Improvement on C.-L. Hsu et al.´s Threshold Proxy Signature Scheme with Known Signers

In a (t,n) threshold proxy signature scheme, the original signer can delegate the signing power to n proxy signers such that any t or more proxy signers can sign messages on behalf of the original signer, but t - 1 or less of the proxy signers cannot generate a proxy signature. Recently, C.-L. Hsu et al. proposed a nonrepudiable threshold proxy signature scheme with known signers. Unfortunately, their threshold proxy signature scheme is insecure. In this paper, we show that the scheme is universally forgeable. The author then proposes a method to remove the weakness.