Abstract:In 1999, Sun proposed a nonrepudiable (t, n) threshold proxy signature scheme with known signers. Latter, Hwang et al. pointed out Sun’s scheme is vulnerable to the so-called collusion attack that (n - 1) malicious proxy signers in the proxy group could conspire to impersonate the remainder one. Hwang et al. further proposed an improvement to withstand such an attack. In this paper, however, we will show that Hwang et al.’s improvement is still vulnerable to their proposed collusion attack and unable to achieve the nonrepudiation requirement as they claimed. Finally, we proposed a new efficient nonrepudiable threshold proxy signature scheme not only to eliminate the security leaks but also to be more efficient than Hwang et al.’s scheme in terms of computational complexities and communication costs.