The Ministry of Edu. Key Lab. of Computer Network and Infor. Security;
Xidian University;
Xi' an;
Shanxi 710071;
China;
Department of lnformation and Computing Science;
Zhengzhou University of Light Industry;
Zhengzhou;
Henan 450002;
China;
Department of Math. & Physics;
Zhumadian Institute of Education;
Zhumadian;
Henan 463000;
China;