Cryptanalysis of Two Proxy Signature Schemes with Restricted Times of Delegation

The proxy signature scheme enables a proxy signer to sign on behalf of the original signer and this practical function becomes useful for many applications. However an unlimited delegation of signing capability might usually lead to a security concern. To address the aforementioned issue, Choi et al. proposed a modified Schnorr signature scheme with restricted signing capability. In this paper, we pointed out that Choi et al.´s scheme failed to satisfy the strong unforgeability property of the ordinary proxy signature scheme.