@abstractmethod<EOL><INDENT>def start(self, properties, initial_response):<DEDENT>
raise NotImplementedError<EOL>
Start the authentication process.

        :Parameters:
            - `properties`: the `authentication properties`_
            - `initial_response`: the initial response send by the client with
              the authentication request.

        :Types:
            - `properties`: mapping
            - `initial_response`: `bytes`

        :return: a challenge, a success or a failure indicator.
        :returntype: `Challenge` or `Failure` or `Success`