Welcoming Message from the General Chair and Organizer of ICCIT 2007 Committee

In this paper, we propose a new efficient model for issuing multiple private keys in IBC (identity-based cryptosystem). In our model, the private key of a user is divided into two components: LTK (long-term key) and STK (short-term key). Both are issued separately by different parties. The LTK is issued in a threshold manner by KPAs (key privacy agency). In contrast, the STK is issued by a single authority called KGC (key generation center). A user can efficiently obtain a new private key containing the same identity information by contacting only the KGC and obtaining a new STK. We also give a security proof of the key issuing model and present a new IBE (identity-based encryption) scheme based on our key issuing model.