Don't share full key history for RTC per-participant encryption (#4406)

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

Also record stats for how many keys have been sent/received and age of those received

* Update src/matrixrtc/MatrixRTCSession.ts

Co-authored-by: Robin <robin@robin.town>

* Add comment about why we track total age of events

---------

Co-authored-by: Robin <robin@robin.town>

spec/unit/matrixrtc/MatrixRTCSession.spec.ts

@@ -594,6 +594,8 @@ describe("MatrixRTCSession", () => {
         });
 
         it("sends keys when joining", async () => {
+            jest.useFakeTimers();
+            try {
                 const eventSentPromise = new Promise((resolve) => {
                     sendEventMock.mockImplementation(resolve);
                 });
@@ -602,7 +604,10 @@ describe("MatrixRTCSession", () => {
 
                 await eventSentPromise;
 
-            expect(sendEventMock).toHaveBeenCalledWith(expect.stringMatching(".*"), "io.element.call.encryption_keys", {
+                expect(sendEventMock).toHaveBeenCalledWith(
+                    expect.stringMatching(".*"),
+                    "io.element.call.encryption_keys",
+                    {
                         call_id: "",
                         device_id: "AAAAAAA",
                         keys: [
@@ -611,7 +616,13 @@ describe("MatrixRTCSession", () => {
                                 key: expect.stringMatching(".*"),
                             },
                         ],
-            });
+                        sent_ts: Date.now(),
+                    },
+                );
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
+            } finally {
+                jest.useRealTimers();
+            }
         });
 
         it("does not send key if join called when already joined", () => {
@@ -619,10 +630,12 @@ describe("MatrixRTCSession", () => {
 
             expect(client.sendStateEvent).toHaveBeenCalledTimes(1);
             expect(client.sendEvent).toHaveBeenCalledTimes(1);
+            expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
 
             sess!.joinRoomSession([mockFocus], mockFocus, { manageMediaKeys: true });
             expect(client.sendStateEvent).toHaveBeenCalledTimes(1);
             expect(client.sendEvent).toHaveBeenCalledTimes(1);
+            expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
         });
 
         it("retries key sends", async () => {
@@ -651,6 +664,7 @@ describe("MatrixRTCSession", () => {
                 await eventSentPromise;
 
                 expect(sendEventMock).toHaveBeenCalledTimes(2);
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(2);
             } finally {
                 jest.useRealTimers();
             }
@@ -684,6 +698,7 @@ describe("MatrixRTCSession", () => {
 
                 sess.joinRoomSession([mockFocus], mockFocus, { manageMediaKeys: true });
                 await keysSentPromise1;
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
 
                 sendEventMock.mockClear();
                 jest.advanceTimersByTime(10000);
@@ -707,6 +722,7 @@ describe("MatrixRTCSession", () => {
                 await keysSentPromise2;
 
                 expect(sendEventMock).toHaveBeenCalled();
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(2);
             } finally {
                 jest.useRealTimers();
             }
@@ -747,14 +763,17 @@ describe("MatrixRTCSession", () => {
                                 key: expect.stringMatching(".*"),
                             },
                         ],
+                        sent_ts: Date.now(),
                     },
                 );
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
 
                 sendEventMock.mockClear();
 
                 // these should be a no-op:
                 sess.onMembershipUpdate();
                 expect(sendEventMock).toHaveBeenCalledTimes(0);
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
             } finally {
                 jest.useRealTimers();
             }
@@ -796,8 +815,10 @@ describe("MatrixRTCSession", () => {
                                 key: expect.stringMatching(".*"),
                             },
                         ],
+                        sent_ts: Date.now(),
                     },
                 );
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
 
                 sendEventMock.mockClear();
 
@@ -832,8 +853,10 @@ describe("MatrixRTCSession", () => {
                                 key: expect.stringMatching(".*"),
                             },
                         ],
+                        sent_ts: Date.now(),
                     },
                 );
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(2);
             } finally {
                 jest.useRealTimers();
             }
@@ -877,8 +900,10 @@ describe("MatrixRTCSession", () => {
                                 key: expect.stringMatching(".*"),
                             },
                         ],
+                        sent_ts: Date.now(),
                     },
                 );
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
 
                 sendEventMock.mockClear();
 
@@ -913,8 +938,10 @@ describe("MatrixRTCSession", () => {
                                 key: expect.stringMatching(".*"),
                             },
                         ],
+                        sent_ts: Date.now(),
                     },
                 );
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(2);
             } finally {
                 jest.useRealTimers();
             }
@@ -946,6 +973,8 @@ describe("MatrixRTCSession", () => {
                 sess.joinRoomSession([mockFocus], mockFocus, { manageMediaKeys: true });
                 const firstKeysPayload = await keysSentPromise1;
                 expect(firstKeysPayload.keys).toHaveLength(1);
+                expect(firstKeysPayload.keys[0].index).toEqual(0);
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
 
                 sendEventMock.mockClear();
 
@@ -962,8 +991,10 @@ 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);
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(2);
             } finally {
                 jest.useRealTimers();
             }
@@ -984,6 +1015,7 @@ describe("MatrixRTCSession", () => {
                 await keysSentPromise1;
 
                 sendEventMock.mockClear();
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
 
                 const onMembershipsChanged = jest.fn();
                 sess.on(MatrixRTCSessionEvent.MembershipsChanged, onMembershipsChanged);
@@ -1002,6 +1034,7 @@ describe("MatrixRTCSession", () => {
                 });
 
                 expect(sendEventMock).not.toHaveBeenCalled();
+                expect(sess!.statistics.counters.roomEventEncryptionKeysSent).toEqual(1);
             } finally {
                 jest.useRealTimers();
             }
@@ -1167,6 +1200,7 @@ describe("MatrixRTCSession", () => {
         const bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
         expect(bobKeys).toHaveLength(1);
         expect(bobKeys[0]).toEqual(Buffer.from("this is the key", "utf-8"));
+        expect(sess!.statistics.counters.roomEventEncryptionKeysReceived).toEqual(1);
     });
 
     it("collects keys at non-zero indices", () => {
@@ -1195,6 +1229,7 @@ describe("MatrixRTCSession", () => {
         expect(bobKeys[2]).toBeFalsy();
         expect(bobKeys[3]).toBeFalsy();
         expect(bobKeys[4]).toEqual(Buffer.from("this is the key", "utf-8"));
+        expect(sess!.statistics.counters.roomEventEncryptionKeysReceived).toEqual(1);
     });
 
     it("collects keys by merging", () => {
@@ -1219,6 +1254,7 @@ describe("MatrixRTCSession", () => {
         let bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
         expect(bobKeys).toHaveLength(1);
         expect(bobKeys[0]).toEqual(Buffer.from("this is the key", "utf-8"));
+        expect(sess!.statistics.counters.roomEventEncryptionKeysReceived).toEqual(1);
 
         sess.onCallEncryption({
             getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
@@ -1239,6 +1275,7 @@ describe("MatrixRTCSession", () => {
         bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
         expect(bobKeys).toHaveLength(5);
         expect(bobKeys[4]).toEqual(Buffer.from("this is the key", "utf-8"));
+        expect(sess!.statistics.counters.roomEventEncryptionKeysReceived).toEqual(2);
     });
 
     it("ignores older keys at same index", () => {
@@ -1279,6 +1316,7 @@ describe("MatrixRTCSession", () => {
         const bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
         expect(bobKeys).toHaveLength(1);
         expect(bobKeys[0]).toEqual(Buffer.from("newer key", "utf-8"));
+        expect(sess!.statistics.counters.roomEventEncryptionKeysReceived).toEqual(2);
     });
 
     it("key timestamps are treated as monotonic", () => {
@@ -1342,5 +1380,73 @@ describe("MatrixRTCSession", () => {
 
         const myKeys = sess.getKeysForParticipant(client.getUserId()!, client.getDeviceId()!)!;
         expect(myKeys).toBeFalsy();
+        expect(sess!.statistics.counters.roomEventEncryptionKeysReceived).toEqual(0);
+    });
+
+    it("tracks total age statistics for collected keys", () => {
+        jest.useFakeTimers();
+        try {
+            const mockRoom = makeMockRoom([membershipTemplate]);
+            sess = MatrixRTCSession.roomSessionForRoom(client, mockRoom);
+
+            // defaults to getTs()
+            jest.setSystemTime(1000);
+            sess.onCallEncryption({
+                getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
+                getContent: jest.fn().mockReturnValue({
+                    device_id: "bobsphone",
+                    call_id: "",
+                    keys: [
+                        {
+                            index: 0,
+                            key: "dGhpcyBpcyB0aGUga2V5",
+                        },
+                    ],
+                }),
+                getSender: jest.fn().mockReturnValue("@bob:example.org"),
+                getTs: jest.fn().mockReturnValue(0),
+            } as unknown as MatrixEvent);
+            expect(sess!.statistics.totals.roomEventEncryptionKeysReceivedTotalAge).toEqual(1000);
+
+            jest.setSystemTime(2000);
+            sess.onCallEncryption({
+                getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
+                getContent: jest.fn().mockReturnValue({
+                    device_id: "bobsphone",
+                    call_id: "",
+                    keys: [
+                        {
+                            index: 0,
+                            key: "dGhpcyBpcyB0aGUga2V5",
+                        },
+                    ],
+                    sent_ts: 0,
+                }),
+                getSender: jest.fn().mockReturnValue("@bob:example.org"),
+                getTs: jest.fn().mockReturnValue(Date.now()),
+            } as unknown as MatrixEvent);
+            expect(sess!.statistics.totals.roomEventEncryptionKeysReceivedTotalAge).toEqual(3000);
+
+            jest.setSystemTime(3000);
+            sess.onCallEncryption({
+                getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
+                getContent: jest.fn().mockReturnValue({
+                    device_id: "bobsphone",
+                    call_id: "",
+                    keys: [
+                        {
+                            index: 0,
+                            key: "dGhpcyBpcyB0aGUga2V5",
+                        },
+                    ],
+                    sent_ts: 1000,
+                }),
+                getSender: jest.fn().mockReturnValue("@bob:example.org"),
+                getTs: jest.fn().mockReturnValue(Date.now()),
+            } as unknown as MatrixEvent);
+            expect(sess!.statistics.totals.roomEventEncryptionKeysReceivedTotalAge).toEqual(5000);
+        } finally {
+            jest.useRealTimers();
+        }
     });
 });

src/matrixrtc/MatrixRTCSession.ts

@@ -144,6 +144,31 @@ 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 statistics for this session.
+     */
+    public statistics = {
+        counters: {
+            /**
+             * The number of times we have sent a room event containing encryption keys.
+             */
+            roomEventEncryptionKeysSent: 0,
+            /**
+             * The number of times we have received a room event containing encryption keys.
+             */
+            roomEventEncryptionKeysReceived: 0,
+        },
+        totals: {
+            /**
+             * The total age (in milliseconds) of all room events containing encryption keys that we have received.
+             * We track the total age so that we can later calculate the average age of all keys received.
+             */
+            roomEventEncryptionKeysReceivedTotalAge: 0,
+        },
+    };
+
     /**
      * The callId (sessionId) of the call.
      *
@@ -467,10 +492,16 @@ 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 +510,9 @@ 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 +523,7 @@ 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 +550,17 @@ 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 +574,33 @@ 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 = 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);
+                sent_ts: Date.now(),
+            };
+
+            this.statistics.counters.roomEventEncryptionKeysSent += 1;
+
+            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) {
@@ -649,6 +695,10 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
             return;
         }
 
+        this.statistics.counters.roomEventEncryptionKeysReceived += 1;
+        const age = Date.now() - (typeof content.sent_ts === "number" ? content.sent_ts : event.getTs());
+        this.statistics.totals.roomEventEncryptionKeysReceivedTotalAge += age;
+
         for (const key of content.keys) {
             if (!key) {
                 logger.info("Ignoring false-y key in keys event");
@@ -674,7 +724,7 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
                 );
             } else {
                 logger.debug(
-                    `Embedded-E2EE-LOG onCallEncryption userId=${userId}:${deviceId} encryptionKeyIndex=${encryptionKeyIndex}`,
+                    `Embedded-E2EE-LOG onCallEncryption userId=${userId}:${deviceId} encryptionKeyIndex=${encryptionKeyIndex} age=${age}ms`,
                     this.encryptionKeys,
                 );
                 this.setEncryptionKey(userId, deviceId, encryptionKeyIndex, encryptionKey, event.getTs());
@@ -1030,9 +1080,9 @@ 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);
     };
 }

src/matrixrtc/types.ts

@@ -23,6 +23,7 @@ export interface EncryptionKeysEventContent {
     keys: EncryptionKeyEntry[];
     device_id: string;
     call_id: string;
+    sent_ts?: number;
 }
 
 export type CallNotifyType = "ring" | "notify";