#!/usr/bin/env python3

"""ElGamal encryption and decryption"""

import random
import sys
import inverse


def main() -> None:
    """"""

    if len(sys.argv) == 1:
        command = input("command ('encrypt' or 'decrypt'): ")
    else:
        command = sys.argv[1]
    if command == "encrypt":
        x = int(input("x : "))
        g = int(input("g : "))
        e = int(input("e : "))
        k = int(input("k (0 for random) : "))
        if k == 0:
            k = random.randint(1, p - 1)
        p = int(input("p : "))
        r, y = encrypt(x, g, e, k, p)
        print(f"({r}, {y})")
    elif command == "decrypt":
        r = int(input("r : "))
        y = int(input("y : "))
        d = int(input("d : "))
        p = int(input("p : "))
        x = decrypt(r, y, d, p)
        print(x)


def encrypt(x: int, g: int, e: int, k: int, p: int) -> tuple[int, int]:
    """Encrypt a message (x) using elgamal"""


    r = pow(g, k, p)
    y = x * pow(e, k, p) % p

    return r, y


def decrypt(r: int, y: int, d: int, p: int) -> int:
    """Decrypt a message (r, y) using elgamal"""

    inv = inverse.inverser(pow(r, d, p), p, quiet=True)
    if inv is None:
        raise ValueError("{p} is not prime")
    x: int = (y * inv) % p

    return x


if __name__ == "__main__":
    main()