A basic Vernam Cipher to demonstrate using XOR to encode a message.

VernamCipher.py

#!/usr/bin/env python3
# 
# A basic cipher using XOR to encode a message using a password of the same
# length. The script takes two arguments, the message and the password.
# Decoding works exactly the same way except the message is replaced with 
# the encoded message, and the same password must be used
#
# Usage:
# ./VernamCipher.py {message} {password}
# 
# Examples:
# ./VernamCipher.py "Hello World" HFEFHYDEFGD
# ABPOJEVKUNA
# ./VernamCipher.py ABPOJEVKUNA HFEFHYDEFGD
# HELLO WORLD
#
# Copyright (C) 2020 Joe Baker (JoeBlakeB)
# This program is free software under the GPLv3 license.
#

import sys

if len(sys.argv) > 1:
    input1 = sys.argv[1].upper()
else:
    print("You didn't specify an input")
    exit()

if len(sys.argv) > 2:
    input2 = sys.argv[2].upper()
else:
    print("You did't specify an OPT")
    exit()
if len(input1) != len(input2):
    print("The OTP is not the same length as the message")
    exit()

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ,. '?!"

for inputs in [input1, input2]:
    for letter in inputs:
        if letter not in alphabet:
            print(letter, "is not a valid character.")
            exit()

output = ""

def xor(a, b):
    a = int(a)
    b = int(b)
    if a + b == 1:
        return 1
    else:
        return 0

def xorDecimal(a, b):
    a = format(a, "05b")
    b = format(b, "05b")
    c = ""
    for i in range(5):
        c += str(xor(a[i], b[i]))
    return int(c, 2)

for i in range(len(input1)):
    input1Letter = input1[i]
    input2Letter = input2[i]

    input1Number = alphabet.index(input1Letter)
    input2Number = alphabet.index(input2Letter)

    outputNumber = xorDecimal(input1Number, input2Number)
    outputLetter = alphabet[outputNumber]
    output += outputLetter

print(output)