Abstract

Abstract. Threshold signature schemes play a vital role in securing digital assets within blockchain and distributed systems. FROST2 stands out as a practical threshold Schnorr signature scheme, noted for its efficiency and compatibility with standard verification processes. However, under the one-more discrete logarithm assumption, with static corruption and centralized key generation settings, FROST2 has been shown by Bellare et al. (in CRYPTO 2022) to achieve only TS-SUF-2 security, which is a consequence of its vulnerability to TS-UF-3 attacks. In this paper, we address this security limitation by presenting an enhanced variant of FROST2, namely, FROST2 `+` which achieves the TSSUF-4 security level under the same computational assumptions as the original FROST2. FROST2 `+` strengthens FROST2 by integrating additional pre-processing token verifications that help mitigate TS-UF-3 and TS-UF-4 vulnerabilities while maintaining practical efficiency. We show that FROST2 `+` can achieve TS-SUF-4 security not only under the same conditions as the original FROST2 analysis, but also when initialized with a distributed key generation protocol such as PedPoP. Our benchmark using ZCashs FROST library shows that the performance of FROST2 `+` is comparable to FROST2 and about 64 __ 79% faster than FROST when precomputation is enabled. **Keywords:** Schnorr Threshold Signature FROST TS-SUF Provable Security