Simple Encryption

simple_encryption.py

encode_table = {
    'a': "01",
    'b': "02",
    'c': "03",
    'd': "04",
    'e': "05",
    'f': "06",
    'g': "07",
    'h': "08",
    'i': "09",
    'j': "10",
    'k': "11",
    'l': "12",
    'm': "13",
    'n': "14",
    'o': "15",
    'p': "16",
    'q': "17",
    'r': "18",
    's': "19",
    't': "20",
    'u': "21",
    'v': "22",
    'w': "23",
    'x': "24",
    'y': "25",
    'z': "26"
}


def gcd(a, b):
    """Calculate the Greatest Common Divisor of a and b.

    Unless b==0, the result will have the same sign as b (so that when
    b is divided by it, the result comes out positive).
    """
    while b:
        a, b = b, a % b
    return a


def is_prime(n):
    d = n - 1
    s = 0
    while not d & 1:
        s += 1
        d >>= 1
    for a in (2, 13, 23, 1662803):
        if pow(a, d, n) != 1 and all(pow(a, (1 << r) * d, n) != n - 1 for r in range(0, s)):
            return False
    return True


def encode_message(msg):  # type: (str) -> int
    encoded = ''
    for i in msg:
        encoded += encode_table[i]
    return int(encoded)


def decode_message(msg):  # type: (int|str) -> str
    if isinstance(msg, int):
        msg = str(msg)

    decoded = ''
    for i in range(0, len(msg), 2):
        value = msg[i:i+2]
        key = ''
        for key_, value_ in encode_table.items():
            if value == value_:
                key = key_
                break

        if len(key) == 0:
            continue

        decoded += key

    return decoded


def encrypt(msg, key):
    msg = int(msg)
    msg = msg * pow(10, 2)

    while not is_prime(msg):
        msg += 1

    return msg * key


if __name__ == '__main__':
    secret = "062114032009151449"
    message = decode_message(secret)
    print("The secret message is: %s" % message)

    key = 131

    msg1 = "ilovejenniwithallmyheart"
    secret1 = encode_message(msg1)
    encoded1 = encrypt(secret1, key)
    print("The encoded message is: %d" % encoded1)

    msg2 = "ilikecodingbecauseitisuseful"
    secret2 = encode_message(msg2)
    encoded2 = encrypt(secret2, key)
    print("The encoded message is: %d" % encoded2)

    divisor = gcd(encoded1, encoded2)
    print("The secret key was found to be: %d" % divisor)