public function toXML(DOMElement $parent) : DOMElement
    {
        $doc = $parent->ownerDocument;

        $e = $doc->createElementNS(XMLSecurityDSig::XMLDSIGNS, 'ds:KeyInfo');
        $parent->appendChild($e);

        if ($this->Id !== null) {
            $e->setAttribute('Id', $this->Id);
        }

        foreach ($this->info as $n) {
            $n->toXML($e);
        }

        return $e;
    }