Paper 2026/1966 Original: eprint.iacr.org/2026/1966 ↗

Design and Analysis of Isogeny-Based Strong Designated Verifier Signature

Abstract

Strong designated-verifier signatures provide authentication while restricting verification to a chosen verifier and protecting the signer from transferable evidence. Designing such signatures in the post-quantum setting is challenging because authentication, signer privacy, simulation, and efficiency must be achieved simultaneously. Recently, Renan proposed CSI-SDVS, a compact post-quantum strong designated-verifier signature scheme built from CSIDH-style commutative isogeny class-group actions. We show that its response design, $z_i=b_i-s_i$, breaks privacy of the signer's identity: because the PSI experiment reveals both candidate signer secret keys, an adversary can reconstruct the signing randomness and identify the actual signer with overwhelming probability. We validate the attack over 30,000 executions, obtaining 100\% signer identification in the main 128-bit experiment and for $\eta\in \{1,2,4,8\}$. In the following, we propose an isogeny-based strong designated verifier signature. We prove correctness, non-transferability, signer privacy, and strong unforgeability under Gap Parallelization in the random-oracle model. For $\eta=1$, the redesigned signature is 113 bytes compared with 49 bytes in CSI-SDVS, while signer key sizes remain unchanged.