Further Cryptanalysis of some Proxy Signature Schemes

Proxy signature is a signature that an original signer delegates his or her signing capability to a proxy signer, and then the proxy signer creates a signature on behalf of the original signer. However, Sun et al. [7] showed that the proxy and multi-proxy signatures of Lee et al. [3], and the strong proxy signature scheme with proxy signer privacy protection of Shum et al. [6] are not against the original signer’s forgery attack, so these schemes do not process the property of unforgeability. In this paper, we present an extensive forgery method on these schemes, following the work due to Sun et al.