Fix MatrixRTC sender key wrapping

spec/unit/matrixrtc/MatrixRTCSession.spec.ts

@@ -1010,6 +1010,48 @@ describe("MatrixRTCSession", () => {
  }
  });
 
+ it("Wraps key index around to 0 when it reaches the maximum", async () => {
+ // this should give us keys with index [0...255, 0, 1]
+ const membersToTest = 258;
+ const members: CallMembershipData[] = [];
+ for (let i = 0; i < membersToTest; i++) {
+ members.push(Object.assign({}, membershipTemplate, { device_id: `DEVICE${i}` }));
+ }
+ jest.useFakeTimers();
+ try {
+ // start with a single member
+ const mockRoom = makeMockRoom(members.slice(0, 1));
+
+ for (let i = 0; i < membersToTest; i++) {
+ const keysSentPromise = new Promise<EncryptionKeysEventContent>((resolve) => {
+ sendEventMock.mockImplementation((_roomId, _evType, payload) => resolve(payload));
+ });
+
+ if (i === 0) {
+ // if first time around then set up the session
+ sess = MatrixRTCSession.roomSessionForRoom(client, mockRoom);
+ sess.joinRoomSession([mockFocus], mockFocus, { manageMediaKeys: true });
+ } else {
+ // otherwise update the state
+ mockRoom.getLiveTimeline().getState = jest
+ .fn()
+ .mockReturnValue(makeMockRoomState(members.slice(0, i + 1), mockRoom.roomId));
+ }
+
+ sess!.onMembershipUpdate();
+
+ // advance time to avoid key throttling
+ jest.advanceTimersByTime(10000);
+
+ const keysPayload = await keysSentPromise;
+ expect(keysPayload.keys).toHaveLength(1);
+ expect(keysPayload.keys[0].index).toEqual(i % 256);
+ }
+ } finally {
+ jest.useRealTimers();
+ }
+ });
+
  it("Doesn't re-send key immediately", async () => {
  const realSetTimeout = setTimeout;
  jest.useFakeTimers();

src/matrixrtc/MatrixRTCSession.ts

@@ -448,13 +448,12 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
  }
 
  private getNewEncryptionKeyIndex(): number {
- const userId = this.client.getUserId();
- const deviceId = this.client.getDeviceId();
-
- if (!userId) throw new Error("No userId!");
- if (!deviceId) throw new Error("No deviceId!");
+ if (this.currentEncryptionKeyIndex === -1) {
+ return 0;
+ }
 
- return (this.getKeysForParticipantInternal(userId, deviceId)?.length ?? 0) % 16;
+ // maximum key index is 255
+ return (this.currentEncryptionKeyIndex + 1) % 256;
  }
 
  /**