1. School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore
2. Department of Mathematics, Nanjing University of Aeronautics and Astronautics, Nanjing, People's Republic of China
3. State Key Laboratory of Information Security, Institute of Information Engineering, Chinese Academy of Sciences, Beijing, People's Republic of China
4. School of Mathematics, Southeast University, Nanjing, People's Republic of China