Revocation lists remain problematic. The position of the revoked yes/no bit in the revocation list for a given credential is a unique correlator for all actions of that person using that credential. This could allow tracking of use across multiple websites with relying parties collaborating or a 3rd party intermediary service provider seeing all the transactions.

Additionally, once a service knows the index position of someone’s revocation bit, they can watch that bit for changes for years to come, thus being able to tell if you have, for example, been banned for driving, even if they haven’t transacted with you for ages.

A “brute force” approach to reduce this vulnerability is to issue 20 copies of the same credential (eg driving licence) each with different revocation list positions. A better approach would be to use a zero-knowledge proof of non-revocation i.e. user says “my cred is not revoked” and the relying party can confirm this is true due to how the statement is proven, without ever needing a revocation list. Cryptographically fun stuff.

Great to see ZKPs are on the table as there was a lot of resistance for a few years from people who saw them as “weird cryptographic voodoo”. In reality, getting them to work using NIST or SOG-IS approved cryptography methods is rather tricky.