Internet-Draft | DVS for JOSE | August 2024 |
Bastian & Kraus | Expires 10 February 2025 | [Page] |
This specification defines designated verifier signatures for JOSE and defines algorithms that use a combination of key agreement and MACs.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://paulbastian.github.io/draft-bastian-dvs-jose/draft-bastian-dvs-jose.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-bastian-dvs-jose/.¶
Discussion of this document takes place on the Javascript Object Signing and Encryption Working Group mailing list (mailto:jose@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/jose/. Subscribe at https://www.ietf.org/mailman/listinfo/jose/.¶
Source for this draft and an issue tracker can be found at https://github.com/paulbastian/draft-bastian-dvs-jose.¶
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 10 February 2025.¶
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.¶
Designated verifier signatures (DVS) are signature schemes in which signatures are generated, that can only be verified 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 ECDH key exchange with an HMAC.¶
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.¶
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.¶
The draft uses "JSON Web Signature", "JOSE Header", "JWS Signature", "JWS Signing Input" as defined by [RFC7515].¶
DVS rely on the following primitives:¶
A Diffie-Hellman Key Agreement (DHKA)¶
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):¶
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)¶
An HPKE algorithm (for the HPKE variants):¶
SealAuth(pkR, info, aad, pt, skS)
: encrypts and authenticates single plaintext pt
with associated data aad
and context info
using a private sender key skS
and public receiver key pkR
.¶
OpenAuth(enc, skR, info, aad, ct, pkS)
: decrypts ciphertext and tag ct
with associated data aad
and context info
using a private receiver key skR
and public sender key pkS
.¶
A designated verifier signature requires three components for an algorithm:¶
a Diffie-Hellman Key Agreement (DHKA)¶
a Key Derivation Function (KDF)¶
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.¶
The following functions are defined to describe a generic, composable Designated Verifier Signature Scheme:¶
def dvsSign(pkR, skS, msg, salt= "", info = "DVS-1") dh = DH(skS, pkR) prk = Extract(salt, dh) k = Expand(prk, info, Nk) mac = MacSign(k, msg) return mac def dvsVerify(skR, pkS, msg, salt = "", info = "DVS-1", mac) dh = DH(skR, pkS) prk = Extract(salt, dh) k = Expand(prk, info, Nk) mac' = MacSign(k, msg) if mac != mac': raise Exception("Designated Verifier Signature invalid") return¶
The generation of the Designated Verifier Signature takes the private key of the Signing Party, the public key of the Verifying Party and the message as inputs. The retrieval and communication of the Verifying Party's public key is out of scope of this specification and subject to the implementing protocols.¶
Input:¶
skS
: private key of the Signing Party¶
pkR
: public key of the Verifying Party¶
msg
: JWS Signing Input¶
salt
: Salt for key derivation¶
info
: optional info for key derivation¶
Steps: * TODO¶
The generation of the Designated Verifier Signature takes the private key of the Signing Party, the public key of the Verifying Party and the message as inputs. The retrieval and communication of the Verifying Party's public key is out of scope of this specification and subject to the implementing protocols.¶
Input:¶
skR
: private key of the Verifying Party¶
pkS
: public key of the Signing Party¶
msg
: JWS Signing Input¶
salt
: Salt for key derivation¶
info
: optional info for key derivation¶
signature
: the Message Authentication Code¶
Steps: * TODO¶
Algorithms MUST follow the naming DVS-<DHKA>-<KDF>-<MAC>
.¶
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]¶
To create a signature, the sender simply calls the single-shot Seal()
method with an empty plaintext value and the message to be signed as AAD.
This produces an encoded key enc and a ciphertext value that contains only the AAD tag. The signature value is the concatenation of the encoded key and the AAD tag.¶
Input:¶
skS
: private key of the Signing Party¶
pkR
: public key of the Verifying Party¶
msg
: JWS Signing Input¶
info
: optional info for key derivation¶
Steps:¶
To verify a signature, the recipient extracts encoded key and the AAD tag from the signature value and calls the single-shor Open()
with the provided ciphertext.
If the AEAD authentication passes, then the signature is valid.¶
Input:¶
skR
: private key of the Verifying Party¶
pkS
: public key of the Signing Party¶
msg
: JWS Signing Input¶
info
: optional info for key derivation¶
signature
: JWS Signature octet string¶
Steps:¶
Decode enc
|| ct
= signature
by length of enc
and ct
. See [HPKE-IANA] for length of ct and enc.¶
Call pt
= OpenAuth(enc, skR, info, aad, ct, pkS)
with¶
aad
= msg¶
the signature is valid, when OpenAuth()
returns pt
= "" with no authentication exception¶
NOTE: ct
contains only a tag. It's length depends on the AEAD algorithm (see Nt values in RFC9180 chapter 7.3.)¶
Algorithms MUST follow the naming DVS-HPKE-<Mode>-<KEM>-<KDF>-<AEAD>
.
"Mode" is Auth (PSKAuth could also be used).
The "KEM", "KDF", and "AEAD" values are chosen from the HPKE IANA registry [HPKE-IANA].¶
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].¶
The JWT/JWS header:¶
{ "typ" : "JWT", "alg" : "DVS-P256-SHA256-HS256", "jwk" : <JWK of the Signing Party>, "rpk" : <JWK of Verifying Party> }¶
The JWT/JWS payload:¶
{ "iss" : "https://example.as.com", "iat" : "1701870613", "given_name" : "Erika", "family_name" : "Mustermann" }¶
The JWT/JWS signature:¶
base64-encoded MAC¶
This specification described instantiations of Designated Verifier Signatures using specific algorithm combinations:¶
+-----------------------+-----------------------------+----------------+ | 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 | | | | ChaCha20Poly1305 AEAD | | +-----------------------+-----------------------------+----------------+ | DVS-HPKE-Auth-P256 | DVS based on HPKE using | | | -SHA256-AES128GCM | DHKEM(P-256, HKDF-SHA256) | Optional | | | HKDF-SHA256 KDF and | | | | AES-128-GCM AEAD | | +-----------------------+-----------------------------+----------------+¶
Verifying Party MUST ensure the freshness of signatures by utilizing ephemeral keys in rpk
or by providing a nonce for nonce
.¶
Define:¶
Thanks to:¶