1. Information Security Center, State Key Laboratory of Networking and Switching Technology, Beijing University of Posts and Telecommunications, Beijing 100876, China
2. School of Computer Science, Communication University of China, Beijing 100024, China
3. School of Computer Science and Engineering, The University of Aizu, Aizu Wakamatsu 965-8580, Japan
4. Department of Information and Electronic Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan