1. School of Cyberspace Security, Beijing University of Posts and Telecommunications, Beijing, China
2. Institute of Information Security, Beijing Electronic Science and Technology Institute, Beijing, China
3. College of Computer Science and Big Data, Fuzhou University, Fuzhou, China
4. Institute 706, Second Academy of CASIC, Beijing, China
5. Department of Computer and Information Science, University of Oregon, Eugene, OR, USA
6. School of Computing and Information Systems, Singapore Management University, Singapore