1. College of Information Science and Engineering, Yanshan University, Qinhuangdao, Hebei 066000, P. R. China
2. Computer Virtual Technology and System Integration Laboratory of Hebei Province, Qinhuangdao, Hebei 066000, P. R. China
3. Beijing Key Laboratory of Software Security Engineering Technique, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, P. R. China