# only work on python3 and install python-ecdsa.
from ecdsa.ellipticcurve import CurveFp, Point
from binascii import hexlify
from random import randint

class ECDH:
    #defined value for P-256 curve (FIPS 186-3)
    p  = 115792089210356248762697446949407573530086143415290314195533631308867097853951
    n  = 115792089210356248762697446949407573529996955224135760342422259061068512044369
    a  = -3
    b  = 0x5ac635d8aa3a93e7b3ebbd55769886bc651d06b0cc53b0f63bce3c3e27d2604b
    c  = 0x7efba1662985be9403cb055c75d4f7e0ce8d84a9c5114abcaf3177680104fa0d
    gx = 0x6b17d1f2e12c4247f8bce6e563a440f277037d812deb33a0f4a13945d898c296
    gy = 0x4fe342e2fe1a7f9b8ee7eb4a7c0f9e162bce33576b315ececbb6406837bf51f5

    c256 = CurveFp(p, a, b)
    p256 = Point(c256, gx, gy, n)

    def generate_public_key(self,ska):
        d = int.from_bytes(ska,"big")
        q = d * self.p256
        return q.x().to_bytes(32,"big"),q.y().to_bytes(32,"big")

    def generate_dhkey(self,sk,pkx,pky):
        d = int.from_bytes(sk,"big")
        coordinates = Point(self.c256,int.from_bytes(pkx,"big"),int.from_bytes(pky,"big"),self.n)
        dhk = d * coordinates
        return dhk.x().to_bytes(32,"big"), dhk.y().to_bytes(32,"big")

if __name__ == '__main__':

    ecdh = ECDH()
    #Secret Key
    ska     = randint(1,ecdh.n-1).to_bytes(32,"big")
    skb     = randint(1,ecdh.n-1).to_bytes(32,"big")

    #Public Key A B for certain coordinate
    pkax,pkay = ecdh.generate_public_key(ska)
    pkbx,pkby = ecdh.generate_public_key(skb)

    print("Secret Key A   : %s" % hexlify(ska).decode())
    print("Secret Key B   : %s\n" % hexlify(skb).decode())
    print("Public Key A-X : %s" % hexlify(pkax).decode())
    print("Public Key A-Y : %s" % hexlify(pkay).decode())
    print("Public Key B-X : %s" % hexlify(pkbx).decode())
    print("Public Key B-Y : %s\n" % hexlify(pkby).decode())

    dhkax, dhkay = ecdh.generate_dhkey(ska,pkbx,pkby)
    dhkbx, dhkby = ecdh.generate_dhkey(skb,pkax,pkay)

    print("P256(Secret-Key A, Public Key B-X, B-Y) :\nDH-Key (X) : %s\n" % hexlify(dhkax).decode())
    print("P256(Secret-Key B, Public Key A-X, A-Y) :\nDH-Key (X) : %s\n\n" % hexlify(dhkbx).decode())
    print("P256(Secret-Key A, Public Key B-X, B-Y) :\nDH-Key (Y) : %s\n" % hexlify(dhkay).decode())
    print("P256(Secret-Key B, Public Key A-X, A-Y) :\nDH-Key (Y) : %s\n" % hexlify(dhkby).decode())