1. Key Laboratory of Network Security and Cryptology, Fujian Normal University, Fuzhou, Fujian 350007, P. R. China
2. School of Mathematics and Computer Science, Fujian Normal University, Fuzhou, Fujian 350007, P. R. China
3. School of Mathematical Sciences, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China
4. Wentsun Wu Key Laboratory of CAS, Hefei, Anhui 230026, P. R. China