#!/usr/bin/env python3
import math
import random
import time
from itertools import combinations


def euclid(a, b):
    """returns the Greatest Common Divisor of a and b.
    Inspired by https://en.wikipedia.org/wiki/Euclidean_algorithm#Implementations"""
    a = abs(a)
    b = abs(b)
    if a < b:
        a, b = b, a
    while b != 0:
        a, b = b, a % b
    return a


def coPrime(x, y):
    """returns 'True' if the values in the list L are all co-prime
       otherwise, it returns 'False'. """
    if euclid(x, y) != 1:
        return False
    return True


def iterative_extended_euclid(a, b):
    """ returns the GCD (in old_r) of a and b as well as the "Bezout's Identity" such that
    a*old_s + b*old_t = GCD(a,b).
    Algorithm is adopted from https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm#Pseudocode
    """
    old_r, r = a, b
    old_s, s = 1, 0
    old_t, t = 0, 1

    while r != 0:
        quotient = old_r // r
        old_r, r = r, old_r - (quotient * r)
        old_s, s = s, old_s - (quotient * s)
        old_t, t = t, old_t - (quotient * t)

    return old_r, old_s, old_t




def modInv(a, m):
    """returns the multiplicative inverse of a in modulo m as a
       positive value between zero and m-1
       Adopted from https://en.wikipedia.org/wiki/Modular_multiplicative_inverse#Extended_Euclidean_algorithm
       """
    if coPrime(a, m):
        linear_combination = iterative_extended_euclid(a, m)
        return linear_combination[1] % m
    else:
        return 0


def miller_rabin(n, k):

    # Implementation uses the Miller-Rabin Primality Test
    # The optimal number of rounds for this test is 40
    # See http://stackoverflow.com/questions/6325576/how-many-iterations-of-rabin-miller-should-i-use-for-cryptographic-safe-primes
    # for justification

    # If number is even, it's a composite number

    if n == 2:
        return True

    if n % 2 == 0:
        return False

    r, s = 0, n - 1
    while s % 2 == 0:
        r += 1
        s //= 2
    for _ in range(k):
        a = random.randrange(2, n - 1)
        x = pow(a, s, n)
        if x == 1 or x == n - 1:
            continue
        for _ in range(r - 1):
            x = pow(x, 2, n)
            if x == n - 1:
                break
        else:
            return False
    return True


def findAPrime(a, b, k):
    """Return a pseudo prime number roughly between a and b,
    (could be larger than b). """
    x = random.randint(a, b)
    while True:
        if miller_rabin(x, k):
            return x
        else:
            x += 1


def newKey(a, b, k):
    """ Try to find two large pseudo primes roughly between a and b.
    Generate public and private keys for RSA encryption."""

    p = findAPrime(a, b, k)
    while True:
        q = findAPrime(a, b, k)
        if q != p:
            break

    n = p * q
    m = (p - 1) * (q - 1)  # Compute phi(n) for n=pq where p and q are prime

    # Find and e that is coprime to phi(n) to be used in the public key
    while True:
        e = random.randint(1, m)
        if coPrime(e, m):
            break

    # Let d be the modular inverse to e, to be used as private key
    d = modInv(e, m)
    return (n, e, d)


def encrypt(message, modN, e):
    """given a string message, public keys and blockSize, encrypt using
    RSA algorithms."""
    start = time.time()
    ret = pow(message, e, modN)
    print("Elapsed time of encryption:", time.time() - start)
    return ret


def decrypt(secret, modN, d):
    """reverse function of encrypt"""
    start = time.time()
    ret = pow(secret, d, modN)
    print("Elapsed time of decryption:", time.time() - start)
    return ret


if __name__ == '__main__':
    n, e, d = newKey(2**2048, 2 ** 2049, 25)
    message = 2**2048

    print("original message is {}".format(message))
    print("-"*80)
    cipher = encrypt(message, n, e)
    print("cipher text is {}".format(cipher))
    print("-"*80)
    deciphered = decrypt(cipher, n, d)
    print("decrypted message is {}".format(deciphered))