Abstract: Recently, YU Y K et al. proposed a series of II}basecf threshold proxy signature in the standard model. This paper constructed three attack algorithms for the latest scheme of YU-ZHENG, with which attacker can forge valid both regular signature on behalf of the original signer and proxy signature of any proxy signer on any message without knowing the signing key of these signers. Our attacking algorithms work well with scheme of YU-ZHENG and the decedent schemes. Attacks show that this series of schemes are unsecure. We analyzed the root cause of attacks and gave some suggestions for modifications in the end.

Key words: Identity based signature, Threshold proxy signature, Forge attack, Bilinear map