import random

import string

​

def shift(current_position, distance, direction: (0, 1)):

    direction = 1 if direction else -1

    return current_position + direction * distance

​

​

def stream_cipher(message, key, do_encrypt=True):

    """

    This function will receive a message, a key, and the

    decision to encrypt or decrypt the message.

    The function uses a stream cipher encryption algorithm 

    which replaces each letter in the message with a 

    pseudo-random character from a given character set.

    Example:

    >>> stream_cipher("This is a test", 1234)

    'wPwV~#5;:D"905'

    >>> stream_cipher('wPwV~#5;:D"905', 1234, False)

    'This is a test'

    """

    random.seed(key)

    # The character set must be multiplied by 2 so

    # a character shifted beyond the end of the 

    # character set will loop back to the beginning.

    characters = 2 * (

                      string.ascii_letters +

                      string.digits +

                      string.punctuation + ' '

                     )

    # I declare this in a variable so the 

    # program can work with a variable length character set

    lenchars = len(characters)//2

    # This will replace each character in the message

    # with a pseudo-random character selected from

    # the character set.

    return ''.join([characters[shift(characters.index(message[each_char]), lenchars - int(lenchars * random.random()), do_encrypt)] for each_char in range(len(message))])

​

​

def main():

    message = input("Input a message: ")

    key = input("Input a key: ")

    while True:

        do_encrypt = input("Encrypt or decrypt the message? (1,0): ")

        if do_encrypt in ('1', '0'):

            break

        print("Please input a valid number,\n"

              "either 0 for decryption or 1 for encryption.")

    print(encrypt(message, key, do_encrypt == '1'))

    input("Press enter to exit.")

​

if __name__ == '__main__':

    main()