Adding AnonCreds Methods to ACA-Py¶
ACA-Py was originally developed to be used with Hyperledger AnonCreds objects (Schemas, Credential Definitions and Revocation Registries) published on Hyperledger Indy networks. However, with the evolution of "ledger-agnostic" AnonCreds, ACA-Py supports publishing AnonCreds objects wherever you want to put them. If you want to add a new "AnonCreds Methods" to publish AnonCreds objects to a new Verifiable Data Registry (VDR) (perhaps to your favorite blockchain, or using a web-based DID method), you'll find the details of how to do that here. We often using the term "ledger" for the location where AnonCreds objects are published, but here will use "VDR", since a VDR does not have to be a ledger.
The information in this document was discussed on an ACA-Py Maintainers call in March 2024. You can watch the call recording by clicking here.
This is an early version of this document and we assume those reading it
are quite familiar with using ACA-Py, have a good understanding of ACA-Py internals, and are
Python experts. See the Questions or Comments section below
for how to get help as you work through this.
Create a Plugin¶
We recommend that if you are adding a new AnonCreds method, you do so by creating an ACA-Py plugin. See the documentation on ACA-Py plugins and use the set of plugins available in the aries-acapy-plugins repository to help you get started. When you finish your AnonCreds method, we recommend that you publish the plugin in the aries-acapy-plugins repository. If you think that the AnonCreds method you create should be part of ACA-Py core, get your plugin complete and raise the question of adding it to ACA-Py. The Maintainers will be happy to discuss the merits of the idea. No promises though.
Your AnonCreds plugin will have an initialization routine that will register your AnonCreds implementation. It will be registering the identifiers that your method will be using such. It will be the identifier constructs that will trigger the appropriate AnonCreds Registrar and Resolver that will be called for any given AnonCreds object identifier. Check out this example of the registration of the "legacy" Indy AnonCreds method for more details.
The Implementation¶
The basic work involved in creating an AnonCreds method is the implementation of both a "registrar" to write AnonCreds objects to a VDR, and a "resolver" to read AnonCreds objects from a VDR. To do that for your new AnonCreds method, you will need to:
The links above are to a specific commit and the code may have been updated since. You might want to
look at the methods in the current version of acapy_agent/anoncreds/base.py in the main branch.
The interface for those methods are very clean, and there are currently two implementations of the methods in the ACA-Py codebase -- the "legacy" Indy implementation, and the did:indy Indy implementation. There is also a did:web resolver implementation.
Models for the API are defined here
Events¶
When you create your AnonCreds method registrar, make sure that your implementations call appropriate
finish_* event (e.g., AnonCredsIssuer.finish_schema, AnonCredsIssuer.finish_cred_def, etc.) in
AnonCreds Issuer. The calls are necessary to trigger the automation of AnonCreds event creation that
is done by ACA-Py, particularly around the handling of Revocation Registries. As you (should) know, when
an Issuer uses ACA-Py to create a Credential Definition that supports revocation, ACA-Py automatically
creates and publishes two Revocation Registries related to the Credential Definition, publishes the tails
file for each, makes one active, and sets the other to be activated as soon as the active one runs out of
credentials. Your AnonCreds method implementation doesn't have to do much to make that happen -- ACA-Py
does it automatically -- but your implementation must call the finish_* to make trigger ACA-Py to continue
the automation. You can see in Revocation Setup the automation setup.
Questions or Comments¶
The ACA-Py maintainers welcome questions from those new to the community that
have the skills to implement a new AnonCreds method. Use the #aca-py channel
on the OpenWallet Foundation Discord Server or open an issue in this repo to get help.
Pull Requests to the ACA-Py repository to improve this content are welcome!