1. Ph.D. Student, Key Laboratory of Computer Network and Information Security of the Ministry of Education, Xidian Univ., Xidian Univ. Mailbox 160, Xi’an 710071, Shaanxi, China (corresponding author).
2. Lecturer, Dept. of Computer Science and Technology, Xidian Univ., Xi’an 710071, Shaanxi, China.
3. Senior Engineer, National Information Security Engineering and Technology Research Center, Beijing 100093, China.
4. Professor, Dept. of Computer Science and Technology, Xidian Univ., Xi’an 710071, Shaanxi, China.