OID4VP - add explicit reference to the DIF PE input evaluation process

The OID4VC spec references the DIF Presentation Exchange multiple times stating that (specifically) the Presentation Definition and Presentation Submission are “as defined in [DIF.PresentationExchange]”. Most of these references solely point to the specific syntax for both the Presentation Definition and Presentation Submission in Sector 5 and 6 respectively.

There is no specific reference to the process described at the start of Sector 8 Input Evaluation, which is quite useful for Wallet implementations.

The following parts of the OID4VC spec do seem to imply the use of such a process though:

Section 3.1:

“(1) The Verifier sends an Authorization Request to the Wallet. It contains a Presentation Definition as defined in [DIF.PresentationExchange] that describes the requirements of the Credential(s) that the Verifier is requesting to be presented. Such requirements could include what type of Credential(s), in what format(s), which individual Claims within those Credential(s) (Selective Disclosure), etc. The Wallet processes the Authorization Request and determines what Credentials are available matching the Verifier's request. The Wallet also authenticates the End-User and gathers consent to present the requested Credentials.”

Section 3.2:
”(2.5) The HTTPS GET response returns the Request Object containing Authorization Request parameters. It especially contains a Presentation Definition as defined in [DIF.PresentationExchange] that describes the requirements of the Credential(s) that the Verifier is requesting to be presented. Such requirements could include what type of Credential(s), in what format(s), which individual Claims within those Credential(s) (Selective Disclosure), etc. The Wallet processes the Request Object and determines what Credentials are available matching the Verifier's request. The Wallet also authenticates the End-User and gathers her consent to present the requested Credentials.”

Section 6.2:

“Note: Some of the processing rules of the Presentation Definition and the Presentation Submission are outlined in [DIF.PresentationExchange].”

Since the input evaluation process in (the start of section 8 in) DIF Presentation Exchange is so useful I think an explicit notion is quite beneficial.

I would suggest to add the following note to the end of Section 5.1:
”Note: Wallet implementations are RECOMMENDED to select candidate Verifiable Credential(s) using the evaluation process as described in Section 8 of [@!DIF.PresentationExchange].”

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