Temporally-limited blind-regroup of anonymous credentials

We propose a new mechanism for digital identity schemes called ‘blind-regroup’, which, given a credential, allows a querying authority to identify all the existing credentials created by the same user. Unlike traceability, blind-regroup does not identify the ground identity of the user. Blind-regroup allows the user to continue creating credentials, and past blind-regroup queries do not compromise the anonymity of credentials created in the future. Blind-regroup is developed in a setting that satisfies authority transparency; that means that authorities can ultimately be held accountable for abuses of their power. We prove the correctness of blind-regroup (that is, it returns all and only all of the existing credentials matching the given credential); and we prove its security (namely, credentials not captured by blind-regroup remain unlinkable, and credentials are unforgeable). Our approach requires highly specialised zero-knowedge proofs; to demonstrate feasibility we provide SNARK implementations for these proofs and a performance analysis.