Cryanalysis on a Nonrepudiable Threshold Proxy Signature Scheme with Known Signers

In the paper, we analyze the security of C.-L. Hsu et al.'s threshold proxy signature scheme. The scheme suffers from universal forgery and does not hold nonrepudiation. An adversary can forge (t, n) threshold proxy signature on any message m. We propose an improvement to remedy the weakness of C.-L Hsu et al.'s scheme.