<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def authenticate(self):
 """Authentication request. """

        unique_id = self.new_unique_id()
        message = {
            'op': 'authentication',
            'id': unique_id,
            'appKey': self.app_key,
            'session': self.session_token,
        }
        self._send(message)
        return unique_id