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 21 April 2025.¶

Copyright Notice

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

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

1. Introduction

Designated verifier signatures (DVS) are signature schemes in which signatures are generated, that can only be verified by a particular party. Unlike conventional digital signature schemes like ECDSA, this enables repudiable signatures.¶

This specification describes a general structure for designated verifier signature schemes and specified a set of instantiations that use a combination of an KA-DH (Diffie-Hellman key aggrement) with an MAC (Message Authentication Code algorithm).¶

The combination of ECKA-DH and MAC is a established mechanism and used, for example, in the mobile driving licence (mDL) application, specified in [ISO-18013-5].¶

This specification and all described algorithms should respect the efforts for Fully Specified Algorithms.¶

This algorithm is intended for use with digital credentials ecosystems, including the Issuer-Holder-Verifier model described by W3C VCDM or IETF SD-JWT-VC.¶

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.¶

3. Terminology

The draft uses "JSON Web Signature", "JOSE Header", "JWS Signature", "JWS Signing Input" as defined by [RFC7515].¶

Signing Party:

The Party that performs the key agreement first and generates the MAC. Similar to a Signer.¶

Verifying Party:

The Party that performs the key agreement second, generates the MAC and compares it to a given value. Similar to a Verifier.¶

4. Cryptographic Dependencies

DVS rely on the following primitives:¶

• A Diffie-Hellman Key Agreement (KA-DH), for example ECKA-DH defined in [BSI-TR-03111]:¶

  • DH(skX, pkY): Perform a non-interactive Diffie-Hellman exchange using the private key skX and public key pkY to produce a Diffie-Hellman shared secret of length Ndh. This function can raise a ValidationError.¶

  • Ndh: The length in bytes of a Diffie-Hellman shared secret produced by DH().¶

  • Nsk: The length in bytes of a Diffie-Hellman private key.¶

• A key derivation function (KDF), for example HKDF defined in [RFC5869]:¶

  • Extract(salt, ikm): Extract a pseudorandom key of fixed length Nh bytes from input keying material ikm and an optional byte string salt.¶

  • Expand(prk, info, L): Expand a pseudorandom key prk using optional string info into L bytes of output keying material.¶

  • Nh: The output size of the Extract() function in bytes.¶

• A Message Authentication Code algorithm (MAC), for example HMAC defined in [RFC2104]:¶

  • MacSign(k, i): Returns an authenticated tag for the given input i and key k.¶

  • Nk: The length in bytes of key k.¶

5. Designated Verifier Signatures

A designated verifier signature requires three components for an algorithm:¶

1. a Diffie-Hellman Key Agreement (DHKA)¶

2. a Key Derivation Function (KDF)¶

3. a Message Authentication Code algorithm (MAC)¶

In general, these parameters are chosen by the Signing Party. These parameters need to be communicated to the Verifying Party before the generation of a Designated Verifier Signature.¶

def dvsSign(skS, pkR, msg, salt= "", info = "DVS-1")

    dh =  DH(skS, pkR)
    prk = Extract(salt, dh)
    k = Expand(prk, info, Nk)
    signature = MacSign(k, msg)
    return signature

def dvsVerify(skR, pkS, msg, salt = "", info = "DVS-1", signature)

    dh =  DH(skR, pkS)
    prk = Extract(salt, dh)
    k = Expand(prk, info, Nk)
    signature' = MacSign(k, msg)
    if signature != signature':
    raise Exception("Designated Verifier Signature invalid")
    return

6. Designated Verifier Signatures for JOSE

Designated Verifier Signatures behave like a digital signature as described in Section 3 of [RFC7518] and are intended for use in JSON Web Signatures (JWS) as described in [RFC7515]. The Generating Party performs the Message Signature or MAC Computation as defined by Section 5.1 of [RFC7515]. The Verifying Party performs the Message Signature or MAC Validation as defined by Section 5.2 of [RFC7515].¶

The following JWS headers are used to convey Designated Verifier Signatures for JOSE:¶

• alg : REQUIRED. The algorithm parameter describes the chosen signature suite, for example the ones described in (#generic_suites) and (#hpke_suites).¶

• rpk : REQUIRED. The rpk (recipient public key) parameter represents the encoded public key of the Verifying Party that was used in the DHKA algorithm as a JSON Web Key according to [RFC7517]. This parameter MUST be present.¶

• nonce : OPTIONAL. The nonce may be provided by the Verifying Party additional to it's public key and ensure additional freshness of the signature. If provided, the Signing Party SHOULD add the nonce to the header.¶

The Signing Party may use existing JWS header parameters like x5c, jwk or kid to represent or reference it's public key according to [RFC7517].¶

{
    "typ" : "JWT",
    "alg" : "DVS-P256-SHA256-HS256",
    "jwk" : <JWK of the Signing Party>,
    "rpk" : <JWK of Verifying Party>
}

{
    "iss" : "https://example.as.com",
    "iat" : "1701870613",
    "given_name" : "Erika",
    "family_name" : "Mustermann"
}

base64-encoded MAC

+-----------------------+-----------------------------+----------------+
| Algorithm Name        | Algorithm Description       |                |
|                       |                             | Requirements   |
+-----------------------+-----------------------------+----------------+
| DVS-P256-SHA256-HS256 | ECDH using NIST P-256,      |   Optional     |
|                       | HKDF using SHA-256 and      |                |
|                       | HMAC using SHA-256          |                |
+-----------------------+-----------------------------+----------------+
| DVS-HPKE-Auth-X25519  | DVS based on HPKE using     |                |
| -SHA256               | DHKEM(X25519, HKDF-SHA256)  |  Optional      |
| -ChaCha20Poly1305     | HKDF-SHA256 KDF and         |  (Appendix A)  |
|                       | ChaCha20Poly1305 AEAD       |                |
+-----------------------+-----------------------------+----------------+
| DVS-HPKE-Auth-P256    | DVS based on HPKE using     |                |
| -SHA256-AES128GCM     | DHKEM(P-256, HKDF-SHA256)   |   Optional     |
|                       | HKDF-SHA256 KDF and         |   (Appendix A) |
|                       | AES-128-GCM AEAD            |                |
+-----------------------+-----------------------------+----------------+

7. Security Considerations

8. IANA Considerations

Define:¶

• define new rpk header parameter¶

• alg values for DVS-P256-SHA256-HS256 and some more¶

9. References

Appendix A. Designated Verifier Signatures using HPKE

This section describes a simple designated verifier signature scheme based on Hybrid Public Key Encryption (HPKE) [RFC9180] in auth mode. It reuses the authentication scheme underlying the AEAD algorithm in use, while using the KEM to establish a one-time authentication key from a pair of KEM public keys. This scheme was described in early specification drafts of HPKE [RFC9180]¶

Appendix B. Acknowledgments

Thanks to:¶

• Brian Campbell¶

• John Bradley¶

Authors' Addresses