"""

        Given a user object (which may be null), username, a plaintext password,

        and a settings object (containing all the keys needed as listed in settings()),

        authenticate this user's login attempt.

        Return None on failure. On success, return a dictionary of the form:

            see: KallitheaAuthPluginBase.auth_func_attrs

        This is later validated for correctness

        """

        if not username or not password:

            log.debug('Empty username or password skipping...')

            return None

        log.debug("Crowd settings: \n%s" % (formatted_json(settings)))

        server = CrowdServer(**settings)

        server.set_credentials(settings["app_name"], settings["app_password"])

        crowd_user = server.user_auth(username, password)

        log.debug("Crowd returned: \n%s" % (formatted_json(crowd_user)))

        if not crowd_user["status"]:

            return None

        res = server.user_groups(crowd_user["name"])

        log.debug("Crowd groups: \n%s" % (formatted_json(res)))

        crowd_user["groups"] = [x["name"] for x in res["groups"]]

        # old attrs fetched from Kallithea database

        admin = getattr(userobj, 'admin', False)

        active = getattr(userobj, 'active', True)

        email = getattr(userobj, 'email', '')

        firstname = getattr(userobj, 'firstname', '')

        lastname = getattr(userobj, 'lastname', '')

        extern_type = getattr(userobj, 'extern_type', '')

        user_attrs = {

            'username': username,

            'firstname': crowd_user["first-name"] or firstname,

            'lastname': crowd_user["last-name"] or lastname,

            'groups': crowd_user["groups"],

            'email': crowd_user["email"] or email,

            'admin': admin,

            'active': active,

            'active_from_extern': crowd_user.get('active'),

            'extern_name': crowd_user["name"],

            'extern_type': extern_type,

        }

        # set an admin if we're in admin_groups of crowd

            if group in user_attrs["groups"]:

                user_attrs["admin"] = True

        log.debug("Final crowd user object: \n%s" % (formatted_json(user_attrs)))

        log.info('user %s authenticated correctly' % user_attrs['username'])

        return user_attrs