NewCipher creates and returns a new cipher.Block.

ReadKeyFromPem will return SM4Key from PEM format data.

ReadKeyFromPemFile will return SM4Key from filename that saved PEM format data.

密码反馈模式（Cipher FeedBack (CFB)）https://blog.csdn.net/zy_strive_2012/article/details/102520356https://blog.csdn.net/sinat_23338865/article/details/72869841.

Paper: The Galois/Counter Mode of Operation (GCM) David A.

输出反馈模式（Output feedback, OFB）https://blog.csdn.net/chengqiuming/article/details/82390910https://blog.csdn.net/sinat_23338865/article/details/72869841.

WriteKeyToPem will convert SM4Key to PEM format data and return it.

WriteKeyToPemFile will convert SM4Key to PEM format data, then write it into the input filename.

Cipher is an instance of SM4 encryption.