基于椭圆曲线 elliptic.P256生成私钥、公钥.

参考：https://blog.csdn.net/inthat/article/details/130630997 HKDF computes a PKM and OKM (where OKM is `l` bytes long) from the provided parameters.

模拟Echd椭圆取消交换秘钥的过程.