Secure key issuing in ID-based cryptography

ID-based cryptosystems have many advantages over PKI based cryptosystems in key distribution, but they also have an inherent drawback of key escrow problem, i.e. users' private keys are known to the key generation center (KGC). Therefore secure key issuing (SKI) is an important issue in ID-based cryptography. In multiple authority approach (Boneh & Franklin 2001, Chen et al. 2002), key generation function is distributed to multiple authorities. Keeping key privacy using user-chosen secret information (Gentry 2003, Al-Riyami & Paterson 2003) is a simple and efficient solution, but it loses the advantages of ID-based cryptosystems. In this