system: You are an expert security code reviewer. You will be shown a source code file. Your task is to name the single function or class that contains the specified CWE vulnerability.

OUTPUT FORMAT: Respond with the exact function or class name (no parentheses, no qualifiers, no module prefixes), wrapped in \boxed{...}. Example: `\boxed{salted_hmac}`. Output the boxed name ONLY -- no explanation, no other text.

user: ```python
# -*- coding: utf-8 -*-
# Copyright 2014 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.


from twisted.internet import defer

from synapse.util.logutils import log_function
from synapse.util.async import run_on_reactor
from synapse.api.events.room import RoomPowerLevelsEvent

from collections import namedtuple

import copy
import logging
import hashlib

logger = logging.getLogger(__name__)


def _get_state_key_from_event(event):
    return event.state_key


KeyStateTuple = namedtuple("KeyStateTuple", ("context", "type", "state_key"))


class StateHandler(object):
    """ Responsible for doing state conflict resolution.
    """

    def __init__(self, hs):
        self.store = hs.get_datastore()

    @defer.inlineCallbacks
    @log_function
    def annotate_event_with_state(self, event, old_state=None):
        """ Annotates the event with the current state events as of that event.

        This method adds three new attributes to the event:
            * `state_events`: The state up to and including the event. Encoded
              as a dict mapping tuple (type, state_key) -> event.
            * `old_state_events`: The state up to, but excluding, the event.
              Encoded similarly as `state_events`.
            * `state_group`: If there is an existing state group that can be
              used, then return that. Otherwise return `None`. See state
              storage for more information.

        If the argument `old_state` is given (in the form of a list of
        events), then they are used as a the values for `old_state_events` and
        the value for `state_events` is generated from it. `state_group` is
        set to None.

        This needs to be called before persisting the event.
        """
        yield run_on_reactor()

        if old_state:
            event.state_group = None
            event.old_state_events = {
                (s.type, s.state_key): s for s in old_state
            }
            event.state_events = event.old_state_events

            if hasattr(event, "state_key"):
                event.state_events[(event.type, event.state_key)] = event

            defer.returnValue(False)
            return

        if hasattr(event, "outlier") and event.outlier:
            event.state_group = None
            event.old_state_events = None
            event.state_events = None
            defer.returnValue(False)
            return

        ids = [e for e, _ in event.prev_events]

        ret = yield self.resolve_state_groups(ids)
        state_group, new_state = ret

        event.old_state_events = copy.deepcopy(new_state)

        if hasattr(event, "state_key"):
            key = (event.type, event.state_key)
            if key in new_state:
                event.replaces_state = new_state[key].event_id
            new_state[key] = event
        elif state_group:
            event.state_group = state_group
            event.state_events = new_state
            defer.returnValue(False)

        event.state_group = None
        event.state_events = new_state

        defer.returnValue(hasattr(event, "state_key"))

    @defer.inlineCallbacks
    def get_current_state(self, room_id, event_type=None, state_key=""):
        """ Returns the current state for the room as a list. This is done by
        calling `get_latest_events_in_room` to get the leading edges of the
        event graph and then resolving any of the state conflicts.

        This is equivalent to getting the state of an event that were to send
        next before receiving any new events.

        If `event_type` is specified, then the method returns only the one
        event (or None) with that `event_type` and `state_key`.
        """
        events = yield self.store.get_latest_events_in_room(room_id)

        event_ids = [
            e_id
            for e_id, _, _ in events
        ]

        res = yield self.resolve_state_groups(event_ids)

        if event_type:
            defer.returnValue(res[1].get((event_type, state_key)))
            return

        defer.returnValue(res[1].values())

    @defer.inlineCallbacks
    @log_function
    def resolve_state_groups(self, event_ids):
        """ Given a list of event_ids this method fetches the state at each
        event, resolves conflicts between them and returns them.

        Return format is a tuple: (`state_group`, `state_events`), where the
        first is the name of a state group if one and only one is involved,
        otherwise `None`.
        """
        state_groups = yield self.store.get_state_groups(
            event_ids
        )

        group_names = set(state_groups.keys())
        if len(group_names) == 1:
            name, state_list = state_groups.items().pop()
            state = {
                (e.type, e.state_key): e
                for e in state_list
            }
            defer.returnValue((name, state))

        state = {}
        for group, g_state in state_groups.items():
            for s in g_state:
                state.setdefault(
                    (s.type, s.state_key),
                    {}
                )[s.event_id] = s

        unconflicted_state = {
            k: v.values()[0] for k, v in state.items()
            if len(v.values()) == 1
        }

        conflicted_state = {
            k: v.values()
            for k, v in state.items()
            if len(v.values()) > 1
        }

        try:
            new_state = {}
            new_state.update(unconflicted_state)
            for key, events in conflicted_state.items():
                new_state[key] = self._resolve_state_events(events)
        except:
            logger.exception("Failed to resolve state")
            raise

        defer.returnValue((None, new_state))

    def _get_power_level_from_event_state(self, event, user_id):
        if hasattr(event, "old_state_events") and event.old_state_events:
            key = (RoomPowerLevelsEvent.TYPE, "", )
            power_level_event = event.old_state_events.get(key)
            level = None
            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)

            return level
        else:
            return 0

    @log_function
    def _resolve_state_events(self, events):
        curr_events = events

        new_powers = [
            self._get_power_level_from_event_state(e, e.user_id)
            for e in curr_events
        ]

        new_powers = [
            int(p) if p else 0 for p in new_powers
        ]

        max_power = max(new_powers)

        curr_events = [
            z[0] for z in zip(curr_events, new_powers)
            if z[1] == max_power
        ]

        if not curr_events:
            raise RuntimeError("Max didn't get a max?")
        elif len(curr_events) == 1:
            return curr_events[0]

        # TODO: For now, just choose the one with the largest event_id.
        return (
            sorted(
                curr_events,
                key=lambda e: hashlib.sha1(
                    e.event_id + e.user_id + e.room_id + e.type
                ).hexdigest()
            )[0]
        )

```

Name the function or class in this code that contains a `CWE-327: Use of a Broken or Risky Cryptographic Algorithm` vulnerability.
Answer: _resolve_state_events