protected function useAssertCredentials($userEmail = '')
    {
        $serviceJsonUrl = array_get($this->config, 'service.file', '');

        if (empty($serviceJsonUrl)) {
            return false;
        }

        $this->client->setAuthConfig($serviceJsonUrl);
        
        if (! empty($userEmail)) {
            $this->client->setSubject($userEmail);
        }

        return true;
    }