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JSON-LD Credentials in ACA-Py

By design ACA-Py is credential format agnostic. This means you can use it for any credential format, as long as an RFC is defined for the specific credential format. ACA-Py currently supports two types of credentials, AnonCreds and JSON-LD credentials. This document describes how to use the latter by making use of W3C Verifiable Credentials using Linked Data Proofs.

Table of Contents

General Concept

The rest of this guide assumes some basic understanding of W3C Verifiable Credentials, JSON-LD and Linked Data Proofs. If you're not familiar with some of these concepts, the following resources can help you get started:

BBS+

BBS+ credentials offer a lot of privacy preserving features over non-ZKP credentials. Therefore we recommend to always use BBS+ credentials over non-ZKP credentials. To get started with BBS+ credentials it is recommended to at least read RFC 0646: W3C Credential Exchange using BBS+ Signatures for a general overview.

Some other resources that can help you get started with BBS+ credentials:

Preparing to Issue a Credential

Contrary to Indy credentials, JSON-LD credentials do not need a schema or credential definition to issue credentials. Everything required to issue the credential is embedded into the credential itself using Linked Data Contexts.

JSON-LD Context

It is required that every property key in the document can be mapped to an IRI. This means the property key must either be an IRI by default, or have the shorthand property mapped in the @context of the document. If you have properties that are not mapped to IRIs, the Issue Credential API will throw the following error:

<x> attributes dropped. Provide definitions in context to correct. [<missing-properties>]

For credentials the https://www.w3.org/2018/credentials/v1 context MUST always be the first context. In addition, when issuing BBS+ credentials the https://w3id.org/security/bbs/v1 URL MUST be present in the context. For convenience this URL will be automatically added to the @context of the credential if not present.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://other-contexts.com"
  ]
}

Writing JSON-LD Contexts

Writing JSON-LD contexts can be a daunting task and is out of scope of this guide. Generally you should try to make use of already existing vocabularies. Some examples are the vocabularies defined in the W3C Credentials Community Group:

Verifiable credentials are not around that long, so there aren't that many vocabularies ready to use. If you can't use one of the existing vocabularies it is still beneficial to lean on already defined lower level contexts. http://schema.org has a large registry of definitions that can be used to build new contexts. The example vocabularies linked above all make use of types from http://schema.org.

For the remainder of this guide, we will be using the example UniversityDegreeCredential type and https://www.w3.org/2018/credentials/examples/v1 context from the Verifiable Credential Data Model. You should not use this for production use cases.

Signature Suite

Before issuing a credential you must determine a signature suite to use. ACA-Py currently supports three signature suites for issuing credentials:

Generally you should always use BbsBlsSignature2020 as it allows the holder to derive a new credential during the proving, meaning it doesn't have to disclose all fields and doesn't have to reveal the signature.

DID Method

Besides the JSON-LD context, we need a DID to use for issuing the credential. ACA-Py currently supports two did methods for issuing credentials:

  • did:sov - Can only be used for Ed25519Signature2018 signature suite.
  • did:key - Can be used for both Ed25519Signature2018 and BbsBlsSignature2020 signature suites.

did:sov

When using did:sov you need to make sure to use a public did so other agents can resolve the did. It is also important the other agent is using the same indy ledger for resolving the did. You can get the public did using the /wallet/did/public endpoint. For backwards compatibility the did is returned without did:sov prefix. When using the did for issuance make sure this prepend this to the did. (so DViYrCMPWfuLiY7LLs8giB becomes did:sov:DViYrCMPWfuLiY7LLs8giB)

did:key

A did:key did is not anchored to a ledger, but embeds the key directly in the identifier part of the did. See the did:key Method Specification for more information.

You can create a did:key using the /wallet/did/create endpoint with the following body. Use ed25519 for Ed25519Signature2018, bls12381g2 for BbsBlsSignature2020.

{
  "method": "key",
  "options": {
    "key_type": "bls12381g2" // or ed25519
  }
}

The above call will return a did that looks something like this: did:key:zUC7FsmhhifDTuYXdwYES2UpCpWwYieJRapC6oEWqyt5KfJ3ztfLzYnbWjuXQ5drYaKaho3FjxrfDB81gtAJKjbM4yAmBuNoj3YKDXqW151KkkYarpEoEVWMMcN5zPfjCrQ8Saj

Issuing Credentials

Issuing JSON-LD credentials is only possible with the issue credential v2 protocol (/issue-credential-2.0)

The format used for exchanging JSON-LD credentials is defined in RFC 0593: JSON-LD Credential Attachment format. The API in ACA-Py exactly matches the formats as described in this RFC, with the most important (from the ACA-Py API perspective) being aries/[email protected]. Read the RFC to see the exact properties required to construct a valid Linked Data Proof VC Detail.

All endpoints in API use the aries/[email protected]. We'll use the /issue-credential-2.0/send as an example, but it works the same for the other endpoints. In contrary to issuing indy credentials, JSON-LD credentials do not require a credential preview. All properties should be directly embedded in the credentials.

The detail should be included under the filter.ld_proof property. To issue a credential call the /issue-credential-2.0/send endpoint, with the example body below and the connection_id and issuer keys replaced. The value of issuer should be the did that you created in the Did Method paragraph above.

If you don't have auto-respond-credential-offer and auto-store-credential enabled in the ACA-Py config, you will need to call /issue-credential-2.0/records/{cred_ex_id}/send-request and /issue-credential-2.0/records/{cred_ex_id}/store to finalize the credential issuance.

See the example body 
{
  "connection_id": "ddc23de9-359f-465c-b66e-f7c5a0cc9a57",
  "filter": {
    "ld_proof": {
      "credential": {
        "@context": [
          "https://www.w3.org/2018/credentials/v1",
          "https://www.w3.org/2018/credentials/examples/v1"
        ],
        "type": ["VerifiableCredential", "UniversityDegreeCredential"],
        "issuer": "did:key:zUC7FsmhhifDTuYXdwYES2UpCpWwYieJRapC6oEWqyt5KfJ3ztfLzYnbWjuXQ5drYaKaho3FjxrfDB81gtAJKjbM4yAmBuNoj3YKDXqW151KkkYarpEoEVWMMcN5zPfjCrQ8Saj",
        "issuanceDate": "2020-01-01T12:00:00Z",
        "credentialSubject": {
          "degree": {
            "type": "BachelorDegree",
            "name": "Bachelor of Science and Arts"
          },
          "college": "Faber College"
        }
      },
      "options": {
        "proofType": "BbsBlsSignature2020"
      }
    }
  }
}

Retrieving Issued Credentials

After issuing the credential, the credentials should be stored inside the wallet. Because the structure of JSON-LD credentials is so different from indy credentials a new endpoint is added to retrieve W3C credentials.

Call the /credentials/w3c endpoint to retrieve all JSON-LD credentials in your wallet. See the detail below for an example response based on the issued credential from the Issuing Credentials paragraph above.

See the example response 
{
  "results": [
    {
      "contexts": [
        "https://www.w3.org/2018/credentials/examples/v1",
        "https://www.w3.org/2018/credentials/v1",
        "https://w3id.org/security/bbs/v1"
      ],
      "types": ["UniversityDegreeCredential", "VerifiableCredential"],
      "schema_ids": [],
      "issuer_id": "did:key:zUC7FsmhhifDTuYXdwYES2UpCpWwYieJRapC6oEWqyt5KfJ3ztfLzYnbWjuXQ5drYaKaho3FjxrfDB81gtAJKjbM4yAmBuNoj3YKDXqW151KkkYarpEoEVWMMcN5zPfjCrQ8Saj",
      "subject_ids": [],
      "proof_types": ["BbsBlsSignature2020"],
      "cred_value": {
        "@context": [
          "https://www.w3.org/2018/credentials/v1",
          "https://www.w3.org/2018/credentials/examples/v1",
          "https://w3id.org/security/bbs/v1"
        ],
        "type": ["VerifiableCredential", "UniversityDegreeCredential"],
        "issuer": "did:key:zUC7FsmhhifDTuYXdwYES2UpCpWwYieJRapC6oEWqyt5KfJ3ztfLzYnbWjuXQ5drYaKaho3FjxrfDB81gtAJKjbM4yAmBuNoj3YKDXqW151KkkYarpEoEVWMMcN5zPfjCrQ8Saj",
        "issuanceDate": "2020-01-01T12:00:00Z",
        "credentialSubject": {
          "degree": {
            "type": "BachelorDegree",
            "name": "Bachelor of Science and Arts"
          },
          "college": "Faber College"
        },
        "proof": {
          "type": "BbsBlsSignature2020",
          "proofPurpose": "assertionMethod",
          "verificationMethod": "did:key:zUC7FsmhhifDTuYXdwYES2UpCpWwYieJRapC6oEWqyt5KfJ3ztfLzYnbWjuXQ5drYaKaho3FjxrfDB81gtAJKjbM4yAmBuNoj3YKDXqW151KkkYarpEoEVWMMcN5zPfjCrQ8Saj#zUC7FsmhhifDTuYXdwYES2UpCpWwYieJRapC6oEWqyt5KfJ3ztfLzYnbWjuXQ5drYaKaho3FjxrfDB81gtAJKjbM4yAmBuNoj3YKDXqW151KkkYarpEoEVWMMcN5zPfjCrQ8Saj",
          "created": "2021-05-03T12:31:28.561945",
          "proofValue": "iUFtRGdLLCWxKx8VD3oiFBoRMUFKhSitTzMsfImXm6OF0d8il+Z40aLz8S7m8EcXPQhRjcWWL9jkfcf1SDifD4CvxVg69NvB7hZyIIz9hwAyi3LmTm0ez4NDRCKyieBuzqKbfM2eACWn/ilhOJBm6w=="
        }
      },
      "cred_tags": {},
      "record_id": "541ddbce5760497d98e68917be8c05bd"
    }
  ]
}

Present Proof

⚠️ TODO: openwallet-foundation#1125

VC-API

In order to support these functions outside of the respective DIDComm protocols, a set of endpoints conforming to the vc-api specification are available. These endpoints should be used by a controller when building an identity platform.

These endpoints include:

  • GET /vc/credentials -> returns a list of all stored json-ld credentials
  • GET /vc/credentials/{id} -> returns a json-ld credential based on it's ID
  • POST /vc/credentials/issue -> signs a credential
  • POST /vc/credentials/verify -> verifies a credential
  • POST /vc/credentials/store -> stores an issued credential
  • POST /vc/presentations/prove -> proves a presentation
  • POST /vc/presentations/verify -> verifies a presentation

To learn more about using these endpoints, please refer to the available postman collection.

External Suite Provider

It is possible to extend the signature suite support, including outsourcing signing JSON-LD Credentials to some other component (KMS, HSM, etc.), using the ExternalSuiteProvider interface. This interface can be implemented and registered via plugin. The plugged in provider will be used by ACA-Py's LDP-VC subsystem to create a LinkedDataProof object, which is responsible for signing normalized credential values.

This interface enables taking advantage of ACA-Py's JSON-LD processing to construct and format the credential while exposing a simple interface to a plugin to make it responsible for signatures. This can also be combined with plugged in DID Methods, VerificationKeyStrategy, and other pluggable components.

See this example project here for more details on the interface and its usage: https://github.com/dbluhm/acapy-ld-signer