code: def _calculate_aes_cipher(key):
      
  
      if len(key) not in [16, 24, 32]:
          raise ValueError(pretty_message(
              '''
              key must be either 16, 24 or 32 bytes (128, 192 or 256 bits)
              long - is %s
              ''',
              len(key)
          ))
  
      if len(key) == 16:
          cipher = 'aes128'
      elif len(key) == 24:
          cipher = 'aes192'
      elif len(key) == 32:
          cipher = 'aes256'
  
      return cipher
docs: Determines if the key is a valid AES 128, 192 or 256 key
  
      :param key:
          A byte string of the key to use
  
      :raises:
          ValueError - when an invalid key is provided
  
      :return:
          A unicode string of the AES variation - "aes128", "aes192" or "aes256"