%%% title = "SD-JWT-based Verifiable Credentials with JSON payloads (SD-JWT VC)" abbrev = "SD-JWT VC" ipr = "trust200902" workgroup = "TODO Working Group" keyword = ["security", "openid", "sd-jwt"]

[seriesInfo] name = "Internet-Draft" value = "draft-terbu-sd-jwt-vc-latest" status = "standard"

[[author]] initials="O." surname="Terbu" fullname="Oliver Terbu" organization="Spruce Systems, Inc." [author.address] email = "oliver.terbu@spruceid.com"

[[author]] initials="D." surname="Fett" fullname="Daniel Fett" organization="Authlete Inc. " [author.address] email = "mail@danielfett.de"

%%%

.# Abstract

This specification describes data formats as well as validation and processing rules to express Verifiable Credentials with JSON payload based on the SD-JWT format [@!I-D.ietf-oauth-selective-disclosure-jwt].

{mainmatter}

Introduction

Three-Party-Model

In the so-called Three-Party-Model, Issuers issue Verifiable Credentials to a Holder, who can then present the Verifiable Credentials to Verifiers. Verifiable Credentials are tamper-evident (usually cryptographically signed) statements about a Subject, typically the Holder.

         +------------+
         |            |
         |   Issuer   |
         |            |
         +------------+
               |
    Issues Verifiable Credential
               |
               v
         +------------+
         |            |
         |   Holder   |
         |            |
         +------------+
               |
  Presents Verifiable Credential
               |
               v
         +-------------+
         |             |+                                  +------------+
         |  Verifiers  ||+                                 |   Status   |
         |             |||---------- optionally ---------->|  Provider  |
         +-------------+||        retrieve status of       |            |
          +-------------+|       Verifiable Credential     +------------+
           +-------------+

Figure: Three-Party-Model with optional Status Provider

Verifiers can check the authenticity of the data in the Verifiable Credentials and optionally enforce Holder Binding, i.e., ask the Holder to prove that they are the intended holder of the Verifiable Credential, for example, by proving possession of a cryptographic key referenced in the credential. This process is further described in [@!I-D.ietf-oauth-selective-disclosure-jwt].

To support revocation of Verifiable Credentials, an optional fourth party can be involved, a Status Provider, who delivers revocation information to Verifiers. (The Verifier can also serve as the Status Provider.)

The three-party-model, i.e., actors, Verifiable Credentials, is also described in [@VC-DATA]. This specification defines Verifiable Credentials based on the SD-JWT format and JWT Claim Sets with JSON payloads. A translation algorithm between the two approaches is provided in this specification.

Rationale

JSON Web Tokens (JWTs) [@!RFC7519] can in principle be used to express Verifiable Credentials in a way that is easy to understand and process as it builds upon established web primitives. However, JWT-based credentials do not support selective disclosure, i.e., the ability for a Holder to disclose only a subset of the claims contained in the JWT, which is a requirement to implement the three-party-model efficiently.

Selective Disclosure JWT (SD-JWT) [@!I-D.ietf-oauth-selective-disclosure-jwt] is a specification that introduces conventions to support selective disclosure for JWTs: For an SD-JWT document, a Holder can decide which claims to release (within bounds defined by the Issuer). This format is therefore perfectly suited for Verifiable Credentials.

SD-JWT itself does not define the claims that must be used within the payload or their semantics. This specification therefore defines how Verifiable Credentials can be expressed using SD-JWT.

JWTs (and SD-JWTs) can contain claims that are registered in "JSON Web Token Claims" registry as defined in [@!RFC7519], as well as public and private claims. Private claims are not relevant for this specification due to the openness of the three-party-model. Since SD-JWTs are based on JWTs, this specification aims to express the basic Verifiable Credential data model purely through JWT Claim Sets, using registered claims while allowing Issuers to use additional registered claims, as well as new or existing public claims, to make statements about the Subject of the Verifiable Credential.

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 RFC 2119 [@!RFC2119].

Terms and Definitions

This specification uses the terms "Holder", "Issuer", "Verifier", defined by [@!I-D.ietf-oauth-selective-disclosure-jwt], Verifiable Credential defined by [@VC-DATA].

SD-JWT-based Verifiable Credential (SD-JWT VC): : A Verifiable Credential encoded using the Issuance format defined in [@!I-D.ietf-oauth-selective-disclosure-jwt].

Unsecured payload of an SD-JWT VC: : A JSON object containing all selectively disclosable and non-selectively disclosable claims of the SD-JWT VC. The unsecured payload acts as the input JSON object to issue an SD-JWT VC complying to this specification.

Status Provider: : An entity that provides status information (e.g. revocation) about a Verifiable Credential.

Scope

This specification defines
- Data model and media types for Verifiable Credentials based on SD-JWTs.
- Validation and processing rules for Verifiers
- Mapping mechanisms to related other data models

Use Cases

TBD: explain use cases of the three-party-model.

TBD: conventional crypt, hardware security, hsm, mobile secure area, compliance with FIPS

Verifiable Credentials based on SD-JWT

This section defines encoding, validation and processing rules for SD-JWT VCs.

Media Type

SD-JWT VCs compliant with this specification MUST use the media type application/vc+sd-jwt as defined in (#application-vc-sd-jwt).

Data Format

SD-JWT VCs MUST be encoded using the SD-JWT Combined Format for Issuance as defined in Section 5.3. of [@!I-D.ietf-oauth-selective-disclosure-jwt].

SD-JWT VCs MUST contain all Disclosures corresponding to their SD-JWT component except for Decoy Digests as per Section 5.1.1.3. of [@!I-D.ietf-oauth-selective-disclosure-jwt].

Header Parameters

This section defines JWT header parameters for the SD-JWT component of the SD-JWT VC.

The typ header parameter of the SD-JWT MUST be present. The typ value MUST use vc+sd-jwt. This indicates that the payload of the SD-JWT contains plain JSON and follows the rules as defined in this specification. It further indicates that the SD-JWT is a SD-JWT component of a SD-JWT VC.

The following is a non-normative example of a decoded SD-JWT header:

{
  "alg": "ES256",
  "typ": "vc+sd-jwt"
}

Claims

This section defines the claims that can be included in the payload of SD-JWT VCs.

`type` claim {#type-claim}

This specification defines the JWT claim type. The type claim is used to express the type of the JSON object that is secured by the JWT. The type value MUST be a case-sensitive StringOrURI value.

The following is a non-normative example of how type is used to express a type:

{
  "type": "SomeType"
}

Registered JWT Claims

SD-JWT VCs MAY use any claim registered in the "JSON Web Token Claims" registry as defined in [@!RFC7519].

If present, the following registered JWT claims MUST be included in the SD-JWT and MUST NOT be included in the Disclosures, i.e. cannot be selectively disclosed:

iss
- REQUIRED. The Issuer of the Verifiable Credential. The value of iss MUST be a URI. See [@!RFC7519] for more information.
iat
- REQUIRED. The time of issuance of the Verifiable Credential. See [@!RFC7519] for more information.
nbf
- OPTIONAL. The time before which the Verifiable Credential MUST NOT be accepted before validating. See [@!RFC7519] for more information.
exp
- OPTIONAL. The expiry time of the Verifiable Credential after which the Verifiable Credential is no longer valid. See [@!RFC7519] for more information.
cnf
- REQUIRED when Cryptographic Holder Binding is to be supported. Contains the confirmation method as defined in [@!RFC7800]. It SHOULD contain a JWK as defined in Section 3.2 of [@!RFC7800] and in this case, the kid (Key ID) member MUST be present in the JWK. For Cryptographic Holder Binding, the Holder Binding JWT in the Combined Format for Presentation MUST be signed by the key identified in this claim.
type
- REQUIRED. The type of the Verifiable Credential, e.g., IdentityCredential, as defined in (#type-claim).
status
- OPTIONAL. The information on how to read the status of the Verifiable Credential. See [TBD] for more information.

The following registered JWT claims MAY be contained in the SD-JWT or in the Disclosures and MAY be selectively disclosed:

sub
- OPTIONAL. The identifier of the Subject of the Verifiable Credential. The value of sub MUST be a URI. The Issuer MAY use it to provide the Subject identifier known by the Issuer. There is no requirement for a binding to exist between sub and cnf claims.

Public JWT claims

Additional public claims MAY be used in SD-JWT VCs depending on the application.

Example

The following is a non-normative example of an unsecured payload of an SD-JWT VC.

<{{examples/01/user_claims.json}}

The following is a non-normative example of how the unsecured payload of the SD-JWT VC above can be used in a SD-JWT where the resulting SD-JWT VC contains only claims about the Subject that are selectively disclosable:

<{{examples/01/sd_jwt_payload.json}}

Note that a cnf claim has been added to the SD-JWT payload to express the confirmation method of the holder binding.

The following are the Disclosures belonging to the SD-JWT payload above:

The SD-JWT and the Disclosures would then be serialized by the Issuer into the following format for issuance to the Holder:

<{{examples/01/combined_issuance.txt}}

Verification and Processing {#vc-sd-jwt-verification-and-processing}

The recipient of the SD-JWT VC MUST process and verify an SD-JWT VC as follows:

REQUIRED. Process and verify the SD-JWT as defined in Section 6. of [@!I-D.ietf-oauth-selective-disclosure-jwt]. For the verification, the iss claim in the SD-JWT MAY be used to retrieve the public key from the JWT Issuer Metadata configuration (as defined in (#jwt-issuer-metadata)) of the SD-JWT VC issuer. A Verifier MAY use alternative methods to obtain the public key to verify the signature of the SD-JWT.
OPTIONAL. If status is present in the verified payload of the SD-JWT, the status SHOULD be checked. It depends on the Verifier policy to reject or accept an SD-JWT VP based on the status of the Verifiable Credential.

Any claims used that are not understood MUST be ignored.

Additional validation rules MAY apply, but their use is out of the scope of this specification.

JWT Issuer Metadata {#jwt-issuer-metadata}

This specification defines the JWT Issuer Metadata to retrieve the JWT Issuer Metadata configuration of the JWT Issuer of the JWT. The JWT Issuer is identified by the iss claim in the JWT. Use of the JWT Issuer Metadata is OPTIONAL.

JWT Issuers publishing JWT Issuer Metadata MUST make a JWT Issuer Metadata configuration available at the path formed by concatenating the string /.well-known/jwt-issuer to the iss claim value in the JWT. The iss MUST be a case-sensitive URL using the HTTPS scheme that contains scheme, host and, optionally, port number and path components, but no query or fragment components.

JWT Issuer Metadata Request

A JWT Issuer Metadata configuration MUST be queried using an HTTP GET request at the path defined in (#jwt-issuer-metadata).

The following is a non-normative example of a HTTP request for the JWT Issuer Metadata configuration when iss is set to https://example.com:

GET /.well-known/jwt-issuer HTTP/1.1
Host: example.com

If the iss value contains a path component, any terminating / MUST be removed before inserting /.well-known/ and the well-known URI suffix between the host component and the path component.

The following is a non-normative example of a HTTP request for the JWT Issuer Metadata configuration when iss is set to https://example.com/user/1234:

GET /.well-known/jwt-issuer/user/1234 HTTP/1.1
Host: example.com

JWT Issuer Metadata Response

A successful response MUST use the 200 OK HTTP and return the JWT Issuer Metadata configuration using the application/json content type.

An error response uses the applicable HTTP status code value.

This specification defines the following JWT Issuer Metadata configuration parameters:

issuer REQUIRED. The JWT Issuer identifier, which MUST be identical to the iss value in the JWT.
jwks_uri
- OPTIONAL. URL string referencing the JWT Issuer's JSON Web Key (JWK) Set [@RFC7517] document which contains the JWT Issuer's public keys. The value of this field MUST point to a valid JWK Set document. Use of this parameter is RECOMMENDED, as it allows for easy key rotation.
jwks
- OPTIONAL. JWT Issuer's JSON Web Key Set [@RFC7517] document value, which contains the JWT Issuer's public keys. The value of this field MUST be a JSON object containing a valid JWK Set. This parameter is intended to be used by JWT Issuer that cannot use the jwks_uri parameter.

JWT Issuer Metadata MUST include either jwks_uri or jwks in their JWT Issuer Metadata, but not both.

It is RECOMMENDED that the JWT contains a kid JWT header parameter that can be used to lookup the public key in the JWK Set included by value or referenced in the JWT Issuer Metadata.

The following is a non-normative example of a JWT Issuer Metadata configuration including jwks:

{
   "issuer":"https://example.com",
   "jwks":{
      "keys":[
         {
            "kid":"doc-signer-05-25-2022",
            "e":"AQAB",
            "n":"nj3YJwsLUFl9BmpAbkOswCNVx17Eh9wMO-_AReZwBqfaWFcfG
   HrZXsIV2VMCNVNU8Tpb4obUaSXcRcQ-VMsfQPJm9IzgtRdAY8NN8Xb7PEcYyk
   lBjvTtuPbpzIaqyiUepzUXNDFuAOOkrIol3WmflPUUgMKULBN0EUd1fpOD70p
   RM0rlp_gg_WNUKoW1V-3keYUJoXH9NztEDm_D2MQXj9eGOJJ8yPgGL8PAZMLe
   2R7jb9TxOCPDED7tY_TU4nFPlxptw59A42mldEmViXsKQt60s1SLboazxFKve
   qXC_jpLUt22OC6GUG63p-REw-ZOr3r845z50wMuzifQrMI9bQ",
            "kty":"RSA"
         }
      ]
   }
}

The following is a non-normative example of a JWT Issuer Metadata configuration including jwks_uri:

{
   "issuer":"https://example.com",
   "jwks_uri":"https://jwt-issuer.example.org/my_public_keys.jwks"
}

Additional JWT Issuer Metadata configuration parameters MAY also be used.

JWT Issuer Metadata Validation

The issuer value returned MUST be identical to the iss value of the JWT. If these values are not identical, the data contained in the response MUST NOT be used.

Presenting Verifiable Credentials

This section defines encoding, validation and processing rules for presentations of SD-JWT VCs.

Data Format

A presentation of an SD-JWT VC MUST be encoded using the SD-JWT Combined Format for Presentation as defined in Section 5.4. of [@!I-D.ietf-oauth-selective-disclosure-jwt].

A presentation of an SD-JWT VC MAY contain a Holder Binding JWT as described in Section 5.4.1. of [@!I-D.ietf-oauth-selective-disclosure-jwt].

Holder Binding JWT

If the presentation of the SD-JWT VC includes a Holder Binding JWT, the following claims are used within the Holder Binding JWT:

nonce
- REQUIRED. String value used to associate a transaction between a Verifier an a Holder, and to mitigate replay attacks. The value is passed through unmodified from the Verifier to the Holder Binding JWT. Sufficient entropy MUST be present in the nonce values used to prevent attackers from guessing values.
aud
- REQUIRED. The intended recipient of the Holder Binding JWT which is typically the Verifier. See [@!RFC7519] for more information.
iat
- REQUIRED. The time of issuance of the Holder Binding JWT. See [@!RFC7519] for more information.
exp
- OPTIONAL. The expiration time of the signature when the Holder Binding is no longer considered valid. See [@!RFC7519] for more information.

The Holder Binding JWT MAY include addtional claims which when not understood MUST be ignored.

Examples

The following is a non-normative example of a presentation of the SD-JWT shown above including a Holder Binding JWT:

<{{examples/01/combined_presentation.txt}}

In this presentation, the Holder provides only the Disclosure for the claim address. Other claims are not disclosed to the Verifier.

The following example shows a presentation of a (different) SD-JWT without a Holder Binding JWT:

<{{examples/02/combined_presentation.txt}}

Verification and Processing {#vp-sd-jwt-verification-and-processing}

The Verifier MUST process and verify an SD-JWT VP as follows:

REQUIRED. When processing and verifying the presentation of the SD-JWT VC, the Verifier MUST follow the same verification and processing rules as defined in (#vc-sd-jwt-verification-and-processing).
OPTIONAL. If provided, the Verifier MUST verify the Holder Binding JWT according to Section 6.2. of [@!I-D.ietf-oauth-selective-disclosure-jwt]. To verify the Holder Binding JWT, the cnf claim of the SD-JWT MUST be used.

Security Considerations {#security-considerations}

TBD: Verifier provided nonce.

Privacy Considerations {#privacy-considerations}

TBD: Holder provided nonce via jti.

Relationships to Other Documents

W3C Verifiable Credential Data Model 2.0

The W3C VCDM 2.0 [@VC-DATA] defines a JSON-LD vocabulary for Verifiable Credentials and Verifiable Presentations. To interop with the W3C VCDM 2.0 data model defined in [@VC-DATA], this specification defines a mapping algorithm for SD-JWT VC and SD-JWT VP to the vocabulary and data model defined W3C VCDM 2.0 which is based on JSON-LD.

W3C VC Specifications Directory

This specification registers the media type application/vc+sd-jwt in the W3C VC Specifications Directory [@VC-DIR].

Mapping Algorithm

The following is a uni-directional transformation algorithm that takes in a SD-JWT VC conformant to this specification and maps it to the corresponding properties in the W3C VCDM 2.0 [@VC-DATA] which is based on a JSON-LD vocabulary. It includes specific handling for JWT claims used in this specification. The function returns a Verifiable Credential object in the W3C VCDM 2.0 format.

Procedure:

Let payload be the unsecured payload of the SD-JWT VC reconstructed from the SD-JWT and Disclosures.
Let vc be an empty JSON object that represents the transformed Verifiable Credential:

Set the @context property of vc to "https://www.w3.org/ns/credentials/v2".

If payload contains the nbf property:

Convert the value of nbf from epoch time to an ISO datetime format.
Assign the converted value to the validFrom property of vc.
Remove the nbf claim from payload.

If payload contains the exp property:

Convert the value of exp from epoch time to an ISO datetime format.
Assign the converted value to the validUntil property of vc.
Remove the exp claim from payload.

If payload contains the jti property:

Assign the value of jti to the id property of vc.
Remove the jti claim from payload.

Set the issuer property of vc to the value of the iss property in payload.

Remove the iss claim from payload.

Set the type property of vc to a String array and set the first array element to "VerifiableCredential". Add the value of the type property in payload as the second array element.

Remove the type claim from payload.

If payload contains the sub property:

Assign the value of sub as the id property of the credentialSubject object in vc.
Remove the sub claim from payload.

Else if payload does not have a sub property, create an empty credentialSubject object.
Add all remaining claims in payload to the credentialSubject object of vc and ignore claims that do not have a corresponding representation.
Output vc which contains the resulting Verifiable Credential.

The following is a non-normative example of a pseudocode algorithm:

function get_credential_from_vc_sd_jwt(vc_sd_jwt):
  // Reconstruct unsecured payload from SD-JWT and Disclosures
  return payload

function transform_vc_sd_jwt_to_w3c_vc(vc_sd_jwt):

  // construct input credential (JSON object)
  payload = get_unsecured_payload_from_vc_sd_jwt(vc_sd_jwt)

  vc = {
    "@context": [
      "https://www.w3.org/ns/credentials/v2"
    ]
  }
  if (payload.hasProperty("iat")) {
    vc.issuedAt = epoch_time_to_ISO_datetime(payload.iat)
    payload = remove_claim_from_json(payload, "iat")
  }

  if (payload.hasProperty("nbf")) {
    vc.validFrom = epoch_time_to_ISO_datetime(payload.nbf)
    payload = remove_claim_from_json(payload, "nbf")
  }

  if (payload.hasProperty("exp")) {
    vc.validUntil = epoch_time_to_ISO_datetime(payload.exp)
    payload = remove_claim_from_json(payload, "exp")
  }

  if (payload.hasProperty("jti")) {
    vc.id = payload.jti
    payload = remove_claim_from_json(payload, "jti")
  }

  vc.issuer = payload.iss
  payload = remove_claim_from_json(payload, "iss")

  vc.type = [ "VerifiableCredential", payload.type ]
  payload = remove_claim_from_json(payload, "type")

  if (payload.hasProperty("sub")) {
    vc.credentialSubject = {
      "id": payload.sub
    }
    payload = remove_claim_from_json(payload, "sub")
  } else {
    vc.credentialSubject = { }
  }

  // add all remaining claims to credentialSubject
  // ignore other claims such as "cnf" where no
  // corresponding representation exists
  vc.credentialSubject = insert_claims_into_credential_subject(vc, payload)

  return vc

{backmatter}

<title>Verifiable Credentials Data Model v2.0</title> Digital Bazaar Digital Bazaar Crossword Cybersecurity PLC <title>VC Specifications Directory</title> Digital Bazaar

IANA Considerations

JSON Web Token Claims Registration

Claim Name: "type"
- Claim Description: Credential Type
- Change Controller: IESG
- Specification Document(s): (#type-claim) of this document

Media Types Registry

application/vc+sd-jwt {#application-vc-sd-jwt}

The Internet media type for a SD-JWT VC is application/vc+sd-jwt.

Type name: : application

Subtype name: : vc+sd-jwt

Required parameters: : n/a

Optional parameters: : n/a

Encoding considerations: : 8-bit code points; SD-JWT VC values are encoded as a series of base64url-encoded values (some of which may be the empty string) separated by period ('.') and tilde ('~') characters.

Security considerations: : See Security Considerations in (#security-considerations).

Interoperability considerations: : n/a

Published specification: : RFC TODO
Applications that use this media type: : Applications that issue, present, verify verifiable credentials and presentations.
Additional information:
- Magic number(s): n/a
- File extension(s): n/a
- Macintosh file type code(s): n/a
- Person & email address to contact for further information: TBD
- Intended usage: COMMON
- Restrictions on usage: none
- Author: Oliver Terbu oliver.terbu@spruceid.com
- Change controller: IETF

Acknowledgements {#Acknowledgements}

We would like to thank Alen Horvat, Andres Uribe, Christian Bormann, Giuseppe De Marco, Torsten Lodderstedt and Kristina Yasuda for their contributions (some of which substantial) to this draft and to the initial set of implementations.

Document History

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Files

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draft-terbu-sd-jwt-vc.md

File metadata and controls

Introduction

Three-Party-Model

Rationale

Requirements Notation and Conventions

Terms and Definitions

Scope

Use Cases

Verifiable Credentials based on SD-JWT

Media Type

Data Format

Header Parameters

Claims

type claim {#type-claim}

Registered JWT Claims

Public JWT claims

Example

Verification and Processing {#vc-sd-jwt-verification-and-processing}

JWT Issuer Metadata {#jwt-issuer-metadata}

JWT Issuer Metadata Request

JWT Issuer Metadata Response

JWT Issuer Metadata Validation

Presenting Verifiable Credentials

Data Format

Holder Binding JWT

Examples

Verification and Processing {#vp-sd-jwt-verification-and-processing}

Security Considerations {#security-considerations}

Privacy Considerations {#privacy-considerations}

Relationships to Other Documents

W3C Verifiable Credential Data Model 2.0

W3C VC Specifications Directory

Mapping Algorithm

IANA Considerations

JSON Web Token Claims Registration

Media Types Registry

application/vc+sd-jwt {#application-vc-sd-jwt}

Acknowledgements {#Acknowledgements}

Document History

`type` claim {#type-claim}