VC-K – Verifiable Credentials Library for Kotlin Multiplatform

This library implements verifiable credentials to support several use cases, i.e. issuing of credentials, presentation of credentials and validation thereof. This library may be shared between backend services issuing credentials, wallet apps holding credentials, and verifier apps validating them.

Credentials may be represented in the W3C VC Data Model or as ISO credentials according to ISO/IEC 18013-5:2021. Issuing may happen according to ARIES RFC 0453 Issue Credential V2 or with OpenID for Verifiable Credential Issuance. Presentation of credentials may happen according to ARIES RFC 0454 Present Proof V2 or with Self-Issued OpenID Provider v2, supporting OpenID for Verifiable Presentations.

Architecture

This library was built with Kotlin Multiplatform and Multiplatform Mobile in mind. Its primary targets are JVM, Android and iOS. In order to achieve smooth usage especially under iOS, there have been some notable design decisions:

Code interfacing with client implementations uses the return type KmmResult to transport the Success case (i.e. a custom data type) as well as potential errors from native implementations as a Failure.
Native implementations can be plugged in by implementing interfaces, e.g. CryptoService and KeyPairAdapter, as opposed to callback functions.
Use of primitive data types for constructor properties instead of e.g. kotlinx datetime types.
This library provides some "default" implementations, e.g. DefaultCryptoService to test as much code as possible from the commonMain module.
Some classes feature additional constructors or factory methods with a shorter argument list because the default arguments are lost when called from Swift.

Notable features for multiplatform are:

Use of Napier as the logging framework
Use of Kotest for unit tests
Use of kotlinx-datetime for date and time classes
Use of kotlinx-serialization for serialization from/to JSON and CBOR (extended CBOR functionality in our fork of kotlinx.serialization)
Implementation of a ZLIB service in Kotlin with native parts, see ZlibService
Implementation of JWS and JWE operations in pure Kotlin (delegating to native crypto), see JwsService
Abstraction of several cryptographic primitives, to be implemented in native code, see CryptoService
Implementation of COSE operations in pure Kotlin (delegating to native crypto), see CoseService

Some parts for increased multiplatform support have been extracted into separate repositories:

Reimplementation of Kotlin's Result called KmmResult for easy use from Swift code (since inline classes are not supported).
Several crypto datatypes including an ASN.1 parser and encoder called Signum.

The main entry point for applications is an instance of HolderAgent, VerifierAgent or IssuerAgent, according to the nomenclature from the W3C VC Data Model.

Many classes define several constructor parameters, some of them with default values, to enable a simple form of dependency injection. Implementers are advised to specify the parameter names of arguments passed to increase readability and prepare for future extensions.

Aries

A single run of an ARIES protocol (for issuing or presentation) is implemented by the *Protocol classes, whereas the *Messenger classes should be used by applications to manage several runs of a protocol. These classes reside in the artifact vck-aries.

OpenId

For SIOPv2 see OidcSiopProtocol, and for OpenId4VCI see WalletService. Most code resides in the artifact/subdirectory vck-openid. Both protocols are able to transport credentials as plain JWTs, SD-JWT or ISO 18013-5.

Limitations

For Verifiable Credentials and Presentations, only the JWT proof mechanism is implemented.
Json Web Keys always use a kid of did:key:mEpA... with a custom, uncompressed representation of secp256r1 keys.
Several parts of the W3C VC Data Model have not been fully implemented, i.e. everything around resolving cryptographic key material.
Anything related to ledgers (e.g. resolving DID documents) is out of scope.
Cryptographic operations are implemented for EC cryptography on the secp256r1 curve to fully support hardware-backed keys on Android and iOS. However, the enum classes for cryptographic primitives may be extended to support other algorithms.

iOS Implementation

The DefaultCryptoService for iOS should not be used in production as it does not implement encryption, decryption, key agreement and message digests correctly. See the Swift Package for details on a more correct iOS implementation.

Credentials

A single credential itself is an instance of CredentialSubject (when using the plain JWT representation with ECDSA signatures) and has no special meaning attached to it. This library implements AtomicAttribute2023 as a trivial sample of a custom credential. For Selective Disclosure JWT and ISO representations, only the claims (holding names and values) exist, without any data class holding the values.

Other libraries using this library may call LibraryInitializer.registerExtensionLibrary() to register that extension with this library. See our implementation of the EU PID credential or our implementation of the Mobile Driving Licence for examples. We also maintain a comprehensive list of all credentials powered by this library over here.

Dataflow for OID4VCI

We'll present an issuing process according to OID4VCI, along with OID4VP, with all terms taken from there.

The credential issuer serves the following metadata:

{
  "issuer": "https://wallet.a-sit.at/credential-issuer",
  "credential_issuer": "https://wallet.a-sit.at/credential-issuer",
  "authorization_servers": [
    "https://wallet.a-sit.at/authorization-server"
  ],
  "credential_endpoint": "https://wallet.a-sit.at/credential-issuer/credential",
  "token_endpoint": "https://wallet.a-sit.at/authorization-server/token",
  "authorization_endpoint": "https://wallet.a-sit.at/authorization-server/authorize",
  "credential_identifiers_supported": true,
  "credential_configurations_supported": {
    "at.a-sit.wallet.atomic-attribute-2023": {
      "format": "mso_mdoc",
      "scope": "at.a-sit.wallet.atomic-attribute-2023",
      "cryptographic_binding_methods_supported": [
        "cose_key"
      ],
      "credential_signing_alg_values_supported": [
        "ES256"
      ],
      "doctype": "at.a-sit.wallet.atomic-attribute-2023.iso",
      "claims": {
        "at.a-sit.wallet.atomic-attribute-2023": {
          "given_name": {},
          "family_name": {},
          "date_of_birth": {}
        }
      }
    },
    "AtomicAttribute2023#jwt_vc_json": {
      "format": "jwt_vc_json",
      "scope": "AtomicAttribute2023",
      "cryptographic_binding_methods_supported": [
        "did:key",
        "urn:ietf:params:oauth:jwk-thumbprint"
      ],
      "credential_signing_alg_values_supported": [
        "ES256"
      ],
      "credential_definition": {
        "type": [
          "VerifiableCredential",
          "AtomicAttribute2023"
        ],
        "credentialSubject": {
          "given_name": {},
          "family_name": {},
          "date_of_birth": {}
        }
      }
    },
    "AtomicAttribute2023#vc+sd-jwt": {
      "format": "vc+sd-jwt",
      "scope": "AtomicAttribute2023",
      "cryptographic_binding_methods_supported": [
        "did:key",
        "urn:ietf:params:oauth:jwk-thumbprint"
      ],
      "credential_signing_alg_values_supported": [
        "ES256"
      ],
      "vct": "AtomicAttribute2023",
      "claims": {
        "given_name": {},
        "family_name": {},
        "date_of_birth": {}
      }
    }
  }
}

The credential issuer starts with a credential offer:

{
  "credential_issuer": "https://wallet.a-sit.at/credential-issuer",
  "credential_configuration_ids": [
    "at.a-sit.wallet.atomic-attribute-2023",
    "AtomicAttribute2023#jwt_vc_json",
    "AtomicAttribute2023#vc+sd-jwt"
  ],
  "grants": {
    "authorization_code": {
      "issuer_state": "18136181-97fd-4af9-9e66-85a51cdea269",
      "authorization_server": "https://wallet.a-sit.at/authorization-server"
    },
    "urn:ietf:params:oauth:grant-type:pre-authorized_code": {
      "pre-authorized_code": "2c74a00b-70b8-4062-9757-75652174bc5d",
      "authorization_server": "https://wallet.a-sit.at/authorization-server"
    }
  }
}

Since the issuer gives an pre-authorized code, the wallet uses this for the token request:

{
  "grant_type": "urn:ietf:params:oauth:grant-type:pre-authorized_code",
  "redirect_uri": "https://wallet.a-sit.at/app/callback",
  "client_id": "https://wallet.a-sit.at/app",
  "authorization_details": [
    {
      "type": "openid_credential",
      "format": "vc+sd-jwt",
      "vct": "AtomicAttribute2023"
    }
  ],
  "pre-authorized_code": "2c74a00b-70b8-4062-9757-75652174bc5d"
}

The credential issuer answers with an access token:

{
  "access_token": "413ed326-107b-4429-8efa-872cb89949d8",
  "token_type": "bearer",
  "expires_in": 3600,
  "c_nonce": "4fc81553-970e-4765-899f-611d7c4173ae",
  "authorization_details": []
}

The wallet creates a credential request, including a proof-of-posession of its private key:

{
  "format": "vc+sd-jwt",
  "vct": "AtomicAttribute2023",
  "proof": {
    "proof_type": "jwt",
    "jwt": "eyJ0eXAiOiJvcGVuaWQ0dmNpLXByb29mK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7ImNydiI6IlAtMjU2Iiwia3R5IjoiRUMiLCJ4IjoiS1R2WlRpX28xeTc5REQwSk1IanZyYUpkaXFEWGpNZlZIZVNYemVEVVV4ZyIsInkiOiJiUkZReC1meG9GOURJOTdUWmRQZmNYUXlEY0V0eEQydlNTNWNZMERHYXJZIn19.eyJpc3MiOiJodHRwczovL3dhbGxldC5hLXNpdC5hdC9hcHAiLCJhdWQiOiJodHRwczovL3dhbGxldC5hLXNpdC5hdC9jcmVkZW50aWFsLWlzc3VlciIsIm5vbmNlIjoiNGZjODE1NTMtOTcwZS00NzY1LTg5OWYtNjExZDdjNDE3M2FlIiwiaWF0IjoxNzIwNDIyNTM3fQ.FNM1BCli6pHEXedxRGOmNzxfpsEwQz67onbeQZfRU4tNxEqYauW45MrFkrYsi0Ly7wvfdoPkONZG7s7EmD-vkA"
  }
}

The JWT included decodes to the following:

{
  "typ": "openid4vci-proof+jwt",
  "alg": "ES256",
  "jwk": {
    "crv": "P-256",
    "kty": "EC",
    "x": "KTvZTi_o1y79DD0JMHjvraJdiqDXjMfVHeSXzeDUUxg",
    "y": "bRFQx-fxoF9DI97TZdPfcXQyDcEtxD2vSS5cY0DGarY"
  }
}
.
{
  "iss": "https://wallet.a-sit.at/app",
  "aud": "https://wallet.a-sit.at/credential-issuer",
  "nonce": "4fc81553-970e-4765-899f-611d7c4173ae",
  "iat": 1720422537
}

The credential issuer issues the following credential:

{
  "format": "vc+sd-jwt",
  "credential": "eyJraWQiOiJkaWQ6a2V5OnpEbmFleVlrcDlqZ0hjN3lheEcybkZTMXc4MXJISkVyS3hZSkVtTExVRTJVU05wWmUiLCJ0eXAiOiJ2YytzZC1qd3QiLCJhbGciOiJFUzI1NiJ9.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.oDdilbVBmoRpy162gcDR8a0vvYHlP7LXvJ3gxmjz4dYKRLhoMwM_tIcu0Dy5_ftXPq5IO1p9GYMkfUhHk881kw~WyI4cWRNYkN6SWZGajQxT1ZVUE13OEI5R2xaN2tiemxxaXg5T1RSU2huWGgwIiwiZ2l2ZW5fbmFtZSIsIkVyaWthIl0~WyIxUDdYQjB5eFFjaVBlZkVrV2o5R2N0MzNUYVZXeGNnVER1N19aTGptWG13IiwiZmFtaWx5X25hbWUiLCJNdXN0ZXJmcmF1Il0~WyJ3a2VlWG4wejAwa2tiaHVaeVBXM2dwZVBpOXhSVWU1cmVrQ3Npc2d6ZXg4Iiwic3ViamVjdCIsInN1YmplY3QiXQ"
}

The SD-JWT included decodes to the following:

{
  "kid": "did:key:zDnaeyYkp9jgHc7yaxG2nFS1w81rHJErKxYJEmLLUE2USNpZe",
  "typ": "vc+sd-jwt",
  "alg": "ES256"
}
.
{
  "sub": "did:key:zDnaeTCvh7YKSyomaRqN4VwNvRdTKw2AjEdGEdrwm4vDQu9Ew",
  "nbf": 1720422537,
  "iss": "did:key:zDnaeyYkp9jgHc7yaxG2nFS1w81rHJErKxYJEmLLUE2USNpZe",
  "exp": 1720422597,
  "iat": 1720422537,
  "jti": "urn:uuid:d815e10d-a44d-4144-bfa5-9f9913cf19fe",
  "_sd": [
    "hpMQ1iyetq93yeB4nEeufx60xv4Y5jlqVq8HskIsZY0",
    "AGGXYE9HyqullSH_bL-6vDSHHreOGrDVQUNtEP3zuBQ",
    "okeZHIQBCPVUquJ8Vb7lwvmkKZsfRKJUA7_EN2Vc_c4"
  ],
  "vct": "AtomicAttribute2023",
  "status": {
    "id": "https://wallet.a-sit.at/backend/credentials/status/1#3",
    "type": "RevocationList2020Status",
    "revocationListIndex": 3,
    "revocationListCredential": "https://wallet.a-sit.at/backend/credentials/status/1"
  },
  "_sd_alg": "sha-256",
  "cnf": {
    "crv": "P-256",
    "kty": "EC",
    "kid": "urn:ietf:params:oauth:jwk-thumbprint:sha256:Zh-r6VlXNTAFay5XV1sXZIPnff7GpeY0P3LsOT2fHYQ=",
    "x": "KTvZTi_o1y79DD0JMHjvraJdiqDXjMfVHeSXzeDUUxg",
    "y": "bRFQx-fxoF9DI97TZdPfcXQyDcEtxD2vSS5cY0DGarY"
  }
}

with the following claims appended:

["8qdMbCzIfFj41OVUPMw8B9GlZ7kbzlqix9OTRShnXh0","given_name","Erika"]

["1P7XB0yxQciPefEkWj9Gct33TaVWxcgTDu7_ZLjmXmw","family_name","Musterfrau"]

["wkeeXn0z00kkbhuZyPW3gpePi9xRUe5rekCsisgzex8","date_of_birth","1980-01-13"]

Dataflow for SIOPv2

We'll present a presentation process according to SIOPv2, along with OID4VP, with all terms taken from there.

The verifier creates a URL to be displayet to the wallet, containing a reference to the authentication request itself:

https://wallet.a-sit.at/mobile
    ?client_id=https://example.com/rp
    &request_uri=https://example.com/request/15a4e87c-8e3e-4368-87a3-48083bac7232

The authentication request is signed as a JWS and contains the following header and payload:

{
  "alg": "ES256",
  "x5c": [
    "MIIBNDCB2qADAgECAgjm7Gfdm0IFUDAKBggqhkjOPQQDAjASMRAwDgYDVQQDDAdEZWZhdWx0MB4XDTI0MDcxNzEzNTUzMVoXDTI0MDcxNzEzNTYwMVowEjEQMA4GA1UEAwwHRGVmYXVsdDBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABCaD9zTN05m6zZSX+QrQzdF4l1Bt3pVSo6VV9VFUSFCkImDkSsyCbgkJoZFkUNObbMiZbccQk6H33HAfxGWpLTyjGjAYMBYGA1UdEQQPMA2CC2V4YW1wbGUuY29tMAoGCCqGSM49BAMCA0kAMEYCIQDSotU4YnsO0Ar5+barXkCn3PQlCaPJEnBwRL60SnHW9wIhAK5mLYgfslZvT5jFmMrS3Ivzm2EpNyjdCPGVT3wpt6/t"
  ]
}
.
{
  "response_type": "id_token vp_token",
  "client_id": "example.com",
  "scope": "openid profile AtomicAttribute2023 AtomicAttribute2023 at.a-sit.wallet.atomic-attribute-2023",
  "state": "36efe035-ecf6-4c56-94c8-3dfe167ddaad",
  "nonce": "148279b8-5222-4db9-bb9b-0614bbe0ef7d",
  "client_metadata": {
    "redirect_uris": [
      "https://example.com/rp"
    ],
    "jwks": {
      "keys": [
        {
          "crv": "P-256",
          "kty": "EC",
          "x": "JoP3NM3TmbrNlJf5CtDN0XiXUG3elVKjpVX1UVRIUKQ",
          "y": "ImDkSsyCbgkJoZFkUNObbMiZbccQk6H33HAfxGWpLTw"
        }
      ]
    },
    "subject_syntax_types_supported": [
      "urn:ietf:params:oauth:jwk-thumbprint",
      "did:key"
    ],
    "vp_formats": {
      "jwt_vp": {
        "alg": [ "ES256", "ES384", "ES512" ]
      },
      "vc+sd-jwt": {
        "alg": [ "ES256", "ES384", "ES512" ]
      },
      "mso_mdoc": {
        "alg": [ "ES256", "ES384", "ES512" ]
      }
    }
  },
  "id_token_type": "subject_signed_id_token",
  "presentation_definition": {
    "id": "b0611eea-89e4-4b78-8f44-40a2cb608d4a",
    "input_descriptors": [
      {
        "id": "9a5511bf-efe4-4ec2-84e5-4cabf1d13000",
        "schema": [
          {
            "uri": "https://wallet.a-sit.at/schemas/1.0.0/AtomicAttribute2023.json"
          }
        ],
        "constraints": {
          "fields": [
            {
              "path": [ "$[\"family_name\"]" ]
            },
            {
              "path": [ "$[\"given_name\"]" ]
            },
            {
              "path": [ "$.vct" ],
              "filter": {
                "type": "string",
                "pattern": "AtomicAttribute2023"
              }
            }
          ]
        }
      }
    ],
    "format": {
      "vc+sd-jwt": {
        "alg": [ "ES256", "ES384", "ES512" ]
      }
    }
  },
  "client_id_scheme": "x509_san_dns",
  "response_mode": "direct_post",
  "response_uri": "https://example.com/response",
  "aud": "https://example.com/rp",
  "iss": "https://example.com/rp"
}

The certificate includes the X.509 SAN extension with the value example.com.

The wallet posts back the response to https://example.com/response with the parameters presentation_submission, vp_token and state.

The value for presentation_submission is:

{
  "id": "be3972bc-18cf-4e24-abf4-fe49eb870bab",
  "definition_id": "b0611eea-89e4-4b78-8f44-40a2cb608d4a",
  "descriptor_map": [
    {
      "id": "9a5511bf-efe4-4ec2-84e5-4cabf1d13000",
      "format": "vc+sd-jwt",
      "path": "$"
    }
  ]
}

The value for vp_token is an SD-JWT, with header and payload:

{
  "kid": "did:key:zDnaezoC5xdfzSdKoCZCZiw52eqnTks3xMKMDRTFKfSiq9obD",
  "typ": "vc+sd-jwt",
  "alg": "ES256"
}
.
{
  "sub": "did:key:zDnaexYu9PCJsDmg7YwyBCJR2qQT8DxqwXhm4BLAJVWUgW1Ms",
  "nbf": 1721224531,
  "iss": "did:key:zDnaezoC5xdfzSdKoCZCZiw52eqnTks3xMKMDRTFKfSiq9obD",
  "exp": 1721224591,
  "iat": 1721224531,
  "jti": "urn:uuid:6ed95e4f-83ca-457c-8d8b-bf8bf93ad09e",
  "_sd": [
    "0nIEwdknjG7aMeKKiVnJNd7lK-dleq0_RPYc84gDg98",
    "DZo-dZ4EQTxcekJqJKvhYZySgxodgH1vvT5su__pJlk"
  ],
  "vct": "AtomicAttribute2023",
  "status": {
    "id": "https://wallet.a-sit.at/backend/credentials/status/1#2",
    "type": "RevocationList2020Status",
    "revocationListIndex": 2,
    "revocationListCredential": "https://wallet.a-sit.at/backend/credentials/status/1"
  },
  "_sd_alg": "sha-256",
  "cnf": {
    "crv": "P-256",
    "kty": "EC",
    "x": "3Tyi-VLN7pmm6kcbRuyD0bYrYllX_cH1fYbf72pNwtI",
    "y": "6K5c5nYUfRBY3vRo-Q043hr8cFCqYj3iwnxwGFF2Ca8"
  }
}

with the following claims appended:

["_bBdCtzFzC1DYL1r6gNq2fZPMvMdR31mo2zp6gq1sZ4","given_name","Susanne"]

["SNDcrxkCBGQrLj0kHiX72Gn4l5dz-sfKo12tbqAoNf8","family_name","Meier"]

["wkeeXn0z00kkbhuZyPW3gpePi9xRUe5rekCsisgzex8","date_of_birth","1990-02-14"]

as well as a key binding (the JWT is decoded):

{
  "typ": "kb+jwt",
  "alg": "ES256",
  "jwk": {
    "crv": "P-256",
    "kty": "EC",
    "x": "3Tyi-VLN7pmm6kcbRuyD0bYrYllX_cH1fYbf72pNwtI",
    "y": "6K5c5nYUfRBY3vRo-Q043hr8cFCqYj3iwnxwGFF2Ca8"
  }
}
.
{
  "iat": 1721224531,
  "aud": "did:key:zDnaeT2KFSXyCSo3WLjrZeQsNwaCQv2r6bV4bLZNbdhZKhBbh",
  "nonce": "148279b8-5222-4db9-bb9b-0614bbe0ef7d",
  "sd_hash": "UkLqyUz3zKR1iMTam7AsP89aEzYvbQqiJ4jfV82A96c"
}

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 959072.

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The Apache License does not apply to the logos, (including the A-SIT logo) and the project/module name(s), as these are the sole property of A-SIT/A-SIT Plus GmbH and may not be used in derivative works without explicit permission!

All modules:

vck

vck-aries

vck-openid