Shift Cipher

shift.py

class ShiftCipher:  
    def __init__(self, N = 13):
        self.original_alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m",
             "n","o","p","q","r","s","t","u","v","w","x","y","z"]
        self.cipher_alphabet =  self.original_alphabet[N:]
        self.cipher_alphabet.extend(self.original_alphabet[0:N])      
        
    def encrypt_message(self, message):
        encrypted = ""
        #lowercase the message as alphabets are lowercase
        message = message.lower()
        for letter in message:
            if self.original_alphabet.count(letter) > 0:
                #if the letter exists in original alphabet, find the index and add encrypted form
                index = self.original_alphabet.index(letter)
                encrypted += self.cipher_alphabet[index]
            else:
                #if it does not exist in cipher alphabet, do not change the character 
                 encrypted += letter   
        return encrypted

    def decrypt_message(self, message):
        decrypted = ""
        #lowercase the message as alphabets are lowercase
        message = message.lower()
        for letter in message:
            if self.original_alphabet.count(letter) > 0:
                #if the letter exists in original alphabet, find the index and add decrypted form
                index = self.cipher_alphabet.index(letter)
                decrypted += self.original_alphabet[index]
            else:
                #if it does not exist in original alphabet, do not change the character 
                decrypted += letter
        return decrypted

import sys

if __name__ == "__main__":
    if len(sys.argv) < 3:
        print("Not enough arguments, please provide e/d followed by the message")
    N = 13
    cipher = ShiftCipher(N)

    if sys.argv[1] == "e":
        message = cipher.encrypt_message(sys.argv[2])
        print("Encrypted message: \n {}".format(message))
    elif sys.argv[1] == "d":
        message = cipher.decrypt_message(sys.argv[2])
        print("Decrypted message: \n {}".format(message))