Improvement of Identity-Based Threshold Proxy Signature Scheme with Known Signers

In 2006, Bao et al proposed an identity-based threshold proxy signature scheme with known signers. In this paper, we show that Bao et al ’s scheme is vulnerable to the forgery attack. An adversary can forge a valid threshold proxy signature for any message with knowing a previously valid threshold proxy signature. In addition, their scheme also suffers from the weakness that the proxy signers might change the threshold value. That is, the proxy signers can arbitrarily modify the threshold strategy without being detected by the original signer or verifiers, which might violate the original signer’s intent. Furthermore, we propose an improved scheme that remedies the weaknesses of Bao et al ’s scheme. The improved scheme satisfies all secure requirements for threshold proxy signature.