Cryptanalysis and Improvement of a Secure Proxy Signature Scheme Based on Elliptic Curve Cryptosystem

Based on the RSA problem, Zhou et al.’s proposed a proxy-protected proxy signature scheme. They claimed that their scheme were secure and efficient. However, this paper shows that their proposed proxy-protected proxy signature scheme is insecure against the replaced warrant attack: the original signer can remove the old warrant from a valid proxy signature and then construct a new warrant, and at the end the original signer can forge a valid proxy signature with the new warrant. To avoid the problem, an improved proxy signature scheme is given and its security is analyzed.