function listRevoked($crl = null)
    {
        if (!isset($crl)) {
            $crl = $this->currentCert;
        }

        if (!isset($crl['tbsCertList'])) {
            return false;
        }

        $result = array();

        if (is_array($rclist = $this->_subArray($crl, 'tbsCertList/revokedCertificates'))) {
            foreach ($rclist as $rc) {
                $result[] = $rc['userCertificate']->toString();
            }
        }

        return $result;
    }