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 el 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 el al.'s scheme is still vulnerable to the impersonation attack According, the current paper first shows the weakness of Feng Cao el al's improved scheme, and then proposes our improvement which reaches the goal of avoiding key escrow problem and enhances efficiency.