Department;
of;
Mathematical;
Science,;
Xidian;
University,;
Xi'an;
710071,;
Shaanxi,;
China;
Key;
Laboratory;
of;
Computer;
Networks;
and;
Information;
Security;
of;
Ministry;
of;
Education,;
Xidian;
University,;
Xi'an;
710071,;
Shaanxi,;
China;
Xi'an;
Communications;
Institute,;
Xi'an;
710106,;
Shaanxi,;
China;