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Security Javascript Object Signing and Encryption next generation unicorn sparkling distributed ledger This specification defines designated verifier signatures for JOSE and defines algorithms that use a combination of key agreement and MACs. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Javascript Object Signing and Encryption Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction 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)[https://www.ietf.org/archive/id/draft-jones-jose-fully-specified-algorithms-00.html]. 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.

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 when, and only when, they appear in all capitals, as shown here.

Terminology The draft uses "JSON Web Signature", "JOSE Header", "JWS Signature", "JWS Signing Input" as defined by .

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.

Cryptographic Dependencies DVS uses the following notation: TODO DVS rely on the following primitives: TODO

Designates 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.

Signature Generation 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. The generation of the signature follows these steps:

1. Perform the key agreement as defined by the DHKA algorithm - use the specified elliptic curve to generate a key pair and set the epk - use the Verifier's public key defined by kid to perform the key agreement - optionally provide a certificate chain defined by x5c
2. Extract and expand the shared secret as defined by KDF algorithm - use the output from the key agreement as an input for the key derivation algorithm - derive the MAC key
3. Generate a MAC as defined by MAC algorithm - use the output from the key derivation algorithm as an input for the MAC algorithm - use the JWS Signing Input as defined in Section 5.1 if as the message input for the MAC algorithm - generate the MAC

The verification of signature follows these steps:

1. Perform key agreement as defined by the DHKA algorithm - use the specified elliptic curve to generate an ephemeral key pair and set the kid - provide the public key kid to the Signing Party - use the Signing Party's public key defined by epk and perform the key agreement - optionally validate the certificate chain defined by x5c
2. Extract and expand the shared secret as defined by KDF algorithm - use the output from the key agreement as an input for the key derivation algorithm - derive the MAC key
3. Generate a MAC as defined by MAC algorithm - use the output from the key derivation algorithm as an input for the MAC algorithm - generate the MAC
4. Compare the generated MAC with the signature value

Designated Verifier Signatures for JOSE Designated Verifier Signatures behave like a digital signature as described in Section 3 of and are intended for use in JSON Web Signatures (JWS) as described in . The Generating Party performs the Message Signature or MAC Computation as defined by Section 5.1 of . The Verifying Party performs the Message Signature or MAC Validation as defined by Section 5.2 of . The following JWS headers are used to convey Designated Verifier Signatures for JOSE:

• alg : The algorithm parameter describes the chosen signature suite, for example the ones described in (#suites)
• jwk : The jwk parameter represents the encoded public key of the Signing Party for the use in the DHKA algorithm as a JSON Web Key according to . It MUST contain only public key parameters and SHOULD contain only the minimum JWK parameters necessary to represent the key. Usage of this parameter MUST be supported.
• x5c : The x5c parameter represents the encoded certificate chain and its leaf public key of the Signing Party for the use in the DHKA algorithm as a X.509 certificate chain according to . Alternatively, the Signing Party may use "x5t", x5t#S256" or "x5u". Usage of this parameter MAY be supported.
• rpk : 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 . This parameter MUST be present.

Example JWT The JWT/JWS header: , "rpk" : } ]]> The JWT/JWS payload: The JWT/JWS signature:

Signature Suites Algorithms MUST follow the naming DVS-<DHKA>-<KDF>-<MAC>. This specification described instantiations of Designated Verifier Signatures using specific algorithm combinations:

Security Considerations TODO Security

IANA Considerations Define:

• rpk header parameter
• alg values for DVS-P256-SHA256-HS256 and some more

Normative References JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification. A JSON Web Key (JWK) is a JavaScript Object Notation (JSON) data structure that represents a cryptographic key. This specification also defines a JWK Set JSON data structure that represents a set of JWKs. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and IANA registries established by that specification. This specification registers cryptographic algorithms and identifiers to be used with the JSON Web Signature (JWS), JSON Web Encryption (JWE), and JSON Web Key (JWK) specifications. It defines several IANA registries for these identifiers. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments TODO acknowledge.