Implement Monoalphabetic cipher encryption-decryption in Python.

Monoalphabetic cipher in Python.

import sys, random

LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def main():

    myMessage = 'All our dreams can come true, if we have the courage to pursue them'

    myKey = 'QWERTYUIOPASDFGHJKLZXCVBNM'

    myMode = 'encrypt' 

    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 sets.')

def encryptMessage(key, message):

    return translateMessage(key, message, 'encrypt')

def decryptMessage(key, message):

    return translateMessage(key, message, 'decrypt')

def translateMessage(key, message, mode):

    translated = ''

    chA = LETTERS

    chB = key

    if mode == 'decrypt':

    

        chA, chB = chB, chA

    

    for symbol in message:

        if symbol.upper() in chA:

           

            symIndex = chA.find(symbol.upper())

            if symbol.isupper():

                translated += chB[symIndex].upper()

            else:

                translated += chB[symIndex].lower()

        else:

            

            translated += symbol

    return translated

def getRandomKey():

    key = list(LETTERS)

    random.shuffle(key)

    return ''.join(key)

main()

OUTPUT

Using key : QWERTYUIOPASDFGHJKLZXCVBNM

The encrypted message is :

Qss gxk rktqdl eqf egdt zkxt, oy vt iqct zit egxkqut zg hxklxt zitd