forked-synapse/synapse/api/auth.py

# -*- coding: utf-8 -*-
# Copyright 2014 - 2016 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""This module contains classes for authenticating the user."""
from canonicaljson import encode_canonical_json
from signedjson.key import decode_verify_key_bytes
from signedjson.sign import verify_signed_json, SignatureVerifyException

from twisted.internet import defer

from synapse.api.constants import EventTypes, Membership, JoinRules
from synapse.api.errors import AuthError, Codes, SynapseError, EventSizeError
from synapse.types import Requester, RoomID, UserID, EventID
from synapse.util.logutils import log_function
from unpaddedbase64 import decode_base64

import logging
import pymacaroons

logger = logging.getLogger(__name__)


AuthEventTypes = (
    EventTypes.Create, EventTypes.Member, EventTypes.PowerLevels,
    EventTypes.JoinRules, EventTypes.RoomHistoryVisibility,
    EventTypes.ThirdPartyInvite,
)


class Auth(object):

    def __init__(self, hs):
        self.hs = hs
        self.store = hs.get_datastore()
        self.state = hs.get_state_handler()
        self.TOKEN_NOT_FOUND_HTTP_STATUS = 401
        self._KNOWN_CAVEAT_PREFIXES = set([
            "gen = ",
            "guest = ",
            "type = ",
            "time < ",
            "user_id = ",
        ])

    def check(self, event, auth_events):
        """ Checks if this event is correctly authed.

        Args:
            event: the event being checked.
            auth_events (dict: event-key -> event): the existing room state.


        Returns:
            True if the auth checks pass.
        """
        self.check_size_limits(event)

        try:
            if not hasattr(event, "room_id"):
                raise AuthError(500, "Event has no room_id: %s" % event)
            if auth_events is None:
                # Oh, we don't know what the state of the room was, so we
                # are trusting that this is allowed (at least for now)
                logger.warn("Trusting event: %s", event.event_id)
                return True

            if event.type == EventTypes.Create:
                # FIXME
                return True

            creation_event = auth_events.get((EventTypes.Create, ""), None)

            if not creation_event:
                raise SynapseError(
                    403,
                    "Room %r does not exist" % (event.room_id,)
                )

            creating_domain = RoomID.from_string(event.room_id).domain
            originating_domain = UserID.from_string(event.sender).domain
            if creating_domain != originating_domain:
                if not self.can_federate(event, auth_events):
                    raise AuthError(
                        403,
                        "This room has been marked as unfederatable."
                    )

            # FIXME: Temp hack
            if event.type == EventTypes.Aliases:
                return True

            logger.debug(
                "Auth events: %s",
                [a.event_id for a in auth_events.values()]
            )

            if event.type == EventTypes.Member:
                allowed = self.is_membership_change_allowed(
                    event, auth_events
                )
                if allowed:
                    logger.debug("Allowing! %s", event)
                else:
                    logger.debug("Denying! %s", event)
                return allowed

            self.check_event_sender_in_room(event, auth_events)
            self._can_send_event(event, auth_events)

            if event.type == EventTypes.PowerLevels:
                self._check_power_levels(event, auth_events)

            if event.type == EventTypes.Redaction:
                self.check_redaction(event, auth_events)

            logger.debug("Allowing! %s", event)
        except AuthError as e:
            logger.info(
                "Event auth check failed on event %s with msg: %s",
                event, e.msg
            )
            logger.info("Denying! %s", event)
            raise

    def check_size_limits(self, event):
        def too_big(field):
            raise EventSizeError("%s too large" % (field,))

        if len(event.user_id) > 255:
            too_big("user_id")
        if len(event.room_id) > 255:
            too_big("room_id")
        if event.is_state() and len(event.state_key) > 255:
            too_big("state_key")
        if len(event.type) > 255:
            too_big("type")
        if len(event.event_id) > 255:
            too_big("event_id")
        if len(encode_canonical_json(event.get_pdu_json())) > 65536:
            too_big("event")

    @defer.inlineCallbacks
    def check_joined_room(self, room_id, user_id, current_state=None):
        """Check if the user is currently joined in the room
        Args:
            room_id(str): The room to check.
            user_id(str): The user to check.
            current_state(dict): Optional map of the current state of the room.
                If provided then that map is used to check whether they are a
                member of the room. Otherwise the current membership is
                loaded from the database.
        Raises:
            AuthError if the user is not in the room.
        Returns:
            A deferred membership event for the user if the user is in
            the room.
        """
        if current_state:
            member = current_state.get(
                (EventTypes.Member, user_id),
                None
            )
        else:
            member = yield self.state.get_current_state(
                room_id=room_id,
                event_type=EventTypes.Member,
                state_key=user_id
            )

        self._check_joined_room(member, user_id, room_id)
        defer.returnValue(member)

    @defer.inlineCallbacks
    def check_user_was_in_room(self, room_id, user_id):
        """Check if the user was in the room at some point.
        Args:
            room_id(str): The room to check.
            user_id(str): The user to check.
        Raises:
            AuthError if the user was never in the room.
        Returns:
            A deferred membership event for the user if the user was in the
            room. This will be the join event if they are currently joined to
            the room. This will be the leave event if they have left the room.
        """
        member = yield self.state.get_current_state(
            room_id=room_id,
            event_type=EventTypes.Member,
            state_key=user_id
        )
        membership = member.membership if member else None

        if membership not in (Membership.JOIN, Membership.LEAVE):
            raise AuthError(403, "User %s not in room %s" % (
                user_id, room_id
            ))

        if membership == Membership.LEAVE:
            forgot = yield self.store.did_forget(user_id, room_id)
            if forgot:
                raise AuthError(403, "User %s not in room %s" % (
                    user_id, room_id
                ))

        defer.returnValue(member)

    @defer.inlineCallbacks
    def check_host_in_room(self, room_id, host):
        curr_state = yield self.state.get_current_state(room_id)

        for event in curr_state.values():
            if event.type == EventTypes.Member:
                try:
                    if UserID.from_string(event.state_key).domain != host:
                        continue
                except:
                    logger.warn("state_key not user_id: %s", event.state_key)
                    continue

                if event.content["membership"] == Membership.JOIN:
                    defer.returnValue(True)

        defer.returnValue(False)

    def check_event_sender_in_room(self, event, auth_events):
        key = (EventTypes.Member, event.user_id, )
        member_event = auth_events.get(key)

        return self._check_joined_room(
            member_event,
            event.user_id,
            event.room_id
        )

    def _check_joined_room(self, member, user_id, room_id):
        if not member or member.membership != Membership.JOIN:
            raise AuthError(403, "User %s not in room %s (%s)" % (
                user_id, room_id, repr(member)
            ))

    def can_federate(self, event, auth_events):
        creation_event = auth_events.get((EventTypes.Create, ""))

        return creation_event.content.get("m.federate", True) is True

    @log_function
    def is_membership_change_allowed(self, event, auth_events):
        membership = event.content["membership"]

        # Check if this is the room creator joining:
        if len(event.prev_events) == 1 and Membership.JOIN == membership:
            # Get room creation event:
            key = (EventTypes.Create, "", )
            create = auth_events.get(key)
            if create and event.prev_events[0][0] == create.event_id:
                if create.content["creator"] == event.state_key:
                    return True

        target_user_id = event.state_key

        creating_domain = RoomID.from_string(event.room_id).domain
        target_domain = UserID.from_string(target_user_id).domain
        if creating_domain != target_domain:
            if not self.can_federate(event, auth_events):
                raise AuthError(
                    403,
                    "This room has been marked as unfederatable."
                )

        # get info about the caller
        key = (EventTypes.Member, event.user_id, )
        caller = auth_events.get(key)

        caller_in_room = caller and caller.membership == Membership.JOIN
        caller_invited = caller and caller.membership == Membership.INVITE

        # get info about the target
        key = (EventTypes.Member, target_user_id, )
        target = auth_events.get(key)

        target_in_room = target and target.membership == Membership.JOIN
        target_banned = target and target.membership == Membership.BAN

        key = (EventTypes.JoinRules, "", )
        join_rule_event = auth_events.get(key)
        if join_rule_event:
            join_rule = join_rule_event.content.get(
                "join_rule", JoinRules.INVITE
            )
        else:
            join_rule = JoinRules.INVITE

        user_level = self._get_user_power_level(event.user_id, auth_events)
        target_level = self._get_user_power_level(
            target_user_id, auth_events
        )

        # FIXME (erikj): What should we do here as the default?
        ban_level = self._get_named_level(auth_events, "ban", 50)

        logger.debug(
            "is_membership_change_allowed: %s",
            {
                "caller_in_room": caller_in_room,
                "caller_invited": caller_invited,
                "target_banned": target_banned,
                "target_in_room": target_in_room,
                "membership": membership,
                "join_rule": join_rule,
                "target_user_id": target_user_id,
                "event.user_id": event.user_id,
            }
        )

        if Membership.INVITE == membership and "third_party_invite" in event.content:
            if not self._verify_third_party_invite(event, auth_events):
                raise AuthError(403, "You are not invited to this room.")
            return True

        if Membership.JOIN != membership:
            if (caller_invited
                    and Membership.LEAVE == membership
                    and target_user_id == event.user_id):
                return True

            if not caller_in_room:  # caller isn't joined
                raise AuthError(
                    403,
                    "%s not in room %s." % (event.user_id, event.room_id,)
                )

        if Membership.INVITE == membership:
            # TODO (erikj): We should probably handle this more intelligently
            # PRIVATE join rules.

            # Invites are valid iff caller is in the room and target isn't.
            if target_banned:
                raise AuthError(
                    403, "%s is banned from the room" % (target_user_id,)
                )
            elif target_in_room:  # the target is already in the room.
                raise AuthError(403, "%s is already in the room." %
                                     target_user_id)
            else:
                invite_level = self._get_named_level(auth_events, "invite", 0)

                if user_level < invite_level:
                    raise AuthError(
                        403, "You cannot invite user %s." % target_user_id
                    )
        elif Membership.JOIN == membership:
            # Joins are valid iff caller == target and they were:
            # invited: They are accepting the invitation
            # joined: It's a NOOP
            if event.user_id != target_user_id:
                raise AuthError(403, "Cannot force another user to join.")
            elif target_banned:
                raise AuthError(403, "You are banned from this room")
            elif join_rule == JoinRules.PUBLIC:
                pass
            elif join_rule == JoinRules.INVITE:
                if not caller_in_room and not caller_invited:
                    raise AuthError(403, "You are not invited to this room.")
            else:
                # TODO (erikj): may_join list
                # TODO (erikj): private rooms
                raise AuthError(403, "You are not allowed to join this room")
        elif Membership.LEAVE == membership:
            # TODO (erikj): Implement kicks.
            if target_banned and user_level < ban_level:
                raise AuthError(
                    403, "You cannot unban user &s." % (target_user_id,)
                )
            elif target_user_id != event.user_id:
                kick_level = self._get_named_level(auth_events, "kick", 50)

                if user_level < kick_level or user_level <= target_level:
                    raise AuthError(
                        403, "You cannot kick user %s." % target_user_id
                    )
        elif Membership.BAN == membership:
            if user_level < ban_level or user_level <= target_level:
                raise AuthError(403, "You don't have permission to ban")
        else:
            raise AuthError(500, "Unknown membership %s" % membership)

        return True

    def _verify_third_party_invite(self, event, auth_events):
        """
        Validates that the invite event is authorized by a previous third-party invite.

        Checks that the public key, and keyserver, match those in the third party invite,
        and that the invite event has a signature issued using that public key.

        Args:
            event: The m.room.member join event being validated.
            auth_events: All relevant previous context events which may be used
                for authorization decisions.

        Return:
            True if the event fulfills the expectations of a previous third party
            invite event.
        """
        if "third_party_invite" not in event.content:
            return False
        if "signed" not in event.content["third_party_invite"]:
            return False
        signed = event.content["third_party_invite"]["signed"]
        for key in {"mxid", "token"}:
            if key not in signed:
                return False

        token = signed["token"]

        invite_event = auth_events.get(
            (EventTypes.ThirdPartyInvite, token,)
        )
        if not invite_event:
            return False

        if event.user_id != invite_event.user_id:
            return False
        try:
            public_key = invite_event.content["public_key"]
            if signed["mxid"] != event.state_key:
                return False
            if signed["token"] != token:
                return False
            for server, signature_block in signed["signatures"].items():
                for key_name, encoded_signature in signature_block.items():
                    if not key_name.startswith("ed25519:"):
                        return False
                    verify_key = decode_verify_key_bytes(
                        key_name,
                        decode_base64(public_key)
                    )
                    verify_signed_json(signed, server, verify_key)

                    # We got the public key from the invite, so we know that the
                    # correct server signed the signed bundle.
                    # The caller is responsible for checking that the signing
                    # server has not revoked that public key.
                    return True
            return False
        except (KeyError, SignatureVerifyException,):
            return False

    def _get_power_level_event(self, auth_events):
        key = (EventTypes.PowerLevels, "", )
        return auth_events.get(key)

    def _get_user_power_level(self, user_id, auth_events):
        power_level_event = self._get_power_level_event(auth_events)

        if power_level_event:
            level = power_level_event.content.get("users", {}).get(user_id)
            if not level:
                level = power_level_event.content.get("users_default", 0)

            if level is None:
                return 0
            else:
                return int(level)
        else:
            key = (EventTypes.Create, "", )
            create_event = auth_events.get(key)
            if (create_event is not None and
                    create_event.content["creator"] == user_id):
                return 100
            else:
                return 0

    def _get_named_level(self, auth_events, name, default):
        power_level_event = self._get_power_level_event(auth_events)

        if not power_level_event:
            return default

        level = power_level_event.content.get(name, None)
        if level is not None:
            return int(level)
        else:
            return default

    @defer.inlineCallbacks
    def get_user_by_req(self, request, allow_guest=False):
        """ Get a registered user's ID.

        Args:
            request - An HTTP request with an access_token query parameter.
        Returns:
            tuple of:
                UserID (str)
                Access token ID (str)
        Raises:
            AuthError if no user by that token exists or the token is invalid.
        """
        # Can optionally look elsewhere in the request (e.g. headers)
        try:
            user_id = yield self._get_appservice_user_id(request.args)
            if user_id:
                request.authenticated_entity = user_id
                defer.returnValue(
                    Requester(UserID.from_string(user_id), "", False)
                )

            access_token = request.args["access_token"][0]
            user_info = yield self._get_user_by_access_token(access_token)
            user = user_info["user"]
            token_id = user_info["token_id"]
            is_guest = user_info["is_guest"]

            ip_addr = self.hs.get_ip_from_request(request)
            user_agent = request.requestHeaders.getRawHeaders(
                "User-Agent",
                default=[""]
            )[0]
            if user and access_token and ip_addr:
                self.store.insert_client_ip(
                    user=user,
                    access_token=access_token,
                    ip=ip_addr,
                    user_agent=user_agent
                )

            if is_guest and not allow_guest:
                raise AuthError(
                    403, "Guest access not allowed", errcode=Codes.GUEST_ACCESS_FORBIDDEN
                )

            request.authenticated_entity = user.to_string()

            defer.returnValue(Requester(user, token_id, is_guest))
        except KeyError:
            raise AuthError(
                self.TOKEN_NOT_FOUND_HTTP_STATUS, "Missing access token.",
                errcode=Codes.MISSING_TOKEN
            )

    @defer.inlineCallbacks
    def _get_appservice_user_id(self, request_args):
        app_service = yield self.store.get_app_service_by_token(
            request_args["access_token"][0]
        )
        if app_service is None:
            defer.returnValue(None)

        if "user_id" not in request_args:
            defer.returnValue(app_service.sender)

        user_id = request_args["user_id"][0]
        if app_service.sender == user_id:
            defer.returnValue(app_service.sender)

        if not app_service.is_interested_in_user(user_id):
            raise AuthError(
                403,
                "Application service cannot masquerade as this user."
            )
        if not (yield self.store.get_user_by_id(user_id)):
            raise AuthError(
                403,
                "Application service has not registered this user"
            )
        defer.returnValue(user_id)

    @defer.inlineCallbacks
    def _get_user_by_access_token(self, token):
        """ Get a registered user's ID.

        Args:
            token (str): The access token to get the user by.
        Returns:
            dict : dict that includes the user and the ID of their access token.
        Raises:
            AuthError if no user by that token exists or the token is invalid.
        """
        try:
            ret = yield self.get_user_from_macaroon(token)
        except AuthError:
            # TODO(daniel): Remove this fallback when all existing access tokens
            # have been re-issued as macaroons.
            ret = yield self._look_up_user_by_access_token(token)
        defer.returnValue(ret)

    @defer.inlineCallbacks
    def get_user_from_macaroon(self, macaroon_str):
        try:
            macaroon = pymacaroons.Macaroon.deserialize(macaroon_str)
            self.validate_macaroon(macaroon, "access", False)

            user_prefix = "user_id = "
            user = None
            guest = False
            for caveat in macaroon.caveats:
                if caveat.caveat_id.startswith(user_prefix):
                    user = UserID.from_string(caveat.caveat_id[len(user_prefix):])
                elif caveat.caveat_id == "guest = true":
                    guest = True

            if user is None:
                raise AuthError(
                    self.TOKEN_NOT_FOUND_HTTP_STATUS, "No user caveat in macaroon",
                    errcode=Codes.UNKNOWN_TOKEN
                )

            if guest:
                ret = {
                    "user": user,
                    "is_guest": True,
                    "token_id": None,
                }
            else:
                # This codepath exists so that we can actually return a
                # token ID, because we use token IDs in place of device
                # identifiers throughout the codebase.
                # TODO(daniel): Remove this fallback when device IDs are
                # properly implemented.
                ret = yield self._look_up_user_by_access_token(macaroon_str)
                if ret["user"] != user:
                    logger.error(
                        "Macaroon user (%s) != DB user (%s)",
                        user,
                        ret["user"]
                    )
                    raise AuthError(
                        self.TOKEN_NOT_FOUND_HTTP_STATUS,
                        "User mismatch in macaroon",
                        errcode=Codes.UNKNOWN_TOKEN
                    )
            defer.returnValue(ret)
        except (pymacaroons.exceptions.MacaroonException, TypeError, ValueError):
            raise AuthError(
                self.TOKEN_NOT_FOUND_HTTP_STATUS, "Invalid macaroon passed.",
                errcode=Codes.UNKNOWN_TOKEN
            )

    def validate_macaroon(self, macaroon, type_string, verify_expiry):
        """
        validate that a Macaroon is understood by and was signed by this server.

        Args:
            macaroon(pymacaroons.Macaroon): The macaroon to validate
            type_string(str): The kind of token this is (e.g. "access", "refresh")
            verify_expiry(bool): Whether to verify whether the macaroon has expired.
                This should really always be True, but no clients currently implement
                token refresh, so we can't enforce expiry yet.
        """
        v = pymacaroons.Verifier()
        v.satisfy_exact("gen = 1")
        v.satisfy_exact("type = " + type_string)
        v.satisfy_general(lambda c: c.startswith("user_id = "))
        v.satisfy_exact("guest = true")
        if verify_expiry:
            v.satisfy_general(self._verify_expiry)
        else:
            v.satisfy_general(lambda c: c.startswith("time < "))

        v.verify(macaroon, self.hs.config.macaroon_secret_key)

        v = pymacaroons.Verifier()
        v.satisfy_general(self._verify_recognizes_caveats)
        v.verify(macaroon, self.hs.config.macaroon_secret_key)

    def _verify_expiry(self, caveat):
        prefix = "time < "
        if not caveat.startswith(prefix):
            return False
        expiry = int(caveat[len(prefix):])
        now = self.hs.get_clock().time_msec()
        return now < expiry

    def _verify_recognizes_caveats(self, caveat):
        first_space = caveat.find(" ")
        if first_space < 0:
            return False
        second_space = caveat.find(" ", first_space + 1)
        if second_space < 0:
            return False
        return caveat[:second_space + 1] in self._KNOWN_CAVEAT_PREFIXES

    @defer.inlineCallbacks
    def _look_up_user_by_access_token(self, token):
        ret = yield self.store.get_user_by_access_token(token)
        if not ret:
            logger.warn("Unrecognised access token - not in store: %s" % (token,))
            raise AuthError(
                self.TOKEN_NOT_FOUND_HTTP_STATUS, "Unrecognised access token.",
                errcode=Codes.UNKNOWN_TOKEN
            )
        user_info = {
            "user": UserID.from_string(ret.get("name")),
            "token_id": ret.get("token_id", None),
            "is_guest": False,
        }
        defer.returnValue(user_info)

    @defer.inlineCallbacks
    def get_appservice_by_req(self, request):
        try:
            token = request.args["access_token"][0]
            service = yield self.store.get_app_service_by_token(token)
            if not service:
                logger.warn("Unrecognised appservice access token: %s" % (token,))
                raise AuthError(
                    self.TOKEN_NOT_FOUND_HTTP_STATUS,
                    "Unrecognised access token.",
                    errcode=Codes.UNKNOWN_TOKEN
                )
            request.authenticated_entity = service.sender
            defer.returnValue(service)
        except KeyError:
            raise AuthError(
                self.TOKEN_NOT_FOUND_HTTP_STATUS, "Missing access token."
            )

    def is_server_admin(self, user):
        return self.store.is_server_admin(user)

    @defer.inlineCallbacks
    def add_auth_events(self, builder, context):
        auth_ids = self.compute_auth_events(builder, context.current_state)

        auth_events_entries = yield self.store.add_event_hashes(
            auth_ids
        )

        builder.auth_events = auth_events_entries

    def compute_auth_events(self, event, current_state):
        if event.type == EventTypes.Create:
            return []

        auth_ids = []

        key = (EventTypes.PowerLevels, "", )
        power_level_event = current_state.get(key)

        if power_level_event:
            auth_ids.append(power_level_event.event_id)

        key = (EventTypes.JoinRules, "", )
        join_rule_event = current_state.get(key)

        key = (EventTypes.Member, event.user_id, )
        member_event = current_state.get(key)

        key = (EventTypes.Create, "", )
        create_event = current_state.get(key)
        if create_event:
            auth_ids.append(create_event.event_id)

        if join_rule_event:
            join_rule = join_rule_event.content.get("join_rule")
            is_public = join_rule == JoinRules.PUBLIC if join_rule else False
        else:
            is_public = False

        if event.type == EventTypes.Member:
            e_type = event.content["membership"]
            if e_type in [Membership.JOIN, Membership.INVITE]:
                if join_rule_event:
                    auth_ids.append(join_rule_event.event_id)

            if e_type == Membership.JOIN:
                if member_event and not is_public:
                    auth_ids.append(member_event.event_id)
            else:
                if member_event:
                    auth_ids.append(member_event.event_id)

            if e_type == Membership.INVITE:
                if "third_party_invite" in event.content:
                    key = (
                        EventTypes.ThirdPartyInvite,
                        event.content["third_party_invite"]["signed"]["token"]
                    )
                    third_party_invite = current_state.get(key)
                    if third_party_invite:
                        auth_ids.append(third_party_invite.event_id)
        elif member_event:
            if member_event.content["membership"] == Membership.JOIN:
                auth_ids.append(member_event.event_id)

        return auth_ids

    @log_function
    def _can_send_event(self, event, auth_events):
        key = (EventTypes.PowerLevels, "", )
        send_level_event = auth_events.get(key)
        send_level = None
        if send_level_event:
            send_level = send_level_event.content.get("events", {}).get(
                event.type
            )
            if send_level is None:
                if hasattr(event, "state_key"):
                    send_level = send_level_event.content.get(
                        "state_default", 50
                    )
                else:
                    send_level = send_level_event.content.get(
                        "events_default", 0
                    )

        if send_level:
            send_level = int(send_level)
        else:
            send_level = 0

        user_level = self._get_user_power_level(event.user_id, auth_events)

        if user_level < send_level:
            raise AuthError(
                403,
                "You don't have permission to post that to the room. " +
                "user_level (%d) < send_level (%d)" % (user_level, send_level)
            )

        # Check state_key
        if hasattr(event, "state_key"):
            if event.state_key.startswith("@"):
                if event.state_key != event.user_id:
                    raise AuthError(
                        403,
                        "You are not allowed to set others state"
                    )
                else:
                    sender_domain = UserID.from_string(
                        event.user_id
                    ).domain

                    if sender_domain != event.state_key:
                        raise AuthError(
                            403,
                            "You are not allowed to set others state"
                        )

        return True

    def check_redaction(self, event, auth_events):
        """Check whether the event sender is allowed to redact the target event.

        Returns:
            True if the the sender is allowed to redact the target event if the
            target event was created by them.
            False if the sender is allowed to redact the target event with no
            further checks.

        Raises:
            AuthError if the event sender is definitely not allowed to redact
            the target event.
        """
        user_level = self._get_user_power_level(event.user_id, auth_events)

        redact_level = self._get_named_level(auth_events, "redact", 50)

        if user_level >= redact_level:
            return False

        redacter_domain = EventID.from_string(event.event_id).domain
        redactee_domain = EventID.from_string(event.redacts).domain
        if redacter_domain == redactee_domain:
            return True

        raise AuthError(
            403,
            "You don't have permission to redact events"
        )

    def _check_power_levels(self, event, auth_events):
        user_list = event.content.get("users", {})
        # Validate users
        for k, v in user_list.items():
            try:
                UserID.from_string(k)
            except:
                raise SynapseError(400, "Not a valid user_id: %s" % (k,))

            try:
                int(v)
            except:
                raise SynapseError(400, "Not a valid power level: %s" % (v,))

        key = (event.type, event.state_key, )
        current_state = auth_events.get(key)

        if not current_state:
            return

        user_level = self._get_user_power_level(event.user_id, auth_events)

        # Check other levels:
        levels_to_check = [
            ("users_default", None),
            ("events_default", None),
            ("state_default", None),
            ("ban", None),
            ("redact", None),
            ("kick", None),
            ("invite", None),
        ]

        old_list = current_state.content.get("users")
        for user in set(old_list.keys() + user_list.keys()):
            levels_to_check.append(
                (user, "users")
            )

        old_list = current_state.content.get("events")
        new_list = event.content.get("events")
        for ev_id in set(old_list.keys() + new_list.keys()):
            levels_to_check.append(
                (ev_id, "events")
            )

        old_state = current_state.content
        new_state = event.content

        for level_to_check, dir in levels_to_check:
            old_loc = old_state
            new_loc = new_state
            if dir:
                old_loc = old_loc.get(dir, {})
                new_loc = new_loc.get(dir, {})

            if level_to_check in old_loc:
                old_level = int(old_loc[level_to_check])
            else:
                old_level = None

            if level_to_check in new_loc:
                new_level = int(new_loc[level_to_check])
            else:
                new_level = None

            if new_level is not None and old_level is not None:
                if new_level == old_level:
                    continue

            if dir == "users" and level_to_check != event.user_id:
                if old_level == user_level:
                    raise AuthError(
                        403,
                        "You don't have permission to remove ops level equal "
                        "to your own"
                    )

            if old_level > user_level or new_level > user_level:
                raise AuthError(
                    403,
                    "You don't have permission to add ops level greater "
                    "than your own"
                )