Key generation, encryption & example

from random import randrange
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad, unpad
from base64 import b64encode, b64decode


def generate_key():
    return randrange(1e15, 1e16 - 2)


def generate_shared(private_key, public_key):
    return str(private_key * public_key)[-16:]


def encrypt_message(message, key):
    message = str(message).encode()
    key = str(key).encode()
    cipher = AES.new(key, AES.MODE_CBC)
    ciphertext_bytes = cipher.encrypt(pad(message, AES.block_size))
    return b64encode(cipher.iv).decode('utf-8'), b64encode(ciphertext_bytes).decode('utf-8')


def decrypt_message(key, iv, ciphertext):
    key = str(key).encode()
    iv = b64decode(iv)
    ciphertext = b64decode(ciphertext)
    cipher = AES.new(key, AES.MODE_CBC, iv)
    return unpad(cipher.decrypt(ciphertext), AES.block_size).decode()


if __name__ == '__main__':  # Testing the theory
    public = 1234567890101112
    client = randrange(1e15, 1e16)
    server = randrange(1e15, 1e16)

    print('Public key: {:d}\nClient key: {:d}\nServer key: {:d}\n'.format(public, client, server))

    pub_cli = int(str(public * client)[-16:])  # push_zero(public * client, 15)
    pub_ser = int(str(public * server)[-16:])  # push_zero(public * server, 15)

    print('Last 16 digits of public * client: {:d}\nLast 16 digits of public * server: {:d}\n'.format(pub_cli, pub_ser))

    shared_key_cli = int(str(pub_ser * client)[-16:])  # push_zero(pub_ser * client, 15)
    shared_key_ser = int(str(pub_cli * server)[-16:])  # push_zero(pub_cli * server, 15)

    print('Last 16 digits of pub_ser * client: {:d}\nLast 16 digits of pub_cli * server: {:d} (This is the shared key)'
          '\nshared_key_cli == shared_key_ser = {}'.format(shared_key_cli, shared_key_ser,
                                                           shared_key_cli == shared_key_ser))