Criptografia

#!/usr/bin/python3.7
# -*- coding: utf-8
'''
Critografia ROT13
@author: aihua.sun
Reescrito por Dalilo Sérgio Gomes Rosas
'''
op = 0
print('''
/$$$$$$$   /$$$$$$  /$$$$$$$$  /$$    /$$$$$$
| $$__  $$ /$$__  $$|__  $$__//$$$$   /$$__  $$
| $$  \ $$| $$  \ $$   | $$  |_  $$  |__/  \ $$
| $$$$$$$/| $$  | $$   | $$    | $$     /$$$$$/
| $$__  $$| $$  | $$   | $$    | $$    |___  $$
\033[31m| $$  \ $$| $$  | $$   | $$    | $$   /$$  \ $$
| $$  | $$|  $$$$$$/   | $$   /$$$$$$|  $$$$$$/
|__/  |__/ \______/    |__/  |______/ \______/ \033[0;0m

''')
print('\033[33m[          Dalilo Sérgio Gomes Rosas          ]\033[0m')
print('\n')
LOWER_LETTERS = [chr(x) for x in range(97, 123)]
UPPER_LETTERS = [chr(x) for x in range(65, 91)]
while op != 3:
    print('Escola uma das opções:\n\n1 - Descriptografar\n2 - Nova descriptografia\n3 - Sair\n\n')
    op = int(input('[Opção]>>> '))

    if op == 1 or op == 2:
        try:
            def rot13():
                sourceString = input("Entre com a string ROT-13: ")
                resultString = ""
                for char in sourceString:
                    if char.isupper():
                        resultString += encrypt(char, UPPER_LETTERS)
                    elif char.islower():
                        resultString += encrypt(char, LOWER_LETTERS)
                    else:
                        resultString += char
                print(f'O resultado da descriptografia ROT-13 é: {resultString}')
                print('\n')
            def encrypt(char, letterList):
                resultchar = ''
                originalIndex = letterList.index(char)
                newIndex = originalIndex + 13
                resultchar += letterList[newIndex % len(letterList)]
                return resultchar
            if __name__ == '__main__':
                rot13()
        except KeyboardInterrupt:
            print('\nInterrompido pelo usuário!\n')

if op == 3:
        print('\nFim... Volte sempre!\n\n')
else:
        print('\n\nOpção inválida, tente novamente.\n')