1. College of Computer Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China, Guangxi Key Lab of Multi-source Information Mining & Security, Guangxi Normal University, Guilin 541004, China, and Collaborative Innovation Center of Novel Software Technology and Industrialization, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China.
2. College of Computer Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China.
3. Guangxi Key Laboratory of Image and Graphic Intelligent Processing, Guilin University of Electronic Technology, Guilin 541004, China, and Guangxi Key Laboratory of Trusted Software, Guilin University of Electronic Technology, Guilin 541004, China.
4. College of Computer Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China. (e-mail: zhe.liu@nuaa.edu.cn)
5. School of Computer Science, Fudan University, Shanghai 200433, China.