Attacks on Provably Secure Proxy-protected Signature Schemes based on Factoring

[[abstract]]Recently, Zhou et al. declared that they have proposed the first RSA-based proxy-protected signature scheme, as well as the first factoring-based proxy-protected signature scheme. They declared both schemes are capable of meeting the basic security properties: secrecy, verifiability, unforgeability and nonrepudiation. However, we found that both of these schemes actually involve several drawbacks, one of which is the concern on unforgeability. In this paper, we demonstrate how to forge the proxy signature in their schemes and show that their schemes are actually insecure