Cryptanalysis of a threshold proxy signature with known signers

A scheme of threshold proxy signature with known signers was proposed by Hwang et al. In their scheme, the receiver can identify the proxy signers that actually generated proxy signatures. Tzeng et al. demonstrated that this signature scheme is insecure and proposed an improvement to mend the information leakage. This paper shows that the improved scheme is still insecure under the original signer's forgery attack, i.e. insider attack. Our demonstration of insider attack shows that the original signer can freely choose message and delegation warrant and then produces a signature on them.