A Holder is an (architectural) function (a functional component in the eSSIF-Lab functional architecture) that handles presentation requests that it receives from verifier components (of other parties, but also of its own owner). Typically, this means looking for the requested data in the principal's wallet, and using it to construct a presentation (=response). However, if the wallet doesn't have it, the holder may negotiate a transaction with a component of the designated issuer for the purpose of obtaining the needed credential, which - when obtained - it can subsequently store in the wallet and use in the presentation.
Typically, a Holder component would access its owner's Wallet to check if it has a credential that it can use to construct a Presentation (i.e. response) that satisfies the received request.
It may happen that the Wallet does not have such a credential. However, for every (credential, issuer) pair, the request should specify the URI that is capable of issuing such a credential. If or when the Holder Policy/Preferences permit this, the Holder then requests its Principal's Transaction Data Collector to initiate a new transaction that will get the credential from that issuer, for which a clean transaction form would then consist of one that contains said credential. The Holder would then store it in its Principal's Wallet, and then proceed to service the presentation request as if it had obtained that credential from its Principal's Wallet.
It may also happen that the Wallet has multiple credentials that satisfy the request, in which case the Holder must choose the one to use for constructing the presentation. Again, the Holder Policy/Preferences will specify how this choice needs to be made, and whether or not this can be done automatically by the Holder. If not, the Holder will need to provide for an interaction with a human Colleague that will make such decisions.
In order to make the Holder component work, a Holder Policy/Preferences object is created by, or on behalf of its Principal, which specifies e.g.:
- whether or not credentials may be collected 'on the fly';
- how to choose between credentials that all satisfy a presentation request (and whether the Holder can make such choices by itself, or whether or not human interaction is required);
- the kinds of events and data to write to a holder-audit-log.