input: Write a function to create an application that performs encryption and decryption of strings using the AES encryption algorithm. The function should define two sub-functions: one for encryption and one for decryption. The input to the encryption function is a plaintext string, and the output should be the encrypted string. The input to the decryption function is the encrypted string, and the output should be the decrypted plaintext string. The key to be used for both encryption and decryption should be defined within the function.
  from Crypto.Cipher import AES
  
  key = b'Sixteen byte key'
  
  def encrypt(plaintext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)
  
  assert encrypt(b'test string to be crypt')==encrypt(b'test string to be crypt')
  assert decrypt(encrypt(b'test string to be crypt'))==b'test string to be crypt'
  assert encrypt(b'')==b''
  assert decrypt(b'')==b''
output: 
  from Crypto.Cipher import AES
  
  key = b'Sixteen byte key'
  
  def encrypt(plaintext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)
  