Security analysis of a security-enhanced certificateless signature scheme

To solve the problem of certificate management and key escrow in traditional signature, certificateless signature scheme has attracted wide attention since it was put forward. Recently, Yuan et al. presented a CLS scheme and gave its security proof in the standard model. It proved that the scheme satisfies the existential unforgeability and its security relies on the CDH assumption. However, we find that the security proof of Yuan et al.'s scheme is seriously flawed. In this paper, we present an attack against Yuan et al.'s scheme. The final security analysis results show that there is a non-negligible probability to distinguish the probability distribution of the simulated signature and the real signature. It means that it can be distinguished between the simulated game and the real game. More importantly, the security of Yuan et al.'s scheme can not be reduced to the CDH assumption.