Encrypt and decrypt using python.py

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()