Towards Functions for Verifiable Credentials in a 2-Holder Model

A trust model commonly used to describe verifiable credential scenarios covers the roles issuer, holder and verifier which in general interact through the activities issue/hold, present/verify and revoke. The use case "German health certificate" discussed here reveals that processes may incorporate more than just one holder and require credential exchange between them. After issuance to one holder other holders occur which also may or even must present the credential in the further course. Therefore, a holder must be able to execute functions on credentials in its wallet such that some other holder also holds this credential and is able to present it successfully. To formally describe such functions and the necessary data structures in credentials, the "1-holder"-trust triangle is extended to a "2-holder"-model with two holders. Based on this extended model possible and relevant functions and their semantics in terms of verification results are defined. A concept to extend SD-JWT data structures to support this semantics is presented and its applicability is shown.