WIP to store missing session event ids

src/matrix/Sync.js

@@ -248,6 +248,7 @@ export class Sync {
         return this._storage.readTxn([
             storeNames.olmSessions,
             storeNames.inboundGroupSessions,
+            storeNames.timelineEvents // to read events that can now be decrypted
         ]);
     }
 

src/matrix/e2ee/README.md

@@ -1,40 +1,41 @@
 ## Integratation within the sync lifetime cycle
 
-### prepareSync
+### session.prepareSync
+
+Decrypt any device messages, and turn them into RoomKey instances.
+Any rooms that are not in the sync response but for which we receive keys will be included in the rooms to sync.
+
+Runs before any room.prepareSync, so the new room keys can be passed to each room prepareSync to use in decryption.
+
+### room.prepareSync
     
     The session can start its own read/write transactions here, rooms only read from a shared transaction
 
-    - session
-        - device handler
-            - txn
-                - write pending encrypted
-            - txn
-                - olm decryption read
-            - olm async decryption
-                - dispatch to worker
-            - txn
-                - olm decryption write / remove pending encrypted
     - rooms (with shared read txn)
-        - megolm decryption read
+        - megolm decryption read using any new keys decrypted by the session.
 
-### afterPrepareSync
+### room.afterPrepareSync
 
     - rooms    
         - megolm async decryption   
             - dispatch to worker
 
-### writeSync
+### room.writeSync
 
     - rooms (with shared readwrite txn)
         - megolm decryption write, yielding decrypted events
         - use decrypted events to write room summary
 
-### afterSync
+### session.writeSync
+
+ - writes any room keys that were received
+
+### room.afterSync
 
     - rooms
         - emit changes
 
-### afterSyncCompleted
+### room.afterSyncCompleted
 
     - session
         - e2ee account

src/matrix/e2ee/RoomEncryption.js

@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import {MEGOLM_ALGORITHM, DecryptionSource} from "./common.js";
-import {groupBy} from "../../utils/groupBy.js";
+import {groupEventsBySession} from "./megolm/decryption/utils.js";
 import {mergeMap} from "../../utils/mergeMap.js";
 import {makeTxnId} from "../common.js";
 
@@ -25,15 +25,6 @@ const ENCRYPTED_TYPE = "m.room.encrypted";
 // note that encrypt could still create a new session
 const MIN_PRESHARE_INTERVAL = 60 * 1000; // 1min
 
-function encodeMissingSessionKey(senderKey, sessionId) {
-    return `${senderKey}|${sessionId}`;
-}
-
-function decodeMissingSessionKey(key) {
-    const [senderKey, sessionId] = key.split("|");
-    return {senderKey, sessionId};
-}
-
 export class RoomEncryption {
     constructor({room, deviceTracker, olmEncryption, megolmEncryption, megolmDecryption, encryptionParams, storage, sessionBackup, notifyMissingMegolmSession, clock}) {
         this._room = room;
@@ -43,32 +34,40 @@ export class RoomEncryption {
         this._megolmDecryption = megolmDecryption;
         // content of the m.room.encryption event
         this._encryptionParams = encryptionParams;
-
         this._megolmBackfillCache = this._megolmDecryption.createSessionCache();
         this._megolmSyncCache = this._megolmDecryption.createSessionCache(1);
-        // session => event ids of messages we tried to decrypt and the session was missing
-        this._missingSessions = new SessionToEventIdsMap();
-        // sessions that may or may not be missing, but that while
-        // looking for a particular session came up as a candidate and were
-        // added to the cache to prevent further lookups from storage
-        this._missingSessionCandidates = new SessionToEventIdsMap();
+        // caches devices to verify events
         this._senderDeviceCache = new Map();
         this._storage = storage;
         this._sessionBackup = sessionBackup;
         this._notifyMissingMegolmSession = notifyMissingMegolmSession;
         this._clock = clock;
-        this._disposed = false;
         this._isFlushingRoomKeyShares = false;
         this._lastKeyPreShareTime = null;
+        this._disposed = false;
     }
 
-    async enableSessionBackup(sessionBackup) {
+    enableSessionBackup(sessionBackup) {
         if (this._sessionBackup) {
             return;
         }
         this._sessionBackup = sessionBackup;
-        for(const {senderKey, sessionId} of this._missingSessions.getSessions()) {
-            await this._requestMissingSessionFromBackup(senderKey, sessionId, null);
+    }
+
+    async restoreMissingSessionsFromBackup(events) {
+        const eventsBySession = groupEventsBySession(events);
+        const groups = Array.from(eventsBySession.values());
+        const txn = this._storage.readTxn([this._storage.storeNames.inboundGroupSessions]);
+        const hasSessions = await Promise.all(groups.map(async group => {
+            return this._megolmDecryption.hasSession(this._room.id, group.senderKey, group.sessionId, txn);
+        }));
+        const missingSessions = groups.filter((_, i) => !hasSessions[i]);
+        if (missingSessions.length) {
+            // start with last sessions which should be for the last items in the timeline
+            for (var i = missingSessions.length - 1; i >= 0; i--) {
+                const session = missingSessions[i];
+                await this._requestMissingSessionFromBackup(session.senderKey, session.sessionId);
+            }
         }
     }
 
@@ -129,16 +128,48 @@ export class RoomEncryption {
         return new DecryptionPreparation(preparation, errors, source, this, events);
     }
 
-    async _processDecryptionResults(events, results, errors, flags, txn) {
-        for (const event of events) {
+    async _processDecryptionResults(events, results, errors, source, txn) {
+        const missingSessionEvents = events.filter(event => {
             const error = errors.get(event.event_id);
-            if (error?.code === "MEGOLM_NO_SESSION") {
-                this._addMissingSessionEvent(event, flags.source);
-            } else {
-                this._missingSessions.removeEvent(event);
-                this._missingSessionCandidates.removeEvent(event);
-            }
+            return error?.code === "MEGOLM_NO_SESSION";
+        });
+        if (!missingSessionEvents.length) {
+            return;
         }
+        const eventsBySession = groupEventsBySession(events);
+        if (source === DecryptionSource.Sync) {
+            await Promise.all(Array.from(eventsBySession.values()).map(async group => {
+                const eventIds = group.events.map(e => e.event_id);
+                return this._megolmDecryption.addMissingKeyEventIds(
+                    this._room.id, group.senderKey, group.sessionId, eventIds, txn);
+            }));
+        }
+
+        // TODO: do proper logging here
+        // run detached
+        Promise.resolve().then(async () => {
+            // if the message came from sync, wait 10s to see if the room key arrives late,
+            // and only after that proceed to request from backup
+            if (source === DecryptionSource.Sync) {
+                await this._clock.createTimeout(10000).elapsed();
+                if (this._disposed) {
+                    return;
+                }
+                // now check which sessions have been received already
+                const txn = this._storage.readTxn([this._storage.storeNames.inboundGroupSessions]);
+                await Promise.all(Array.from(eventsBySession).map(async ([key, group]) => {
+                    if (await this._megolmDecryption.hasSession(this._room.id, group.senderKey, group.sessionId, txn)) {
+                        eventsBySession.delete(key);
+                    }
+                }));
+            }
+            await Promise.all(Array.from(eventsBySession.values()).map(group => {
+                return this._requestMissingSessionFromBackup(group.senderKey, group.sessionId);
+            }));
+        }).catch(err => {
+            console.log("failed to fetch missing session from key backup");
+            console.error(err);
+        });
     }
 
     async _verifyDecryptionResult(result, txn) {
@@ -154,24 +185,7 @@ export class RoomEncryption {
         }
     }
 
-    _addMissingSessionEvent(event, source) {
-        const isNewSession = this._missingSessions.addEvent(event);
-        if (isNewSession) {
-            const senderKey = event.content?.["sender_key"];
-            const sessionId = event.content?.["session_id"];
-            this._requestMissingSessionFromBackup(senderKey, sessionId, source);
-        }
-    }
-
-    async _requestMissingSessionFromBackup(senderKey, sessionId, source) {
-        // if the message came from sync, wait 10s to see if the room key arrives,
-        // and only after that proceed to request from backup
-        if (source === DecryptionSource.Sync) {
-            await this._clock.createTimeout(10000).elapsed();
-            if (this._disposed || !this._missingSessions.hasSession(senderKey, sessionId)) {
-                return;
-            }
-        }
+    async _requestMissingSessionFromBackup(senderKey, sessionId) {
         // show prompt to enable secret storage
         if (!this._sessionBackup) {
             this._notifyMissingMegolmSession();
@@ -211,43 +225,19 @@ export class RoomEncryption {
                 console.info(`Backed-up session of unknown algorithm: ${session.algorithm}`);
             }
         } catch (err) {
-            console.error(`Could not get session ${sessionId} from backup`, err);
+            if (!(err.name === "HomeServerError" && err.errcode === "M_NOT_FOUND")) {
+                console.error(`Could not get session ${sessionId} from backup`, err);
+            }
         }
     }
 
     /**
      * @param  {RoomKey} roomKeys
-     * @return {Array<string>} the event ids that should be retried to decrypt
+     * @param {Transaction} txn
+     * @return {Promise<Array<string>>} the event ids that should be retried to decrypt
      */
-    getEventIdsForRoomKey(roomKey) {
-        // TODO: we could concat both results here, and only put stuff in
-        // candidates if it is not in missing sessions to use a bit less memory
-        let eventIds = this._missingSessions.getEventIds(roomKey.senderKey, roomKey.sessionId);
-        if (!eventIds) {
-            eventIds = this._missingSessionCandidates.getEventIds(roomKey.senderKey, roomKey.sessionId);
-        }
-        return eventIds;
-    }
-
-    /**
-     * caches mapping of session to event id of all encrypted candidates
-     * and filters to return only the candidates for the given room key
-     */
-    findAndCacheEntriesForRoomKey(roomKey, candidateEntries) {
-        const matches = [];
-
-        for (const entry of candidateEntries) {
-            if (entry.eventType === ENCRYPTED_TYPE) {
-                this._missingSessionCandidates.addEvent(entry.event);
-                const senderKey = entry.event?.content?.["sender_key"];
-                const sessionId = entry.event?.content?.["session_id"];
-                if (senderKey === roomKey.senderKey && sessionId === roomKey.sessionId) {
-                    matches.push(entry);
-                }
-            }
-        }
-        
-        return matches;
+    getEventIdsForMissingKey(roomKey, txn) {
+        return this._megolmDecryption.getEventIdsForMissingKey(this._room.id, roomKey.senderKey, roomKey.sessionId, txn);
     }
 
     /** shares the encryption key for the next message if needed */
@@ -485,58 +475,3 @@ class BatchDecryptionResult {
         }));
     }
 }
-
-class SessionToEventIdsMap {
-    constructor() {
-        this._eventIdsBySession = new Map();
-    }
-
-    addEvent(event) {
-        let isNewSession = false;
-        const senderKey = event.content?.["sender_key"];
-        const sessionId = event.content?.["session_id"];
-        const key = encodeMissingSessionKey(senderKey, sessionId);
-        let eventIds = this._eventIdsBySession.get(key);
-        // new missing session
-        if (!eventIds) {
-            eventIds = new Set();
-            this._eventIdsBySession.set(key, eventIds);
-            isNewSession = true;
-        }
-        eventIds.add(event.event_id);
-        return isNewSession;
-    }
-
-    getEventIds(senderKey, sessionId) {
-        const key = encodeMissingSessionKey(senderKey, sessionId);
-        const entriesForSession = this._eventIdsBySession.get(key);
-        if (entriesForSession) {
-            return [...entriesForSession];
-        }
-    }
-
-    getSessions() {
-        return Array.from(this._eventIdsBySession.keys()).map(decodeMissingSessionKey);
-    }
-
-    hasSession(senderKey, sessionId) {
-        return this._eventIdsBySession.has(encodeMissingSessionKey(senderKey, sessionId));
-    }
-
-    removeEvent(event) {
-        let hasRemovedSession = false;
-        const senderKey = event.content?.["sender_key"];
-        const sessionId = event.content?.["session_id"];
-        const key = encodeMissingSessionKey(senderKey, sessionId);
-        let eventIds = this._eventIdsBySession.get(key);
-        if (eventIds) {
-            if (eventIds.delete(event.event_id)) {
-                if (!eventIds.length) {
-                    this._eventIdsBySession.delete(key);
-                    hasRemovedSession = true;
-                }
-            }
-        }
-        return hasRemovedSession;
-    }
-}

src/matrix/e2ee/megolm/Decryption.js

@@ -15,25 +15,13 @@ limitations under the License.
 */
 
 import {DecryptionError} from "../common.js";
-import {groupBy} from "../../../utils/groupBy.js";
 import * as RoomKey from "./decryption/RoomKey.js";
 import {SessionInfo} from "./decryption/SessionInfo.js";
 import {DecryptionPreparation} from "./decryption/DecryptionPreparation.js";
 import {SessionDecryption} from "./decryption/SessionDecryption.js";
 import {SessionCache} from "./decryption/SessionCache.js";
 import {MEGOLM_ALGORITHM} from "../common.js";
-
-function getSenderKey(event) {
-    return event.content?.["sender_key"];
-}
-
-function getSessionId(event) {
-    return event.content?.["session_id"];
-}
-
-function getCiphertext(event) {
-    return event.content?.ciphertext;
-}
+import {validateEvent, groupEventsBySession} from "./decryption/utils.js";
 
 export class Decryption {
     constructor({pickleKey, olm, olmWorker}) {
@@ -46,6 +34,37 @@ export class Decryption {
         return new SessionCache(size);
     }
 
+    async addMissingKeyEventIds(roomId, senderKey, sessionId, eventIds, txn) {
+        let sessionEntry = await txn.inboundGroupSessions.get(roomId, senderKey, sessionId);
+        // we never want to overwrite an existing key
+        if (sessionEntry?.session) {
+            return;
+        }
+        if (sessionEntry) {
+            const uniqueEventIds = new Set(sessionEntry.eventIds);
+            for (const id of eventIds) {
+                uniqueEventIds.add(id);
+            }
+            sessionEntry.eventIds = Array.from(uniqueEventIds);
+        } else {
+            sessionEntry = {roomId, senderKey, sessionId, eventIds};
+        }
+        txn.inboundGroupSessions.set(sessionEntry);
+    }
+
+    async getEventIdsForMissingKey(roomId, senderKey, sessionId, txn) {
+        const sessionEntry = await txn.inboundGroupSessions.get(roomId, senderKey, sessionId);
+        if (sessionEntry && !sessionEntry.session) {
+            return sessionEntry.eventIds;
+        }
+    }
+
+    async hasSession(roomId, senderKey, sessionId, txn) {
+        const sessionEntry = await txn.inboundGroupSessions.get(roomId, senderKey, sessionId);
+        const isValidSession = typeof sessionEntry?.session === "string";
+        return isValidSession;
+    }
+
     /**
      * Reads all the state from storage to be able to decrypt the given events.
      * Decryption can then happen outside of a storage transaction.
@@ -61,28 +80,22 @@ export class Decryption {
         const validEvents = [];
 
         for (const event of events) {
-            const isValid = typeof getSenderKey(event) === "string" &&
-                            typeof getSessionId(event) === "string" &&
-                            typeof getCiphertext(event) === "string";
-            if (isValid) {
+            if (validateEvent(event)) {
                 validEvents.push(event);
             } else {
                 errors.set(event.event_id, new DecryptionError("MEGOLM_INVALID_EVENT", event))
             }
         }
 
-        const eventsBySession = groupBy(validEvents, event => {
-            return `${getSenderKey(event)}|${getSessionId(event)}`;
-        });
+        const eventsBySession = groupEventsBySession(validEvents);
 
         const sessionDecryptions = [];
-        await Promise.all(Array.from(eventsBySession.values()).map(async eventsForSession => {
-            const firstEvent = eventsForSession[0];
-            const sessionInfo = await this._getSessionInfoForEvent(roomId, firstEvent, newKeys, sessionCache, txn);
+        await Promise.all(Array.from(eventsBySession.values()).map(async group => {
+            const sessionInfo = await this._getSessionInfo(roomId, group.senderKey, group.sessionId, newKeys, sessionCache, txn);
             if (sessionInfo) {
-                sessionDecryptions.push(new SessionDecryption(sessionInfo, eventsForSession, this._olmWorker));
+                sessionDecryptions.push(new SessionDecryption(sessionInfo, group.events, this._olmWorker));
             } else {
-                for (const event of eventsForSession) {
+                for (const event of group.events) {
                     errors.set(event.event_id, new DecryptionError("MEGOLM_NO_SESSION", event));
                 }
             }
@@ -91,9 +104,7 @@ export class Decryption {
         return new DecryptionPreparation(roomId, sessionDecryptions, errors);
     }
 
-    async _getSessionInfoForEvent(roomId, event, newKeys, sessionCache, txn) {
-        const senderKey = getSenderKey(event);
-        const sessionId = getSessionId(event);
+    async _getSessionInfo(roomId, senderKey, sessionId, newKeys, sessionCache, txn) {
         let sessionInfo;
         if (newKeys) {
             const key = newKeys.find(k => k.roomId === roomId && k.senderKey === senderKey && k.sessionId === sessionId);
@@ -110,7 +121,7 @@ export class Decryption {
         }
         if (!sessionInfo) {
             const sessionEntry = await txn.inboundGroupSessions.get(roomId, senderKey, sessionId);
-            if (sessionEntry) {
+            if (sessionEntry && sessionEntry.session) {
                 let session = new this._olm.InboundGroupSession();
                 try {
                     session.unpickle(this._pickleKey, sessionEntry.session);

src/matrix/e2ee/megolm/decryption/RoomKey.js

@@ -33,7 +33,7 @@ export class BaseRoomKey {
     async _isBetterThanKnown(session, olm, pickleKey, txn) {
         let isBetter = true;
         const existingSessionEntry = await txn.inboundGroupSessions.get(this.roomId, this.senderKey, this.sessionId);
-        if (existingSessionEntry) {
+        if (existingSessionEntry?.session) {
             const existingSession = new olm.InboundGroupSession();
             try {
                 existingSession.unpickle(pickleKey, existingSessionEntry.session);

src/matrix/e2ee/megolm/decryption/utils.js

@@ -0,0 +1,57 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+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.
+*/
+
+import {groupByWithCreator} from "../../../../utils/groupBy.js";
+
+function getSenderKey(event) {
+    return event.content?.["sender_key"];
+}
+
+function getSessionId(event) {
+    return event.content?.["session_id"];
+}
+
+function getCiphertext(event) {
+    return event.content?.ciphertext;
+}
+
+export function validateEvent(event) {
+    return typeof getSenderKey(event) === "string" &&
+           typeof getSessionId(event) === "string" &&
+           typeof getCiphertext(event) === "string";
+}
+
+class SessionKeyGroup {
+    constructor() {
+        this.events = [];
+    }
+
+    get senderKey() {
+        return getSenderKey(this.events[0]);
+    }
+
+    get sessionId() {
+        return getSessionId(this.events[0]);
+    }
+}
+
+export function groupEventsBySession(events) {
+    return groupByWithCreator(events,
+        event => `${getSenderKey(event)}|${getSessionId(event)}`,
+        () => new SessionKeyGroup(),
+        (group, event) => group.events.push(event)
+    );
+}

src/matrix/room/Room.js

@@ -58,56 +58,36 @@ export class Room extends EventEmitter {
         this._observedEvents = null;
     }
 
-    _readRetryDecryptCandidateEntries(sinceEventKey, txn) {
-        if (sinceEventKey) {
-            return readRawTimelineEntriesWithTxn(this._roomId, sinceEventKey,
-                Direction.Forward, Number.MAX_SAFE_INTEGER, this._fragmentIdComparer, txn);
-        } else {
-            // all messages for room ...
-            // if you haven't decrypted any message in a room yet,
-            // it's unlikely you will have tons of them.
-            // so this should be fine as a last resort
-            return readRawTimelineEntriesWithTxn(this._roomId, this._syncWriter.lastMessageKey,
-                Direction.Backward, Number.MAX_SAFE_INTEGER, this._fragmentIdComparer, txn);
-        }
-    }
-
-    async notifyRoomKey(roomKey) {
-        if (!this._roomEncryption) {
-            return;
-        }
-        const retryEventIds = this._roomEncryption.getEventIdsForRoomKey(roomKey);
-        const stores = [
-            this._storage.storeNames.timelineEvents,
-            this._storage.storeNames.inboundGroupSessions,
-        ];
-        let txn;
-        let retryEntries;
+    async _getRetryDecryptEntriesForKey(roomKey, txn) {
+        const retryEventIds = await this._roomEncryption.getEventIdsForMissingKey(roomKey, txn);
+        const retryEntries = [];
         if (retryEventIds) {
-            retryEntries = [];
-            txn = this._storage.readTxn(stores);
             for (const eventId of retryEventIds) {
                 const storageEntry = await txn.timelineEvents.getByEventId(this._roomId, eventId);
                 if (storageEntry) {
                     retryEntries.push(new EventEntry(storageEntry, this._fragmentIdComparer));
                 }
             }
-        } else {
-            // we only look for messages since lastDecryptedEventKey because
-            // the timeline must be closed (otherwise getEventIdsForRoomKey would have found the event ids)
-            // and to update the summary we only care about events since lastDecryptedEventKey
-            const key = this._summary.data.lastDecryptedEventKey;
-            // key might be missing if we haven't decrypted any events in this room
-            const sinceEventKey = key && new EventKey(key.fragmentId, key.entryIndex);
-            // check we have not already decrypted the most recent event in the room
-            // otherwise we know that the messages for this room key will not update the room summary
-            if (!sinceEventKey || !sinceEventKey.equals(this._syncWriter.lastMessageKey)) {
-                txn = this._storage.readTxn(stores.concat(this._storage.storeNames.timelineFragments));
-                const candidateEntries = await this._readRetryDecryptCandidateEntries(sinceEventKey, txn);
-                retryEntries = this._roomEncryption.findAndCacheEntriesForRoomKey(roomKey, candidateEntries);
-            }
         }
-        if (retryEntries?.length) {
+        return retryEntries;
+    }
+
+    /**
+     * Used for keys received from other sources than sync, like key backup.
+     * @internal
+     * @param  {RoomKey} roomKey
+     * @return {Promise}
+     */
+    async notifyRoomKey(roomKey) {
+        if (!this._roomEncryption) {
+            return;
+        }
+        const txn = this._storage.readTxn([
+            this._storage.storeNames.timelineEvents,
+            this._storage.storeNames.inboundGroupSessions,
+        ]);
+        const retryEntries = this._getRetryDecryptEntriesForKey(roomKey, txn);
+        if (retryEntries.length) {
             const decryptRequest = this._decryptEntries(DecryptionSource.Retry, retryEntries, txn);
             // this will close txn while awaiting decryption
             await decryptRequest.complete();
@@ -197,11 +177,10 @@ export class Room extends EventEmitter {
         if (roomEncryption) {
             // also look for events in timeline here
             let events = roomResponse?.timeline?.events || [];
-            // when new keys arrive, also see if any events currently loaded in the timeline
-            // can now be retried to decrypt
-            if (this._timeline && newKeys) {
-                retryEntries = roomEncryption.filterEventEntriesForKeys(
-                    this._timeline.remoteEntries, newKeys);
+            // when new keys arrive, also see if any events that can now be retried to decrypt
+            if (newKeys) {
+                const nestedEntries = await Promise.all(newKeys.map(key => this._getRetryDecryptEntriesForKey(key, txn)));
+                const retryEntries = nestedEntries.reduce((allEntries, entries) => allEntries.concat(entries), []);
                 if (retryEntries.length) {
                     log.set("retry", retryEntries.length);
                     events = events.concat(retryEntries.map(entry => entry.event));
@@ -228,7 +207,7 @@ export class Room extends EventEmitter {
 
     async afterPrepareSync(preparation, parentLog) {
         if (preparation.decryptPreparation) {
-            await parentLog.wrap("afterPrepareSync decrypt", async log => {
+            await parentLog.wrap("decrypt", async log => {
                 log.set("id", this.id);
                 preparation.decryptChanges = await preparation.decryptPreparation.decrypt();
                 preparation.decryptPreparation = null;
@@ -247,10 +226,8 @@ export class Room extends EventEmitter {
                 await decryption.verifySenders(txn);
             }
             if (retryEntries?.length) {
-                // TODO: this will modify existing timeline entries (which we should not do in writeSync), 
-                // but it is a temporary way of reattempting decryption while timeline is open
-                // won't need copies when tracking missing sessions properly
-                // prepend the retried entries, as we know they are older (not that it should matter much for the summary)
+                // prepend the retried entries, as we know they are older
+                // (not that it should matter much for the summary)
                 entries.unshift(...retryEntries);
             }
             decryption.applyToEntries(entries);
@@ -554,6 +531,10 @@ export class Room extends EventEmitter {
 
     enableSessionBackup(sessionBackup) {
         this._roomEncryption?.enableSessionBackup(sessionBackup);
+        if (this._timeline) {
+            const timelineEvents = this._timeline.remoteEntries.filter(e => e.event).map(e => e.event);
+            this._roomEncryption.restoreMissingSessionsFromBackup(timelineEvents);
+        }
     }
 
     get isTrackingMembers() {

src/matrix/room/RoomSummary.js

@@ -122,36 +122,6 @@ function processTimelineEvent(data, eventEntry, isInitialSync, canMarkUnread, ow
             data = data.cloneIfNeeded();
             data.isUnread = true;
         }
-        const {content} = eventEntry;
-        const body = content?.body;
-        const msgtype = content?.msgtype;
-        if (msgtype === "m.text" && !eventEntry.isEncrypted) {
-            data = data.cloneIfNeeded();
-            data.lastMessageBody = body;
-        }
-    }
-    // store the event key of the last decrypted event so when decryption does succeed,
-    // we can attempt to re-decrypt from this point to update the room summary
-    if (!!data.encryption && eventEntry.isEncrypted && eventEntry.isDecrypted) {
-        let hasLargerEventKey = true;
-        if (data.lastDecryptedEventKey) {
-            try {
-                hasLargerEventKey = eventEntry.compare(data.lastDecryptedEventKey) > 0;
-            } catch (err) {
-                // TODO: load the fragments in between here?
-                // this could happen if an earlier event gets decrypted that
-                // is in a fragment different from the live one and the timeline is not open.
-                // In this case, we will just read too many events once per app load
-                // and then keep the mapping in memory. When eventually an event is decrypted in
-                // the live fragment, this should stop failing and the event key will be written.
-                hasLargerEventKey = false;
-            }
-        }
-        if (hasLargerEventKey) {
-            data = data.cloneIfNeeded();
-            const {fragmentId, entryIndex} = eventEntry;
-            data.lastDecryptedEventKey = {fragmentId, entryIndex};
-        }
     }
     return data;
 }
@@ -182,12 +152,9 @@ class SummaryData {
     constructor(copy, roomId) {
         this.roomId = copy ? copy.roomId : roomId;
         this.name = copy ? copy.name : null;
-        this.lastMessageBody = copy ? copy.lastMessageBody : null;
         this.lastMessageTimestamp = copy ? copy.lastMessageTimestamp : null;
         this.isUnread = copy ? copy.isUnread : false;
         this.encryption = copy ? copy.encryption : null;
-        this.lastDecryptedEventKey = copy ? copy.lastDecryptedEventKey : null;
-        this.isDirectMessage = copy ? copy.isDirectMessage : false;
         this.membership = copy ? copy.membership : null;
         this.inviteCount = copy ? copy.inviteCount : 0;
         this.joinCount = copy ? copy.joinCount : 0;