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 x_P 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 .