Revocable TACO: Revocable Threshold based Anonymous Credentials over Blockchains

Abstract

The anonymity, multi-show unlinkability, and selective disclosure property of anonymous credentials enables users to access third-party services without revealing unnecessary details or being profiled. Threshold-based anonymous credentials provide a distributed framework to issue these credentials. While all of this enhances privacy, anonymity can also be misused and some schemes, therefore, have an opening mechanism, which enables the authorities to trace a user's identity. Nevertheless, relying solely on this measure is inadequate as the users can continue to use their previously issued anonymous credentials to authenticate themselves successfully with the service providers. To address this issue, we propose a revocation mechanism for such schemes using dynamic threshold accumulators (DTA) (Helminger et al. 2021). We first formally define a generic threshold-based anonymous credentials with an opening scheme (TACO) and subsequently propose an extension of TACO, "Revocable TACO (RTACO)" - revocable threshold-based credentials over blockchains - that integrates a revocation mechanism based on DTAs to the TACO system. In RTACO, we integrate a revocation handle into the credential as an extra attribute, allowing the disclosure of this attribute during the opening phase, which is then used to blocklist the credential, preventing its further use. We formally prove the security of this scheme in the universal composability (UC) framework. We also give a proof-of-concept implementation of RTACO over the Ethereum blockchain.