Supported by the National Natural Science Foundation of China under Grant Nos.90604004, 60773103 (国家自然科学基金); the National Basic Research Program of China under Grant No.2010CB328104 (国家重点基础研究发展计划(973)); the Research Foundation for the Doctoral Program of the Ministry of Education of China under Grant No.200802860031 (国家教育部高等学校博士点学科专项科研基金); the Jiangsu Provincial Natural Science Foundation of China under Grant Nos.BK2007708, BK2008030 (江苏省自然科学基金); the Key Laboratory of Network and Information Security of Jiangsu Province of China under Grant No.BM2003201 (江苏省网络与信息安全重点实验室资助项目); the Key Laboratory of Computer Network and Information Integration (Southeast University), the Ministry of Education of China under Grant No.93K-9 (计算机网络和信息集成教育部重点实验室(东南大学))