Dept.of Computer Science and Eng.,Shanghai Jiaotong Univ.,Shanghai 200030,China;
National Lab.for Secure Communications,Chengdu 610000;
Dept.of Computer Science and Eng.,Shanghai Jiaotong Univ.,Shanghai 200030,China;
National Lab.for Secure Communications,Chengdu 610000;
Dept.of Computer Science,Qinhai Univ.for Nationalities,Xining 810000;