add clarification to RFC 0510 on how to use with JWT VPs

[TimoGlastra]

When using JWTs as the VP format, how should it be added to the attachment?

Maybe we can make a clarification that the json field is also allowed to be a string value if you want to submit a JWT-VP, and in that case the JWT is encoded and the presentation_submission is encoded within the presenation.

Then for a V2 of the attachment format I'd like to propose the following changes:

• update to PEX v2
• make nonce/challenge required (to prevent replay attacks)
• allow the submission attachment json to be either a string (one JWT-VP) an object (one data integrity VP) or an array (multiple VPs submitted , which is allowed with PEX: Can a single presentation definition result in a single presentation submission with multiple VPs? decentralized-identity/presentation-exchange#462).

In the case of multiple VPs the presentation submission should also be extracted as there will be one presentation submission with multiple VPs. In that case it mights make sense to make the presentation message always an object with the following structure:

{
  // could optionally be a string/object in case of one VP, but I think always making it an array makes implementations simpler
  "vps": [],
  "presentation_submission": {
    "id": "a30e3b91-fb77-4d22-95fa-871689c322e2",
    "definition_id": "32f54163-7166-48f1-93d8-ff217bdb0653",
    "descriptor_map": [
      {
        "id": "banking_input",
        "format": "jwt_vp",
        // references first VP in vps array
        "path": "$.vps[0]",
        "path_nested": {
          "id": "banking_input",
          "format": "jwt_vc",
          "path": "$.vc"
        }
      },
      {
        "id": "citizenship_input",
        "format": "jwt_vp",
        // references second VP in vps array
        "path": "$.vps[1]",
        "path_nested": {
          "id": "citizenship_input",
          "format": "jwt_vc",
          "path": "$.vc"
        }
      }
    ]
  }
}

This way it will work with multiple formats (JSON-LD, JWT, SD-JWT) and also allow multiple VPs to be submitted for a single presentation definition, and make it leverage all features of PEx.

One thing that OID4VP supports is including multiple presentation definitions in the request. I thin that could be solved by adding multiple of the request attachment for PEX as the presentations will be separate from each other.