A simple demonstration of Caeser Cipher algorithm implemented using Python.

caeser.py

# Caesar Cipher

import sys

MAX_KEY_SIZE = 26

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

def getKey():
    key = 0
    while True:
        key = int(sys.argv[1])
        if (key >= 1 and key <= MAX_KEY_SIZE):
            return key

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

    for symbol in message:
        if symbol.isalpha():
            num = ord(symbol)
            num += key

            if symbol.isupper():
                if num > ord('Z'):
                    num -= 26
                elif num < ord('A'):
                    num += 26
            elif symbol.islower():
                if num > ord('z'):
                    num -= 26
                elif num < ord('a'):
                    num += 26
                        
            translated += chr(num)
        else:
            translated += symbol
    return translated

mode = getMode()
key = getKey()

print('Enter your input filename: ')
file_in = input()
fin = open(file_in, 'r')

print('Enter your output filename: ')
file_in = input()
fout = open(file_in, 'w')

for line in fin:
    fout.write(getTranslatedMessage(mode, line, key))

fout.close()