Threshold Issuance Selective Disclosure Credentials With Equivalence Class Signature

Anonymous credentials offer privacy-preserving authentication and authorization by making assertions about identity in the digital realm. To overcome the reliance on a single trusted issuer, decentralized variants have emerged. A classic approach is to split the responsibility of issuing credentials among multiple issuers in a threshold manner (e.g.,t-out-of-n). Unfortunately, among existing threshold protocols, non-interactive practical constructions can only guarantee security in honest majority settings, while interactive constructions face two primary efficiency bottlenecks: either they require an excessive number of interaction rounds, or they fail to support constant-size credential showings for selective disclosure. In this work, we address these challenges by presenting a threshold issuance anonymous credential (TIAC) protocol, built upon the recent advanced signatures, i.e., Equivalence Class Signatures (EQS). Our proposed solution involves a three-round protocol that realizes a standard threshold issuing functionality, providing composable security against a malicious adversary corrupting the majority of issuers. We thereafter introduce a provably secure construction of the TIAC protocol with constant-size showings by combining the proposed threshold issuing protocol and set commitments. We rigorously prove our protocol in the universal composability (UC) framework. The practicality of our protocol is demonstrated through benchmark comparisons with the state-of-the-art EQS-based solution (ASIACRYPT ’24). The benchmarking results show that, with 64 participating issuers, our improvements go up to 6.72× for the threshold issuance phase when observed over WAN.