boolean value
            which will be true if the user should be prompted for the
            passphrase twice and the callback should verify that the two values
            supplied are equal. Third, the value given as the *userdata*
            parameter to :meth:`set_passwd_cb`.  The *callback* must return
            a byte string. If an error occurs, *callback* should return a false
            value (e.g. an empty string).
        :param userdata: (optional) A Python object which will be given as
                         argument to the callback
        :return: None
        ú