1. School of Cyberspace Security, Northwestern Polytechnical University, 26487 Xi'an, Shaanxi, China, (e-mail: wuxiaoxue00@gmail.com)
2. School of Software, Northwestern Polytechnical University, 26487 Xi'an, Shannxi, China, (e-mail: wzheng@nwpu.edu.cn)
3. Faculty of Information Technology, Monash University Faculty of Information Technology, 224480 Clayton, Victoria, Australia, 3800 (e-mail: xxia@zju.edu.cn)
4. School of Information Systems, Singapore Management University, Singapore, Singapore, Singapore, 17890 (e-mail: davidlo@smu.edu.sg)