Cryptanalysis and Improvement on a Threshold Proxy Signature Scheme

In a (t, n) threshold proxy signature scheme, an original signer can delegate the signature authority to a proxy group of n member such that t or more than t proxy signers can cooperatively sign messages on behalf of the original signer, but t-1 or fewer proxy signers cannot generate a valid proxy signature. In this paper, we review the security of C.L. Hsu et al. 's threshold proxy signature schemes with known signers. We show that the threshold proxy signature scheme is insecure against forgery attack. C.L. Hsu et al.'s threshold proxy signature scheme is universally forgeable. A new improvement scheme is proposed. The new scheme remedies its weakness. Our proposed threshold proxy signature is secure against chosen message attacks and chosen warrant attacks in the random oracle model under DL assumption.