private function check_timestamp($timestamp)
    {
        if( ! $timestamp )
            throw new Exception(
                'Missing timestamp parameter. The parameter is required'
            );

        // verify that timestamp is recentish
        $now = time();
        if (abs($now - $timestamp) > $this->timestamp_threshold) {
            throw new Exception(
                "Expired timestamp, yours $timestamp, ours $now"
            );
        }
    }