An improved designated-verifier proxy signature scheme

As a special signature, proxy signature allows a entity called original signer to delegate his signing capability to another entity to produce signature on behalf of him. By combining the ideas of proxy signatures and designated-verifier signatures, G:L.Wang et.al. proposed a strong designated-verifier proxy signature scheme (for short DVP) and claimed that the security of their scheme was based on the computational Diffie-Hellman assumption. Unfortunately, in the paper, we give security analysis of Wang et.al's strong designated-verifier proxy signature scheme and show that their scheme is forgeable. Namely, an adversary can forge a DVP signature on arbitrary message m without the knowledge of the proxy secret key xp or the designated verifier's secret key x(C). At the same time, we give two attacks on the Strong designated-verifier proxy signature scheme. Finally, we give an improved scheme to overcome the above forgery attacks, and show the scheme to be secure in random oracle model.