Acknowledgements We would like to thank Professor W.W. Hager and Professor H. Zhang for the CG_DESCENT code. We are grateful to the anonymous referees and editor for their useful comments, which have made the paper clearer and more comprehensive than the earlier version. This work is supported by the National Natural Science Foundation of China (11601012, 11431002, 11625105, 71771030, 11861002), Natural Science Foundation of Ningxia (NZ17103), Natural Science Foundation of Guangxi (2018GXNS-FAA138169), Guangxi Key Laboratory of Cryptography and Information Security (GCIS201708), Project of Shandong Province Higher Education Science and Technology (J17KA166), the key project of North Minzu University (ZDZX201804).