Don't share full key history for RTC per-participant encryption

matrix-org · Sep 16, 2024 · a52fb3e · a52fb3e
1 parent 53b599f
commit a52fb3e
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Showing 2 changed files with 38 additions and 16 deletions.
diff --git a/spec/unit/matrixrtc/MatrixRTCSession.spec.ts b/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
@@ -946,6 +946,7 @@ describe("MatrixRTCSession", () => {
                 sess.joinRoomSession([mockFocus], mockFocus, { manageMediaKeys: true });
                 const firstKeysPayload = await keysSentPromise1;
                 expect(firstKeysPayload.keys).toHaveLength(1);
+                expect(firstKeysPayload.keys[0].index).toEqual(0);
 
                 sendEventMock.mockClear();
 
@@ -962,7 +963,8 @@ describe("MatrixRTCSession", () => {
 
                 const secondKeysPayload = await keysSentPromise2;
 
-                expect(secondKeysPayload.keys).toHaveLength(2);
+                expect(secondKeysPayload.keys).toHaveLength(1);
+                expect(secondKeysPayload.keys[0].index).toEqual(1);
                 expect(onMyEncryptionKeyChanged).toHaveBeenCalledTimes(2);
             } finally {
                 jest.useRealTimers();

diff --git a/src/matrixrtc/MatrixRTCSession.ts b/src/matrixrtc/MatrixRTCSession.ts
@@ -144,6 +144,8 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
     // if it looks like a membership has been updated.
     private lastMembershipFingerprints: Set<string> | undefined;
 
+    private currentEncryptionKeyIndex = -1;
+
     /**
      * The callId (sessionId) of the call.
      *
@@ -467,10 +469,16 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
             const useKeyTimeout = setTimeout(() => {
                 this.setNewKeyTimeouts.delete(useKeyTimeout);
                 logger.info(`Delayed-emitting key changed event for ${participantId} idx ${encryptionKeyIndex}`);
+                if (userId === this.client.getUserId() && deviceId === this.client.getDeviceId()) {
+                    this.currentEncryptionKeyIndex = encryptionKeyIndex;
+                }
                 this.emit(MatrixRTCSessionEvent.EncryptionKeyChanged, keyBin, encryptionKeyIndex, participantId);
             }, USE_KEY_DELAY);
             this.setNewKeyTimeouts.add(useKeyTimeout);
         } else {
+            if (userId === this.client.getUserId() && deviceId === this.client.getDeviceId()) {
+                this.currentEncryptionKeyIndex = encryptionKeyIndex;
+            }
             this.emit(MatrixRTCSessionEvent.EncryptionKeyChanged, keyBin, encryptionKeyIndex, participantId);
         }
     }
@@ -479,8 +487,9 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
      * Generate a new sender key and add it at the next available index
      * @param delayBeforeUse - If true, wait for a short period before setting the key for the
      *                         media encryptor to use. If false, set the key immediately.
+     * @returns The index of the new key
      */
-    private makeNewSenderKey(delayBeforeUse = false): void {
+    private makeNewSenderKey(delayBeforeUse = false): number {
         const userId = this.client.getUserId();
         const deviceId = this.client.getDeviceId();
 
@@ -491,6 +500,7 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
         const encryptionKeyIndex = this.getNewEncryptionKeyIndex();
         logger.info("Generated new key at index " + encryptionKeyIndex);
         this.setEncryptionKey(userId, deviceId, encryptionKeyIndex, encryptionKey, Date.now(), delayBeforeUse);
+        return encryptionKeyIndex;
     }
 
     /**
@@ -517,17 +527,17 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
     /**
      * Re-sends the encryption keys room event
      */
-    private sendEncryptionKeysEvent = async (): Promise<void> => {
+    private sendEncryptionKeysEvent = async (indexToSend?: number): Promise<void> => {
         if (this.keysEventUpdateTimeout !== undefined) {
             clearTimeout(this.keysEventUpdateTimeout);
             this.keysEventUpdateTimeout = undefined;
         }
         this.lastEncryptionKeyUpdateRequest = Date.now();
 
-        logger.info("Sending encryption keys event");
-
         if (!this.isJoined()) return;
 
+        logger.info(`Sending encryption keys event. indexToSend=${indexToSend}`);
+
         const userId = this.client.getUserId();
         const deviceId = this.client.getDeviceId();
 
@@ -541,20 +551,30 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
             return;
         }
 
+        if (typeof indexToSend !== "number" && this.currentEncryptionKeyIndex === -1) {
+            logger.warn("Tried to send encryption keys event but no current key index found!");
+            return;
+        }
+
+        const keyIndexToSend = typeof indexToSend === "number" ? indexToSend : this.currentEncryptionKeyIndex;
+        const keyToSend = myKeys[keyIndexToSend];
+
         try {
-            await this.client.sendEvent(this.room.roomId, EventType.CallEncryptionKeysPrefix, {
-                keys: myKeys.map((key, index) => {
-                    return {
-                        index,
-                        key: encodeUnpaddedBase64(key),
-                    };
-                }),
+            const content: EncryptionKeysEventContent = {
+                keys: [
+                    {
+                        index: keyIndexToSend,
+                        key: encodeUnpaddedBase64(keyToSend),
+                    },
+                ],
                 device_id: deviceId,
                 call_id: "",
-            } as EncryptionKeysEventContent);
+            };
+
+            await this.client.sendEvent(this.room.roomId, EventType.CallEncryptionKeysPrefix, content);
 
             logger.debug(
-                `Embedded-E2EE-LOG updateEncryptionKeyEvent participantId=${userId}:${deviceId} numSent=${myKeys.length}`,
+                `Embedded-E2EE-LOG updateEncryptionKeyEvent participantId=${userId}:${deviceId} numKeys=${myKeys.length} currentKeyIndex=${this.currentEncryptionKeyIndex} keyIndexToSend=${keyIndexToSend}`,
                 this.encryptionKeys,
             );
         } catch (error) {
@@ -1030,9 +1050,9 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
 
         this.makeNewKeyTimeout = undefined;
         logger.info("Making new sender key for key rotation");
-        this.makeNewSenderKey(true);
+        const newKeyIndex = this.makeNewSenderKey(true);
         // send immediately: if we're about to start sending with a new key, it's
         // important we get it out to others as soon as we can.
-        this.sendEncryptionKeysEvent();
+        this.sendEncryptionKeysEvent(newKeyIndex);
     };
 }