1. School of Mathematics and Computer Science, Fujian Normal University, Fuzhou 350007, P. R. China
2. School of Computing and Mathematics, University of Ulster, Northern Ireland BT37 0QB, UK
3. Key Lab of Network Security and Cryptography, Fujian Normal University, Fuzhou 350007, P. R. China
4. School of Mathematics and Information Science, Baoji University of Arts and Sciences, Baoji 721013, Shaanxi, P. R. China