Many-Time Linkable Ring Signatures

Linkable ring signatures (Liu et al., ACISP’04) is a ring signature scheme with a linking mechanism for detecting signatures from the same signer. This functionality has found many practical applications in electronic voting, cryptocurrencies, and whistleblowing systems. However, existing linkable ring signature schemes impose a fundamental limitation: users can issue only one signature, and after that their anonymity is not guaranteed. This limited number of usage is inadequate for many real-world scenarios.This work introduces the notion of Many-time Linkable Ring Signatures, extending the anonymity guarantees of standard linkable ring signatures. Specifically, many-time linkable ring signatures ensure that signers remain anonymous as long as the number of their signatures is smaller than a system-global threshold. Only when a signer exceeds this threshold the anonymity is lost. We formalize this via a security notion called T$$\textsf{T}$$-anonymity, which guarantees that adversaries cannot distinguish signatures from users who have each produced at most T$$\textsf{T}$$ signatures. This new notion of anonymity generalizes one-time anonymity in previous linkable schemes, while providing stronger guarantees than existing constructions. We also present a lattice-based construction with proven security in the quantum random oracle model (QROM).<p></p>