public function getSignature($domain, $user)
    {
        $url = self::BASE_URL . "{$domain}/{$user}/signature";
        $request = new \Google_Http_Request($url, 'GET', null, null);

        $httpRequest = $this->helper->getClient()
           ->getAuth()
           ->authenticatedRequest($request);
        if ($httpRequest->getResponseHttpCode() == 200) {
            $xmlResponse = new \SimpleXMLElement($httpRequest->getResponseBody(), 0, false, 'apps', true);

            return (string)$xmlResponse->children('apps', true)
               ->attributes()->value;
        } else {
            // An error occurred.
            return null;
        }
    }