1. Key Laboratory of Data Engineering and Knowledge Engineering, School of Information; Renmin University of China; Beijing China
2. State Key Laboratory of Integrated Services Networks; Xidian University; Xi' an China
3. Institute of Information Engineering; Chinese Academy of Sciences; Beijing 100093 China
4. School of Computer; Wuhan University; Wuhan China
5. Department of Computer Science; City University of Hong Kong; Kowloon Hong Kong
6. The Key Laboratory of Cryptographic Technology and Information Security, Ministry of Education; Shandong University; Shandong China
7. School of Electronic and Information Engineering; Beihang University; Beijing China