An Improvement of an Identity-Based Key Issuing Protocol

In 2005, Sui et al. proposed two separable and anonymous identity-based key issuing schemes, but the impersonation attack is proposed to show that Sui et al. ´s second protocol is not free from the key-escrow problem. In order to solve such problem, Feng Cao et al. proposed an improved scheme in 2006. However, in the paper, we find that Feng Cao et al. ´s scheme is still vulnerable to the impersonation attack. According, the current paper first shows the weakness of Feng Cao et al´s improved scheme, and then proposes our improvement which reaches the goal of avoiding key escrow problem and enhances efficiency.