Check that the client key only contains safe characters
        and is no shorter than lower and no longer than upper.

def check_client_key(self, client_key):
        """Check that the client key only contains safe characters
        and is no shorter than lower and no longer than upper.
        """
        lower, upper = self.client_key_length
        return (set(client_key) <= self.safe_characters and
                lower <= len(client_key) <= upper)