[1] Beijing Inst Technol, Sch Math & Stat, Beijing 102400, Peoples R China
[2] Beijing Inst Technol, Beijing Key Lab MCAACI, Beijing 102400, Peoples R China
[3] Minist Ind & Informat Technol, Key Lab Math Theory & Computat Informat Secur, Beijing 102400, Peoples R China
[4] Imperial Coll London, Dept Comp, London SW7 2AZ, England