1. School of Mathematics, Hangzhou Normal University, Hangzhou 311121, China
2. Key Laboratory of Cryptography of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121, China
3. Department of Mathematics, Guru Ghasidas Vishwavidyalaya, Bilaspur 495009, India
4. Section of Mathematics, Department of Information Technology, University of Technology and Applied Sciences, P.O. Box 77, Shinas 324, Oman