def _setNewPass(self, np):
        """
        Called back when we are choosing a new password.  Get the old password
        and send the authentication message with both.

        @param np: the new password as entered by the user
        @type np: L{bytes}
        """
        op = self._oldPass
        self._oldPass = None
        self.askForAuth(b'password', b'\xff' + NS(op) + NS(np))

        Called back when we are choosing a new password.  Get the old password
        and send the authentication message with both.

        @param np: the new password as entered by the user
        @type np: L{bytes}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/ssh/userauth.py