1. School of Computer Science, Wuhan University, Wuhan, China
2. School of Computer Science and the Key Laboratory of Aerospace Information Security and Trusted Computing, Ministry of Education, School of Cyber Science and Engineering, Wuhan University, Wuhan, China
3. Department of Computer and Information Science, University of Oregon, Eugene, OR, USA
4. Department of Computer Science and Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong