Monoalphabetic Cipher

monoalphabetic.py

# Simple Substitution Cipher
# http://inventwithpython.com/hacking (BSD Licensed)

import sys, random

LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def main():
    myMessage = 'When you do the common things in life in an uncommon way, you will command the attention of the world'
    myKey = 'QWERTYUIOPASDFGHJKLZXCVBNM'
    myMode = 'encrypt' # set to 'encrypt' or 'decrypt'

    checkValidKey(myKey)

    if myMode == 'encrypt':
        translated = encryptMessage(myKey, myMessage)
    elif myMode == 'decrypt':
        translated = decryptMessage(myKey, myMessage)
    print('Using key %s' % (myKey))
    print('The %sed message is:' % (myMode))
    print(translated)


def checkValidKey(key):
    keyList = list(key)
    lettersList = list(LETTERS)
    keyList.sort()
    lettersList.sort()
    if keyList != lettersList:
        sys.exit('There is an error in the key or symbol set.')


def encryptMessage(key, message):
    return translateMessage(key, message, 'encrypt')


def decryptMessage(key, message):
    return translateMessage(key, message, 'decrypt')


def translateMessage(key, message, mode):
    translated = ''
    charsA = LETTERS
    charsB = key
    if mode == 'decrypt':
        # For decrypting, we can use the same code as encrypting. We
        # just need to swap where the key and LETTERS strings are used.
        charsA, charsB = charsB, charsA

    # loop through each symbol in the message
    for symbol in message:
        if symbol.upper() in charsA:
            # encrypt/decrypt the symbol
            symIndex = charsA.find(symbol.upper())
            if symbol.isupper():
                translated += charsB[symIndex].upper()
            else:
                translated += charsB[symIndex].lower()
        else:
            # symbol is not in LETTERS, just add it
            translated += symbol

    return translated


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

main()

basmah