1. Department of Computing, The Hong Kong Polytechnic University, Hong Kong, China
2. State Key Laboratory of ISN, School of Cyber Engineering, Xidian University, Xi'an, China
3. State Key Laboratory of ISN, the Shaanxi Key Laboratory of Blockchain and Secure Computing, Xidian University, Xi'an, China
4. Big Data Research Group at Western Norway Research Institute, Vestland, Norway