Ring Referral: Efficient Publicly Verifiable Ad hoc Credential Scheme with Issuer and Strong User Anonymity for Decentralized Identity and More

In this paper, we present a ring referral scheme, by which a user can publicly prove her knowledge of a valid signature for a private message that is signed by one of an ad hoc set of authorized issuers, without revealing the signing issuer. Ring referral is a natural extension to traditional ring signature by allowing a prover to obtain a signature from a third-party signer. Our scheme is useful for diverse applications, such as certificate-hiding decentralized identity, privacy-enhancing federated authentication, anonymous endorsement and privacy -preserving referral marketing. In contrast with prior issuer-hiding credential schemes, our ring referral scheme supports more distinguishing features, such as (1) public verifiability over an ad hoc ring, (2) strong user anonymity against collusion among the issuers and verifier to track a user, (3) transparent setup, (4) message hiding, (5) efficient multi-message logarithmic verifiability, (6) threshold scheme for requiring multiple co-signing issuers. Finally, we implemented our ring referral scheme with extensive empirical evaluation.