input: Write a function to create an application in Python for encryption and decryption of strings using the AES cipher. The function should take a string as input for encryption and another string for decryption. The function should use a predefined key for the cipher.
  
  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'Hello World 1234')==b'!\x83/\xff\x80+\xff\xdd"?\x12\xeby\nZ"\xc5'
  assert decrypt(b'!\x83/\xff\x80+\xff\xdd"?\x12\xeby\nZ"\xc5')==b'Hello World 1234'
  assert encrypt(b'')==b''
  assert decrypt(b'')==b''
output: 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)