Encrypt-and-Decrypt-GUI-With-Python

from tkinter import messagebox, simpledialog, Tk

def is_even(number):
return number % 2 == 0

def get_even_letters(message):
even_letters = []
for counter in range(0, len(message)):
if is_even(counter):
even_letters.append(message[counter])
return even_letters

def get_odd_letters(message):
odd_letters = []
for counter in range(0, len(message)):
if not is_even(counter):
odd_letters.append(message[counter])
return odd_letters

def swap_letters(message):
letter_list = []
if not is_even(len(message)):
message = message + 'x'
even_letters = get_even_letters(message)
odd_letters = get_odd_letters(message)
for counter in range(0, int(len(message)/2)):
letter_list.append(odd_letters[counter])
letter_list.append(even_letters[counter])
new_message = ''.join(letter_list)
return new_message

def get_task():
task = simpledialog.askstring('Task', 'Do you want to encrypt or decrypt?')
return task

def get_message():
message = simpledialog.askstring('Message', 'Enter the secret message: ')
return message

root = Tk()
while True:
task = get_task()
if task == 'encrypt':
message = get_message()
encrypted = swap_letters(message)
messagebox.showinfo('Ciphertext of the secret message is:', encrypted)

elif task == 'decrypt':
    message = get_message()
    decrypted = swap_letters(message)
    messagebox.showinfo('Plaintext of the secret message is:', decrypted)
else:
    break
root.mainloop()