Proxy-protected signature secure against the undelegated proxy signature attack

The proxy signature scheme enables an original signer to delegate his/her signing capability to a designated proxy signer, thereby the proxy signer can sign messages on behalf of the original signer. Recently, Zhou et al. proposed two proxy-protected signature schemes. One is based on the RSA problem and the other is based on the integer factorization problem. In this paper, however, we point out that Zhou et al.'s schemes are insecure against undelegated proxy signature attack because any user without the delegation of the original signer can generate a valid proxy signature. To solve this problem, an improved scheme is proposed and its security is analyzed.