An Improvement of an Identity-Based Key Issuing Protocol

At TCP ADS 2005, Sui et al. proposed a novel separable and anonymous ID-based key issuing scheme without secure channel, and claimed the key-escrow problem has been solved. In this paper, we present a security analysis on their schemes and point out that their two protocols are not secure and they do not solve the key-escrow problem. Further, we present an improved key issuing protocol which is proved to be secure against adaptive chosen message attack without the random oracle model under the q - SDH assumption