main.py

#!/usr/bin/python3
# -*- coding: utf-8 -*-

# =============================================================================
#
#        FILE: main.py
#      AUTHOR: Tan Duc Mai
#       EMAIL: henryfromvietnam@gmail.com
#        DATE: 2021-10-21
# DESCRIPTION: Use Caesar Cipher technique to encrypt or decrypt an
#              inputted message.
#   I hereby declare that I completed this work without any improper help
#   from a third party and without using any aids other than those cited.
#
# =============================================================================


# ------------------------------- Module Imports ------------------------------
# To randomise the offset value, hence unpredictable encryption.
import random

# To implement Design by Contract.
import icontract


# ---------------------------- Function Definitions ---------------------------
@icontract.ensure(lambda result: result is None)
def menu_driven_program():
    print(
        '-------------------',
        '     MAIN MENU',
        '-------------------',
        '1. Enter Message',
        '2. Encrypt Message',
        '3. Decrypt Message',
        '4. Quit',
        sep='\n',
        end='\n\n',
    )


@icontract.ensure(lambda result: isinstance(result, int))
def validate_option():
    """Prompt the user for a number and validate it.

    Returns
    -------
    int
        The valid guess taken from the user.
    """
    option = None

    while option is None or option not in range(1, 5):
        try:
            option = float(input('Enter an option (1,2,3,4): '))
        except ValueError as e:
            print(f'Invalid choice: {e}.')
        else:
            if option not in range(1, 5):
                print('Invalid choice: option should be between 1 and 4.')

    return int(option)


@icontract.ensure(lambda result: isinstance(result, str))
def option_1():
    """Prompt for and display the user message to the screen.

    Returns
    -------
    str
        The message received from the user.
    """
    message = input('Please enter a new message: ')
    if message != '':
        print(f'Your message is: {repr(message)}.', end='\n\n')
    else:
        print()
    return message


@icontract.ensure(lambda message: isinstance(message, str))
@icontract.ensure(lambda result: isinstance(result, str))
def option_2(message):
    """Encrypt the user message (no encryption if message is empty)

    Parameters
    ----------
    str
        Original message received from the user, could be empty if the user
        chooses this option before the first one.

    Returns
    -------
    str
        The message that has been successfully encrypted.
    """
    if message == '':
        print('Error: Cannot encrypt an empty message.', end='\n\n')
        return message
    else:
        offset = random.randint(32, 126)
        encrypted_message = ''
        for i in message:
            unicode_value = ord(i) + offset
            while unicode_value > 126:
                unicode_value -= 95
            encrypted_message += chr(unicode_value)
        encrypted_message += chr(offset)
        print('Your message was successfully encrypted.')
        print(f'Your message is: {repr(encrypted_message)}.', end='\n\n')
        return encrypted_message


@icontract.ensure(lambda encrypted_message: isinstance(encrypted_message, str))
@icontract.ensure(lambda result: isinstance(result, str))
def option_3(encrypted_message):
    """Decrypt the user message (no decryption if message is empty)

    Parameters
    ----------
    str
        The message that has been encrypted, could be empty if the user chooses
        this option before the first one.

    Returns
    -------
    str
        The message that has been successfully decrypted.
    """
    if encrypted_message == '':
        print('Error: Cannot decrypt an empty message.', end='\n\n')
        return encrypted_message
    else:
        offset = ord(encrypted_message[-1])
        decrypted_message = ''
        for i in encrypted_message:
            unicode_value = ord(i) - offset
            while unicode_value < 32:
                unicode_value += 95
            decrypted_message += chr(unicode_value)
        print('Your message was successfully decrypted.')
        print(f'Your message is: {repr(decrypted_message[:-1])}.', end='\n\n')
        return decrypted_message[:-1]


# ------------------------------- Main Function -------------------------------
def main():
    # Display the menu.
    menu_driven_program()

    # Initialise an empty message variable.
    message = ''

    # Validate option chosen by user.
    option = validate_option()

    # Now we have a valid option.
    while option != 4:
        if option == 1:
            message = option_1()
        elif option == 2:
            message = option_2(message)
        else:
            message = option_3(message)
        # Display the menu again.
        menu_driven_program()
        # Validate option chosen by user.
        option = validate_option()

    # Exit the loop, meaning option == 4:
    if message != '':
        print(f'Your message is: {repr(message)}.')
    print('\nGoodbye')


# --------------------------- Call the Main Function --------------------------
if __name__ == '__main__':
    main()