simple_substitution_cipher.py

import random
import sys

LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"


def main():
    message = input("Enter message: ")
    key = "LFWOAYUISVKMNXPBDCRJTQEGHZ"
    resp = input("Encrypt/Decrypt [e/d]: ")

    checkValidKey(key)

    if resp.lower().startswith("e"):
        mode = "encrypt"
        translated = encryptMessage(key, message)
    elif resp.lower().startswith("d"):
        mode = "decrypt"
        translated = decryptMessage(key, message)

    print("\n{}ion: \n{}".format(mode.title(), translated))


def checkValidKey(key: str) -> None:
    keyList = list(key)
    lettersList = list(LETTERS)
    keyList.sort()
    lettersList.sort()

    if keyList != lettersList:
        sys.exit("Error in the key or symbol set.")


def encryptMessage(key: str, message: str) -> str:
    """
    >>> encryptMessage('LFWOAYUISVKMNXPBDCRJTQEGHZ', 'Harshil Darji')
    'Ilcrism Olcvs'
    """
    return translateMessage(key, message, "encrypt")


def decryptMessage(key: str, message: str) -> str:
    """
    >>> decryptMessage('LFWOAYUISVKMNXPBDCRJTQEGHZ', 'Ilcrism Olcvs')
    'Harshil Darji'
    """
    return translateMessage(key, message, "decrypt")


def translateMessage(key: str, message: str, mode: str) -> str:
    translated = ""
    charsA = LETTERS
    charsB = key

    if mode == "decrypt":
        charsA, charsB = charsB, charsA

    for symbol in message:
        if symbol.upper() in charsA:
            symIndex = charsA.find(symbol.upper())
            if symbol.isupper():
                translated += charsB[symIndex].upper()
            else:
                translated += charsB[symIndex].lower()
        else:
            translated += symbol

    return translated


def getRandomKey():
    key = list(LETTERS)
    random.shuffle(key)
    return "".join(key)


if __name__ == "__main__":
    main()