1. ECDH
ECDH是用来交换密码的，Alice和Boc使用椭圆函数secp256k1,生成各自的公钥和私钥，然后用自己的私钥和对方的公钥，生成一个screate key, 这个screate key是一样的。
1.1 secp256k1
公钥分为压缩和非压缩的，这个看前缀，如果是0x04开头，是非压缩。
压缩的：只有x,因为y可以推断出。
非压缩：把椭圆曲线上的x和y相连，就是x+y的hex字符串相连，就是这么简单。有的地方如果接受64个字节，那么不要0x04前缀；有的地方接受65个字节，那么需要加上0x04前缀。
Actually the compressed form in hex is 02 + X_coordinate if Y coordinate is even, or 03 + X_coordinate if Y coordinate is odd; comparing to the uncompressed form is 04 + X_coordinate + Y_coordinate.
So to retrieve the coordinates from the compressed form, just calculate the Y coordinate by curve equation Y^2 = X^3 + a*X + b, there will be two Y values, then choose the correct one by the prefix which is 02 or 03.
You will get the actual coordinates of the point.
https://www.zhihu.com/question/22399196
https://zhuanlan.zhihu.com/p/69042756
https://bitcoin.stackexchange.com/questions/19666/why-do-keys-need-both-x-and-y-coordinates-if-x-can-be-solved-for-y-using-the-cu
https://crypto.stackexchange.com/questions/90151/verify-that-a-point-belongs-to-secp256r1