AC-TIR: Anonymous Credentials with Threshold Issuance and Flexible Revocations

We present AC-TIR, a new anonymous credential with threshold issuance and revocation (AC-TIR). Our AC-TIR augments the state-of-the-art threshold AC with a new variant of Pointcheval-Sanders (PS) signature and flexible revocations. The new PS-type signature achieves strong privacy while the size of public-key is optimal. We give two concrete AC-TIR schemes. Our first scheme is practical during the prove protocol, but the cost of verify protocol grows linearly with the number of revoked users. Our second scheme offers sub-linear complexity for both the prove and verify protocols. We provide formal security analysis of both schemes and we provide benchmarks to show that our AC-TIR scheme is comparable to the existing threshold AC schemes.