cipher.py

SYMBOLS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz 1234567890!@#$%^&*()"
MAX_KEY_SIZE = len(SYMBOLS)


def get_mode():
    while True:
        print("Do you wish to encrypt or decrypt a message?")
        mode = input().lower()
        if mode in ["encrypt", "e", "decrypt", "d"]:
            return mode
        else:
            print('Enter either "encrypt" or "e" or "decrypt" or "d".')


def get_message():
    print("Enter your message:")
    return input()


def get_key():
    key = 0
    while True:
        print('Enter the key number (1-%s)' % (MAX_KEY_SIZE))
        key = int(input())
        if (key >= 1 and key <= MAX_KEY_SIZE):
            return key


def get_translated_message(mode, message, key):
    if mode[0] == 'd':
        key = -key
    translated = ''

    for symbol in message:
        symbol_index = SYMBOLS.find(symbol)
        if symbol_index == -1:  # Symbol not found in SYMBOLS.
            # Just add this symbol without any change.
            translated += symbol
        else:
            # Encrypt or decrypt.
            symbol_index += key
            if symbol_index >= len(SYMBOLS):
                symbol_index -= len(SYMBOLS)
            elif symbol_index < 0:
                symbol_index += len(SYMBOLS)

        translated += SYMBOLS[symbol_index]
    return translated


mode = get_mode()
message = get_message()
key = get_key()
print('Your translated text is:')
print(get_translated_message(mode, message, key))