RE: Proposal: Anchored Resources and Hashlinks for VCs

I haven’t had a chance to look at the very latest VC specification but are the credential claims supposed to be embedded inside a credentialSubject? …has something changed? …what am I missing?  Thk u

Michael Herman
Founder
Trusted Digital Web

From: Steve Capell <steve.capell@gmail.com>
Sent: Wednesday, November 3, 2021 5:13 PM
To: dzagidulin@gmail.com
Cc: Credentials Community Group <public-credentials@w3.org>; public-vc-edu@w3.org
Subject: Re: Proposal: Anchored Resources and Hashlinks for VCs

Very supportive of this

Our use cases is supply chain traceability where credentials issued upstream are referenced by credentials issued downstream to make a traceability thread.  Like pulling on a piece of string from a tangled web and finding just the claims relevant for the final product you have purchased

See the 7th slide at https://unece.org/sites/default/files/2021-10/Inter-ledgerInteroperability-Preferential-CoOs.pdf


Was going to use a custom field in the body to hold the hash link but if there’s some agreement on a standard way to do it, then all the better

Steven Capell
Mob: 0410 437854


On 3 Nov 2021, at 12:58 pm, Dmitri Zagidulin <dzagidulin@gmail.com<mailto:dzagidulin@gmail.com>> wrote:

I've been hearing the subject of cryptographically binding links between VCs and other VCs (or other external resources like PDFs) come up more and more lately (it's especially been prevalent in the VC Edu community). I'd like to put forth a proposal that hopefully addresses the use case, or at very least starts a conversation.
Motivation

  1.  Provide a mechanism with which to link multiple VCs together (as peers or as parent/child relationships).
  2.  Provide examples of a general purpose hashlink<https://datatracker.ietf.org/doc/html/draft-sporny-hashlink> mechanism (useful for linking VCs, but also for linking to external images, PDFs, and other files).

Background / Use Cases

There are many use cases that involve binding multiple Verifiable Credentials together. For example, a simple Student ID credential can consist of an overall container credential, which links to several individual credentials (such as a student Photo credential, a proof of enrollment at a particular university, a proof of age, etc).

Note that this is different than binding multiple credentials together in a Verifiable Presentation (and having the presenter sign the VP). In the VP case, the binding just means "this presenter is authenticating the handing over of these unrelated credentials". Whereas in the linked VC case, the credentials are aware of each other, and the peer or hierarchical relationship is built into the VC itself.

Proposal

This proposal introduces two new related properties: anchoredResource and contentHash.

Anchored Resource

An anchored resource points to one or more linked or bound resources. It can appear in the credentialSubject section (which implies that the resource is linked to the subject), or in the top level credential (same level as issuer, issuanceDate, etc), which implies that the resource is linked to the credential itself.

Content Hash

A contentHash property provides a way to refer to an external resource via a Hashlink.

Example

Here's what this would look like, using a mock Student ID credential.

{

  "@context": [

    "https://www.w3.org/2018/credentials/v1",

    "https://w3id.org/security/suites/ed25519-2020/v1"

  ],



  "id": "urn:uuid:123445",

  "type": ["VerifiableCredential", "StudentIdCredential"],

  "issuer": {

    "id": "did:key:....",

    "image": {

       "id": "https://example-university.edu/logo.png",

       "contentHash": "hl:<hashlink to the logo url above>"

    },

    "name": "Example University"

  },

  "issuanceDate": "2020-04-03T00:00:00.000Z",

  "expirationDate": "2020-12-15T00:00:00.000Z",

  "name": "Student Identification Document",

  "anchoredResource": [{

     "id": "urn:uuid:<URN of the related photo id credential>",

     "contentHash": "hl:<hashlink to the related photo id credential>"

   }],

   "proof": {

      // ed25519 signature proof goes here

   }

}

Note the two usages of contentHash in the example. In the issuer, the hashlink<https://datatracker.ietf.org/doc/html/draft-sporny-hashlink> is to a resolvable resource (a regular .png hosted somewhere), and its only purpose is to cryptographically bind the exact version of the logo. (The alternative way to do that is to embed the full image in a data:image/png;base64,iVBORw0KGgoAA... url. Except those tend to make the VC too large for some use cases, such as fitting onto a QR Code.)

Whereas in the anchoredResource, the hashlink is to a non-resolvable urn:uuid: type URN, which only makes sense if both the student id and the photo credential were presented to the verifier.



Let's continue the conversation over at https://github.com/w3c/vc-data-model/issues/831 !