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

        :Parameters:
            - `properties`: the `authentication properties`_
        :Types:
            - `properties`: mapping

        :return: the initial response to send to the server or a failuer
            indicator.
        :returntype: `Response` or `Failure`