[1] Shandong Univ, Sch Comp Sci & Technol, Jinan, Peoples R China
[2] Shandong Univ, Sch Cyber Sci & Technol, Minist Educ, Key Lab Cryptol Technol & Informat Secur, Jinan, Peoples R China
[3] CISPA Helmholtz Ctr Informat Secur, Saarland Informat Campus, Saarbrucken, Germany