test_elgamal

test_elgamal.py

from useless.math import *
from Crypto.Util.number import *
import random


def gen_key(bits):

    p = getPrime(bits)
    g = random.randint(1, p-1)
    x = random.randint(1, p-1)
    h = pow(g, x, p)

    return g, h, p, x

def encrypt(m):
    r = random.randint(1, p-1)
    c1 = pow(g, r, p)
    c2 = m * pow(h, r, p) % p
    return c1, c2

def decrypt(c1, c2):
    c1 = modinv(c1, p)
    return c2 * pow(c1, x, p) % p

g, h, p, x = gen_key(512)

m = 114514
c1, c2 = encrypt(m)

t = 5

assert decrypt(c1, c2) == m
assert decrypt(c1, t*c2) == t*m