* @throws IOException
*/
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
if (req.getParameter("op") != null) {
resp.setHeader("WWW-Authenticate", new AuthorizationHeaderBuilder()
.forIdentifier(req.getParameter("op"))
.includeStandardAttributes()
.buildHeader());
resp.sendError(401, "Authentication required");
} else {