Pass to-device messages into rust crypto-sdk

We need a separate API, because `ClientEvent.ToDeviceEvent` is only emitted for
successfully decrypted to-device events

Author: richvdh

spec/unit/rust-crypto.spec.ts

@@ -22,14 +22,15 @@ import {
     KeysClaimRequest,
     KeysQueryRequest,
     KeysUploadRequest,
+    OlmMachine,
     SignatureUploadRequest,
 } from "@matrix-org/matrix-sdk-crypto-js";
 import { Mocked } from "jest-mock";
 import MockHttpBackend from "matrix-mock-request";
 
 import { RustCrypto } from "../../src/rust-crypto/rust-crypto";
 import { initRustCrypto } from "../../src/rust-crypto";
-import { HttpApiEvent, HttpApiEventHandlerMap, IHttpOpts, MatrixHttpApi } from "../../src";
+import { HttpApiEvent, HttpApiEventHandlerMap, IHttpOpts, IToDeviceEvent, MatrixHttpApi } from "../../src";
 import { TypedEventEmitter } from "../../src/models/typed-event-emitter";
 
 afterEach(() => {
@@ -57,6 +58,47 @@ describe("RustCrypto", () => {
         });
     });
 
+    describe("to-device messages", () => {
+        let rustCrypto: RustCrypto;
+
+        beforeEach(async () => {
+            const mockHttpApi = {} as MatrixHttpApi<IHttpOpts>;
+            rustCrypto = (await initRustCrypto(mockHttpApi, TEST_USER, TEST_DEVICE_ID)) as RustCrypto;
+        });
+
+        it("should pass through unencrypted to-device messages", async () => {
+            const inputs: IToDeviceEvent[] = [
+                { content: { key: "value" }, type: "org.matrix.test", sender: "@alice:example.com" },
+            ];
+            const res = await rustCrypto.preprocessToDeviceMessages(inputs);
+            expect(res).toEqual(inputs);
+        });
+
+        it("should pass through bad encrypted messages", async () => {
+            const olmMachine: OlmMachine = rustCrypto["olmMachine"];
+            const keys = olmMachine.identityKeys;
+            const inputs: IToDeviceEvent[] = [
+                {
+                    type: "m.room.encrypted",
+                    content: {
+                        algorithm: "m.olm.v1.curve25519-aes-sha2",
+                        sender_key: "IlRMeOPX2e0MurIyfWEucYBRVOEEUMrOHqn/8mLqMjA",
+                        ciphertext: {
+                            [keys.curve25519.toBase64()]: {
+                                type: 0,
+                                body: "ajyjlghi",
+                            },
+                        },
+                    },
+                    sender: "@alice:example.com",
+                },
+            ];
+
+            const res = await rustCrypto.preprocessToDeviceMessages(inputs);
+            expect(res).toEqual(inputs);
+        });
+    });
+
     describe("outgoing requests", () => {
         /** the RustCrypto implementation under test */
         let rustCrypto: RustCrypto;

src/common-crypto/CryptoBackend.ts

@@ -15,6 +15,7 @@ limitations under the License.
 */
 
 import type { IEventDecryptionResult, IMegolmSessionData } from "../@types/crypto";
+import type { IToDeviceEvent } from "../sync-accumulator";
 import { MatrixEvent } from "../models/event";
 
 /**
@@ -74,6 +75,20 @@ export interface CryptoBackend extends SyncCryptoCallbacks {
 
 /** The methods which crypto implementations should expose to the Sync api */
 export interface SyncCryptoCallbacks {
+    /**
+     * Called by the /sync loop whenever there are incoming to-device messages.
+     *
+     * The implementation may preprocess the received messages (eg, decrypt them) and return an
+     * updated list of messages for dispatch to the rest of the system.
+     *
+     * Note that, unlike {@link ClientEvent.ToDeviceEvent} events, this is called on the raw to-device
+     * messages, rather than the results of any decryption attempts.
+     *
+     * @param events - the received to-device messages
+     * @returns A list of preprocessed to-device messages.
+     */
+    preprocessToDeviceMessages(events: IToDeviceEvent[]): Promise<IToDeviceEvent[]>;
+
     /**
      * Called by the /sync loop after each /sync response is processed.
      *

src/crypto/index.ts

@@ -85,7 +85,7 @@ import { CryptoStore } from "./store/base";
 import { IVerificationChannel } from "./verification/request/Channel";
 import { TypedEventEmitter } from "../models/typed-event-emitter";
 import { IContent } from "../models/event";
-import { ISyncResponse } from "../sync-accumulator";
+import { ISyncResponse, IToDeviceEvent } from "../sync-accumulator";
 import { ISignatures } from "../@types/signed";
 import { IMessage } from "./algorithms/olm";
 import { CryptoBackend, OnSyncCompletedData } from "../common-crypto/CryptoBackend";
@@ -3198,6 +3198,21 @@ export class Crypto extends TypedEventEmitter<CryptoEvent, CryptoEventHandlerMap
         }
     };
 
+    public async preprocessToDeviceMessages(events: IToDeviceEvent[]): Promise<IToDeviceEvent[]> {
+        // all we do here is filter out encrypted to-device messages with the wrong algorithm. Decryption
+        // happens later in decryptEvent, via the EventMapper
+        return events.filter((toDevice) => {
+            if (
+                toDevice.type === EventType.RoomMessageEncrypted &&
+                !["m.olm.v1.curve25519-aes-sha2"].includes(toDevice.content?.algorithm)
+            ) {
+                logger.log("Ignoring invalid encrypted to-device event from " + toDevice.sender);
+                return false;
+            }
+            return true;
+        });
+    }
+
     private onToDeviceEvent = (event: MatrixEvent): void => {
         try {
             logger.log(

src/event-mapper.ts

@@ -60,6 +60,9 @@ export function eventMapperFor(client: MatrixClient, options: MapperOpts): Event
             event.setThread(thread);
         }
 
+        // TODO: once we get rid of the old libolm-backed crypto, we can restrict this to room events (rather than
+        //   to-device events), because the rust implementation decrypts to-device messages at a higher level.
+        //   Generally we probably want to use a different eventMapper implementation for to-device events because
         if (event.isEncrypted()) {
             if (!preventReEmit) {
                 client.reEmitter.reEmit(event, [MatrixEventEvent.Decrypted]);

src/rust-crypto/rust-crypto.ts

@@ -24,6 +24,7 @@ import {
 } from "@matrix-org/matrix-sdk-crypto-js";
 
 import type { IEventDecryptionResult, IMegolmSessionData } from "../@types/crypto";
+import type { IToDeviceEvent } from "../sync-accumulator";
 import { MatrixEvent } from "../models/event";
 import { CryptoBackend, OnSyncCompletedData } from "../common-crypto/CryptoBackend";
 import { logger } from "../logger";
@@ -93,6 +94,25 @@ export class RustCrypto implements CryptoBackend {
     //
     ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////
 
+    /** called by the sync loop to preprocess incoming to-device messages
+     *
+     * @param events - the received to-device messages
+     * @returns A list of preprocessed to-device messages.
+     */
+    public async preprocessToDeviceMessages(events: IToDeviceEvent[]): Promise<IToDeviceEvent[]> {
+        // send the received to-device messages into receiveSyncChanges. We have no info on device-list changes,
+        // one-time-keys, or fallback keys, so just pass empty data.
+        const result = await this.olmMachine.receiveSyncChanges(
+            JSON.stringify(events),
+            new RustSdkCryptoJs.DeviceLists(),
+            new Map(),
+            new Set(),
+        );
+
+        // receiveSyncChanges returns a JSON-encoded list of decrypted to-device messages.
+        return JSON.parse(result);
+    }
+
     /** called by the sync loop after processing each sync.
      *
      * TODO: figure out something equivalent for sliding sync.

src/sliding-sync-sdk.ts

@@ -14,6 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import type { SyncCryptoCallbacks } from "./common-crypto/CryptoBackend";
 import { NotificationCountType, Room, RoomEvent } from "./models/room";
 import { logger } from "./logger";
 import * as utils from "./utils";
@@ -127,7 +128,7 @@ type ExtensionToDeviceResponse = {
 class ExtensionToDevice implements Extension<ExtensionToDeviceRequest, ExtensionToDeviceResponse> {
     private nextBatch: string | null = null;
 
-    public constructor(private readonly client: MatrixClient) {}
+    public constructor(private readonly client: MatrixClient, private readonly cryptoCallbacks?: SyncCryptoCallbacks) {}
 
     public name(): string {
         return "to_device";
@@ -150,6 +151,8 @@ class ExtensionToDevice implements Extension<ExtensionToDeviceRequest, Extension
 
     public async onResponse(data: ExtensionToDeviceResponse): Promise<void> {
         const cancelledKeyVerificationTxns: string[] = [];
+        data["events"] = data["events"] || [];
+        await this.cryptoCallbacks?.preprocessToDeviceMessages(data["events"]);
         data.events
             ?.map(this.client.getEventMapper())
             .map((toDeviceEvent) => {
@@ -373,7 +376,7 @@ export class SlidingSyncSdk {
         this.slidingSync.on(SlidingSyncEvent.Lifecycle, this.onLifecycle.bind(this));
         this.slidingSync.on(SlidingSyncEvent.RoomData, this.onRoomData.bind(this));
         const extensions: Extension<any, any>[] = [
-            new ExtensionToDevice(this.client),
+            new ExtensionToDevice(this.client, this.syncOpts.cryptoCallbacks),
             new ExtensionAccountData(this.client),
             new ExtensionTyping(this.client),
             new ExtensionReceipts(this.client),

src/sync.ts

@@ -48,6 +48,7 @@ import {
     IStrippedState,
     ISyncResponse,
     ITimeline,
+    IToDeviceEvent,
 } from "./sync-accumulator";
 import { MatrixEvent } from "./models/event";
 import { MatrixError, Method } from "./http-api";
@@ -1170,19 +1171,15 @@ export class SyncApi {
         }
 
         // handle to-device events
-        if (Array.isArray(data.to_device?.events) && data.to_device!.events.length > 0) {
-            const cancelledKeyVerificationTxns: string[] = [];
-            data.to_device!.events.filter((eventJSON) => {
-                if (
-                    eventJSON.type === EventType.RoomMessageEncrypted &&
-                    !["m.olm.v1.curve25519-aes-sha2"].includes(eventJSON.content?.algorithm)
-                ) {
-                    logger.log("Ignoring invalid encrypted to-device event from " + eventJSON.sender);
-                    return false;
-                }
+        if (data.to_device && Array.isArray(data.to_device.events) && data.to_device.events.length > 0) {
+            let toDeviceMessages: IToDeviceEvent[] = data.to_device.events;
 
-                return true;
-            })
+            if (this.syncOpts.cryptoCallbacks) {
+                toDeviceMessages = await this.syncOpts.cryptoCallbacks.preprocessToDeviceMessages(toDeviceMessages);
+            }
+
+            const cancelledKeyVerificationTxns: string[] = [];
+            toDeviceMessages
                 .map(client.getEventMapper({ toDevice: true }))
                 .map((toDeviceEvent) => {
                     // map is a cheap inline forEach