6tBJLGdN

1#!/bin/env python
2# -*- coding: utf-8 -*-
3
4'''
5homomorphic encryption using ElGamal
6'''
7
8
9GLOBAL=25
10PRIME=997
11SECRET_KEY=20
12
13
14class ElGamal:
15    def __init__(self, g, p, r, x):
16        self.g = g  # global
17        self.p = p  # prime
18        self.r = r  # random
19        self._x = x  # secret key
20        self.y = (g ** x) % p  # public key
21
22    def enc(self, m):
23        c1 = (self.g ** self.r) % self.p
24        c2 = m * ((self.y ** self.r)) % self.p
25        return (c1, c2)
26
27    def dec(self, c1, c2):
28        return (c2 * (c1 ** (self.p - 1 - self._x))) % self.p
29
30
31if __name__ == '__main__':
32    elgamal_a = ElGamal(GLOBAL, PRIME, 14, SECRET_KEY)
33    elgamal_b = ElGamal(GLOBAL, PRIME, 33, SECRET_KEY)
34    assert elgamal_a.dec(*elgamal_a.enc(10)) == 10
35    assert elgamal_a.dec(*elgamal_a.enc(12)) == 12
36    assert elgamal_b.dec(*elgamal_b.enc(20)) == 20
37    assert elgamal_b.dec(*elgamal_b.enc(22)) == 22
38
39    c1, c2 = elgamal_a.enc(2)
40    assert elgamal_a.dec(*(c1, c2 * 3)) == 6
41    assert elgamal_a.dec(*(c1, c2 * 8)) == 16
42    assert elgamal_a.dec(*(c1, c2 * 11)) == 22
43    assert elgamal_a.dec(*(c1, c2 * 15)) == 30
44
45    c1a, c2a = elgamal_a.enc(4)
46    c1b, c2b = elgamal_b.enc(19)
47    c1 = c1a * c1b
48    c2 = c2a * c2b
49    assert elgamal_a.dec(*(c1, c2)) == 76
50    assert elgamal_b.dec(*(c1, c2)) == 76