1. Ministry of Education Key Laboratory for Intelligent Networks and Network Security, Xi’an Jiaotong University, Xi’an, China
2. Department of Computer Science and Engineering, University of California, Riverside, CA, USA
3. State Key Laboratory of Information Security, Institute of Information Engineering, Chinese Academy of Sciences, Beijing, China
4. Department of Computer Science, University of California, Davis, CA, USA
5. State Key Laboratory of Computer Science, Institute of Software, Chinese Academy of Sciences, Beijing, China