An Improvement of Sui et al´s Second ID-Based Key Issuing Protocol

In 2005, Sui et al. proposed two separable and anonymous ID-based key issuing protocols, and claimed the key-escrow problem has been solved in their second protocol. However, in this paper, we point out that Sui et al´s second protocol dose not solve the key escrow problem either by presenting impersonation attack. Further, we present an improvement to resolve such problems, in which KGC1 (or KGC2) confirms a user´s identity by a short signature signed by the user´s password key.