input: 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)
  
  """
  Encrypt and decrypt strings using AES encryption in ECB mode. The key is a fixed 16-byte string.
  
  >>> encrypt(b'Hello, world!')
  b'\x0f\x0e\x0d\x0c\x0b\x0a\x09\x08\x07\x06\x05\x04\x03\x02\x01\x00'
  >>> decrypt(b'\x0f\x0e\x0d\x0c\x0b\x0a\x09\x08\x07\x06\x05\x04\x03\x02\x01\x00')
  b'Hello, world!'
  """
output:     
      plaintext = plaintext + b'\0' * (AES.block_size - len(plaintext) % AES.block_size)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      cipher = AES.new(key, AES.MODE_ECB)
      decrypted = cipher.decrypt(ciphertext)
      return decrypted.rstrip(b'\0')
  