Cryptanalysis of an Improved Forward Secure Proxy Signature Scheme

In 2005, Wang et al. proposed a forward secure proxy signature scheme, they claimed that an adversary could not forge the former proxy signature even if proxy signerpsilas current secret key is lost. However, Zhang et al. pointed out that their scheme is insecure and doesnpsilat satisfy the forward security. Furthermore, Zhang et al. presented a modification scheme. Unfortunately, we find that their modification scheme is also insecure. Anyone can forge a valid proxy signature, and the scheme doesnpsilat even meet basic property of proxy signature. To overcome these weakness, a new forward secure proxy signature is proposed in this paper, its security is analyzed in detail.