Cryptanalysis of Two Nonrepudiable Threshold Proxy Signature Schemes

Abstract

This paper will show that Hsu and Wu’s efficient nonrepudiable threshold proxy signature scheme with known signers and Yang, Tzeng and Hwang’s efficient nonrepudiable threshold proxy signature scheme with known signers are insecure. The malicious original signer can forge a valid threshold proxy signature for any message and any warrant after getting a valid proxy signature. In addition, since Yang et al.’s scheme is more efficient than Hsu and Wu’s scheme in terms of computational complexities and communication costs, this paper only presents a simple countermeasure to improve Yang et al.’s scheme.