Anonymous Credentials

Overview

This document talks about building anonymous credentials using mainly 2 primitives, BBS+ signature scheme which issuer uses to sign the credential and accumulators for membership check needed for revocation. BBS+ implementation comes from this Typescript package which uses this WASM wrapper which itself uses our Rust crypto library.

For an overview of these primitives, see this.

Implementation

On chain, there are 2 modules, one for BBS+ and the other for accumulator. The modules store the BBS+ params, public keys, accumulator params, accumulator public keys and some accumulator details like current accumulated value, last updated, etc. They are somewhat agnostic to the cryptographic details and treat the values as bytes with some size bounds.

  • BBS+ module

    • At path src/modules/bbs-plus.js in the repo.
    • Used to create and remove signature parameters and public keys.
    • The public keys can either refer the signature params or not pass the reference while creating.
    • The params and public keys are owned by a DID and can be only removed by that DID.
    • See the tests at tests/integration/anoncreds/bbs-plus.test.js on how to create, query and remove these.

  • Accumulator module

    • At path src/modules/accumulator.js in the repo.
    • The parameters and public keys are managed in the same way as BBS+ signatures.
    • Accumulators are owned by a DID and can be only removed by that DID.
    • Accumulators are identified by a unique id and that id is used to send updates or remove it.
    • The accumulator update contains the additions, removals and the witness update info and these are not stored in chain state but are present in the blocks and the accumulated value corresponding to the update is logged in the event.
    • In the chain state, only the most recent accumulated value is stored (along with some metadata like creation time, last update, etc), which is sufficient to verify the witness or the proof of knowledge.
    • To update the witness, the updates and witness update info should be parsed from the blocks and the accumulator module provides the functions get the updates and necessary events from the block,
    • See the tests at tests/integration/anoncreds/accumulator.test.js on how to create, query and remove params and keys as well as the accumulator.

  • Composite proofs

    • Proofs that use BBS+ signatures and accumulator
    • The SDK itself doesn't include the Typescript package containing the crypto as a dependency. But it can be used with the SDK to issue, prove, verify and revoke credentials as shown in tests mentioned below.
    • See the test tests/integration/anoncreds/demo.test.js for an example of how a BBS+ signature can be used with an accumulator for anonymous credentials. The accumulator is used to hold a user/credential id. Presence of the id in accumulator means the credential is valid and absence means invalid.

  • Verifiable encryption

    • Encrypt messages from BBS+ signatures for a 3rd party and prove that the encryption was done correctly.
    • See the test tests/integration/anoncreds/saver-and-bound-check.test.js

  • Bound check/Range proof

    • Prove that messages under a BBS+ signature satisfy some bounds.
    • See the test tests/integration/anoncreds/saver-and-bound-check.test.js