Two modified nominative proxy signature schemes for mobile communication

The nominative proxy signature scheme is a method in which the designated proxy signer generates a nominative signature and transmits it to a verifier, instead of the original signer. Recently, Seung-Hyun, et al., proposed a nominative proxy signature scheme for mobile communication and claimed that the scheme was non-repudiation. Unfortunately, in this work we show that the scheme is not non-repudiation. Note that a malicious original signer can forge a message signature which was produced by the proxy signer. Finally, we also present two improved vision of the scheme to repair the security flaw of our scheme based on the difficulty of solving the discrete logarithm.