crypto: Add new ClientEvent.ReceivedToDeviceMessage with proper OlmEncryptionInfo support (#4891)

* crypto: Add new ClientEvent.ReceivedToDeviceMessage

refactor rename ProcessedToDeviceEvent to ReceivedToDeviceEvent

* fix: Restore legacy isEncrypted() for to-device messages

* Update test for new preprocessToDeviceMessages API

* quick fix on doc

* quick update docs and renaming

* review: Better doc and names for OlmEncryptionInfo

* review: Remove IToDeviceMessage alias and only keep IToDeviceEvent

* review: improve comments of processToDeviceMessages

* review: pass up encrypted event when no crypto callbacks

* review: use single payload for ReceivedToDeviceMessage

* fix linter

* review: minor comment update

spec/integ/crypto/to-device-messages.spec.ts

@@ -17,13 +17,22 @@ limitations under the License.
 import fetchMock from "fetch-mock-jest";
 import "fake-indexeddb/auto";
 import { IDBFactory } from "fake-indexeddb";
+import Olm from "@matrix-org/olm";
 
 import { getSyncResponse, syncPromise } from "../../test-utils/test-utils";
-import { createClient, type MatrixClient } from "../../../src";
+import {
+    ClientEvent,
+    createClient,
+    type IToDeviceEvent,
+    type MatrixClient,
+    type MatrixEvent,
+    type ReceivedToDeviceMessage,
+} from "../../../src";
 import * as testData from "../../test-utils/test-data";
 import { E2EKeyResponder } from "../../test-utils/E2EKeyResponder";
 import { SyncResponder } from "../../test-utils/SyncResponder";
 import { E2EKeyReceiver } from "../../test-utils/E2EKeyReceiver";
+import { encryptOlmEvent, establishOlmSession, getTestOlmAccountKeys } from "./olm-utils.ts";
 
 afterEach(() => {
     // reset fake-indexeddb after each test, to make sure we don't leak connections
@@ -43,6 +52,8 @@ describe("to-device-messages", () => {
 
     /** an object which intercepts `/keys/query` requests on the test homeserver */
     let e2eKeyResponder: E2EKeyResponder;
+    let e2eKeyReceiver: E2EKeyReceiver;
+    let syncResponder: SyncResponder;
 
     beforeEach(
         async () => {
@@ -59,8 +70,8 @@ describe("to-device-messages", () => {
             });
 
             e2eKeyResponder = new E2EKeyResponder(homeserverUrl);
-            new E2EKeyReceiver(homeserverUrl);
-            const syncResponder = new SyncResponder(homeserverUrl);
+            e2eKeyReceiver = new E2EKeyReceiver(homeserverUrl);
+            syncResponder = new SyncResponder(homeserverUrl);
 
             // add bob as known user
             syncResponder.sendOrQueueSyncResponse(getSyncResponse([testData.BOB_TEST_USER_ID]));
@@ -149,4 +160,111 @@ describe("to-device-messages", () => {
             // for future: check that bob's device can decrypt the ciphertext?
         });
     });
+
+    describe("receive to-device-messages", () => {
+        it("Should receive decrypted to-device message via ClientEvent", async () => {
+            // create a test olm device which we will use to communicate with alice. We use libolm to implement this.
+            await Olm.init();
+            const testOlmAccount = new Olm.Account();
+            testOlmAccount.create();
+
+            const testDeviceKeys = getTestOlmAccountKeys(testOlmAccount, "@bob:xyz", "DEVICE_ID");
+            e2eKeyResponder.addDeviceKeys(testDeviceKeys);
+
+            await aliceClient.startClient();
+            await syncPromise(aliceClient);
+
+            syncResponder.sendOrQueueSyncResponse(getSyncResponse(["@bob:xyz"]));
+            await syncPromise(aliceClient);
+
+            const p2pSession = await establishOlmSession(aliceClient, e2eKeyReceiver, syncResponder, testOlmAccount);
+
+            const toDeviceEvent = encryptOlmEvent({
+                sender: "@bob:xyz",
+                senderKey: testDeviceKeys.keys[`curve25519:DEVICE_ID`],
+                senderSigningKey: testDeviceKeys.keys[`ed25519:DEVICE_ID`],
+                p2pSession: p2pSession,
+                recipient: aliceClient.getUserId()!,
+                recipientCurve25519Key: e2eKeyReceiver.getDeviceKey(),
+                recipientEd25519Key: e2eKeyReceiver.getSigningKey(),
+                plaincontent: {
+                    body: "foo",
+                },
+                plaintype: "m.test.type",
+            });
+
+            const processedToDeviceResolver: PromiseWithResolvers<ReceivedToDeviceMessage> = Promise.withResolvers();
+
+            aliceClient.on(ClientEvent.ReceivedToDeviceMessage, (payload) => {
+                processedToDeviceResolver.resolve(payload);
+            });
+
+            const oldToDeviceResolver: PromiseWithResolvers<MatrixEvent> = Promise.withResolvers();
+
+            aliceClient.on(ClientEvent.ToDeviceEvent, (event) => {
+                oldToDeviceResolver.resolve(event);
+            });
+
+            expect(toDeviceEvent.type).toBe("m.room.encrypted");
+
+            syncResponder.sendOrQueueSyncResponse({ to_device: { events: [toDeviceEvent] } });
+            await syncPromise(aliceClient);
+
+            const { message, encryptionInfo } = await processedToDeviceResolver.promise;
+
+            expect(message.type).toBe("m.test.type");
+            expect(message.content["body"]).toBe("foo");
+
+            expect(encryptionInfo).not.toBeNull();
+            expect(encryptionInfo!.senderVerified).toBe(false);
+            expect(encryptionInfo!.sender).toBe("@bob:xyz");
+            expect(encryptionInfo!.senderDevice).toBe("DEVICE_ID");
+
+            const oldFormat = await oldToDeviceResolver.promise;
+            expect(oldFormat.isEncrypted()).toBe(true);
+            expect(oldFormat.getType()).toBe("m.test.type");
+            expect(oldFormat.getContent()["body"]).toBe("foo");
+        });
+
+        it("Should receive clear to-device message via ClientEvent", async () => {
+            await aliceClient.startClient();
+            await syncPromise(aliceClient);
+
+            const toDeviceEvent: IToDeviceEvent = {
+                sender: "@bob:xyz",
+                type: "m.test.type",
+                content: {
+                    body: "foo",
+                },
+            };
+
+            const processedToDeviceResolver: PromiseWithResolvers<ReceivedToDeviceMessage> = Promise.withResolvers();
+
+            aliceClient.on(ClientEvent.ReceivedToDeviceMessage, (payload) => {
+                processedToDeviceResolver.resolve(payload);
+            });
+
+            const oldToDeviceResolver: PromiseWithResolvers<MatrixEvent> = Promise.withResolvers();
+
+            aliceClient.on(ClientEvent.ToDeviceEvent, (event) => {
+                oldToDeviceResolver.resolve(event);
+            });
+
+            syncResponder.sendOrQueueSyncResponse({ to_device: { events: [toDeviceEvent] } });
+            await syncPromise(aliceClient);
+
+            const { message, encryptionInfo } = await processedToDeviceResolver.promise;
+
+            expect(message.type).toBe("m.test.type");
+            expect(message.content["body"]).toBe("foo");
+
+            // When the message is not encrypted, we don't have the encryptionInfo.
+            expect(encryptionInfo).toBeNull();
+
+            const oldFormat = await oldToDeviceResolver.promise;
+            expect(oldFormat.isEncrypted()).toBe(false);
+            expect(oldFormat.getType()).toBe("m.test.type");
+            expect(oldFormat.getContent()["body"]).toBe("foo");
+        });
+    });
 });

spec/unit/rust-crypto/rust-crypto.spec.ts

@@ -551,11 +551,11 @@ describe("RustCrypto", () => {
             const inputs: IToDeviceEvent[] = [
                 { content: { key: "value" }, type: "org.matrix.test", sender: "@alice:example.com" },
             ];
-            const res = await rustCrypto.preprocessToDeviceMessages(inputs);
+            const res = (await rustCrypto.preprocessToDeviceMessages(inputs)).map((p) => p.message);
             expect(res).toEqual(inputs);
         });
 
-        it("should pass through bad encrypted messages", async () => {
+        it("should fail to process bad encrypted messages", async () => {
             const olmMachine: OlmMachine = rustCrypto["olmMachine"];
             const keys = olmMachine.identityKeys;
             const inputs: IToDeviceEvent[] = [
@@ -576,7 +576,7 @@ describe("RustCrypto", () => {
             ];
 
             const res = await rustCrypto.preprocessToDeviceMessages(inputs);
-            expect(res).toEqual(inputs);
+            expect(res.length).toEqual(0);
         });
 
         it("emits VerificationRequestReceived on incoming m.key.verification.request", async () => {

src/client.ts

@@ -87,7 +87,12 @@ import { type IIdentityServerProvider } from "./@types/IIdentityServerProvider.t
 import { type MatrixScheduler } from "./scheduler.ts";
 import { type BeaconEvent, type BeaconEventHandlerMap } from "./models/beacon.ts";
 import { type AuthDict } from "./interactive-auth.ts";
-import { type IMinimalEvent, type IRoomEvent, type IStateEvent } from "./sync-accumulator.ts";
+import {
+    type IMinimalEvent,
+    type IRoomEvent,
+    type IStateEvent,
+    type ReceivedToDeviceMessage,
+} from "./sync-accumulator.ts";
 import type { EventTimelineSet } from "./models/event-timeline-set.ts";
 import * as ContentHelpers from "./content-helpers.ts";
 import {
@@ -885,7 +890,9 @@ const EVENT_ID_PREFIX = "$";
 export enum ClientEvent {
     Sync = "sync",
     Event = "event",
+    /** @deprecated Use {@link ReceivedToDeviceMessage}. */
     ToDeviceEvent = "toDeviceEvent",
+    ReceivedToDeviceMessage = "receivedToDeviceMessage",
     AccountData = "accountData",
     Room = "Room",
     DeleteRoom = "deleteRoom",
@@ -1088,6 +1095,20 @@ export type ClientEventHandlerMap = {
      * ```
      */
     [ClientEvent.ToDeviceEvent]: (event: MatrixEvent) => void;
+    /**
+     * Fires whenever the SDK receives a new to-device message.
+     * @param payload - The message and encryptionInfo for this message (See {@link ReceivedToDeviceMessage}) which caused this event to fire.
+     * @example
+     * ```
+     * matrixClient.on("receivedToDeviceMessage", function(payload){
+     *   const { message, encryptionInfo } = payload;
+     *   var claimed_sender = encryptionInfo ? encryptionInfo.sender : message.sender;
+     *   var isVerified = encryptionInfo ? encryptionInfo.verified : false;
+     *   var type = message.type;
+     * });
+     * ```
+     */
+    [ClientEvent.ReceivedToDeviceMessage]: (payload: ReceivedToDeviceMessage) => void;
     /**
      * Fires if a to-device event is received that cannot be decrypted.
      * Encrypted to-device events will (generally) use plain Olm encryption,

src/common-crypto/CryptoBackend.ts

@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import type { IDeviceLists, IToDeviceEvent } from "../sync-accumulator.ts";
+import type { IDeviceLists, IToDeviceEvent, ReceivedToDeviceMessage } from "../sync-accumulator.ts";
 import { type IClearEvent, type MatrixEvent } from "../models/event.ts";
 import { type Room } from "../models/room.ts";
 import { type CryptoApi, type DecryptionFailureCode, type ImportRoomKeysOpts } from "../crypto-api/index.ts";
@@ -96,9 +96,11 @@ export interface SyncCryptoCallbacks {
      * messages, rather than the results of any decryption attempts.
      *
      * @param events - the received to-device messages
-     * @returns A list of preprocessed to-device messages.
+     * @returns A list of preprocessed to-device messages. This will not map 1:1 to the input list, as some messages may be invalid or
+     * failed to decrypt, and so will be omitted from the output list.
+     *
      */
-    preprocessToDeviceMessages(events: IToDeviceEvent[]): Promise<IToDeviceEvent[]>;
+    preprocessToDeviceMessages(events: IToDeviceEvent[]): Promise<ReceivedToDeviceMessage[]>;
 
     /**
      * Called by the /sync loop when one time key counts and unused fallback key details are received.

src/crypto-api/index.ts

@@ -1365,6 +1365,27 @@ export interface OwnDeviceKeys {
     curve25519: string;
 }
 
+/**
+ * Information about the encryption of a successfully decrypted to-device message.
+ */
+export interface OlmEncryptionInfo {
+    /** The user ID of the event sender, note this is untrusted data unless `isVerified` is true **/
+    sender: string;
+    /**
+     * The device ID of the device that sent us the event.
+     * Note this is untrusted data unless {@link senderVerified} is true.
+     * If the device ID is not known, this will be `null`.
+     **/
+    senderDevice?: string;
+    /** The sender device's public Curve25519 key, base64 encoded **/
+    senderCurve25519KeyBase64: string;
+    /**
+     *  If true, this message is guaranteed to be authentic as it is coming from a device belonging to a user that we have verified.
+     *  This is the state at the time of decryption (the user could be verified later).
+     */
+    senderVerified: boolean;
+}
+
 export * from "./verification.ts";
 export type * from "./keybackup.ts";
 export * from "./recovery-key.ts";

src/rust-crypto/rust-crypto.ts

@@ -19,7 +19,7 @@ import * as RustSdkCryptoJs from "@matrix-org/matrix-sdk-crypto-wasm";
 
 import type { IEventDecryptionResult, IMegolmSessionData } from "../@types/crypto.ts";
 import { KnownMembership } from "../@types/membership.ts";
-import type { IDeviceLists, IToDeviceEvent } from "../sync-accumulator.ts";
+import { type IDeviceLists, type IToDeviceEvent, type ReceivedToDeviceMessage } from "../sync-accumulator.ts";
 import type { ToDevicePayload, ToDeviceBatch } from "../models/ToDeviceMessage.ts";
 import { type MatrixEvent, MatrixEventEvent } from "../models/event.ts";
 import { type Room } from "../models/room.ts";
@@ -1481,7 +1481,7 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, CryptoEventH
      * @param oneTimeKeysCounts - the received one time key counts
      * @param unusedFallbackKeys - the received unused fallback keys
      * @param devices - the received device list updates
-     * @returns A list of preprocessed to-device messages.
+     * @returns A list of processed to-device messages.
      */
     private async receiveSyncChanges({
         events,
@@ -1493,15 +1493,13 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, CryptoEventH
         oneTimeKeysCounts?: Map<string, number>;
         unusedFallbackKeys?: Set<string>;
         devices?: RustSdkCryptoJs.DeviceLists;
-    }): Promise<IToDeviceEvent[]> {
-        const result = await this.olmMachine.receiveSyncChanges(
+    }): Promise<RustSdkCryptoJs.ProcessedToDeviceEvent[]> {
+        return await this.olmMachine.receiveSyncChanges(
             events ? JSON.stringify(events) : "[]",
             devices,
             oneTimeKeysCounts,
             unusedFallbackKeys,
         );
-
-        return result.map((processed) => JSON.parse(processed.rawEvent));
     }
 
     /** called by the sync loop to preprocess incoming to-device messages
@@ -1509,22 +1507,56 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, CryptoEventH
      * @param events - the received to-device messages
      * @returns A list of preprocessed to-device messages.
      */
-    public async preprocessToDeviceMessages(events: IToDeviceEvent[]): Promise<IToDeviceEvent[]> {
+    public async preprocessToDeviceMessages(events: IToDeviceEvent[]): Promise<ReceivedToDeviceMessage[]> {
         // 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 processed = await this.receiveSyncChanges({ events });
 
-        // look for interesting to-device messages
+        const received: ReceivedToDeviceMessage[] = [];
+
         for (const message of processed) {
-            if (message.type === EventType.KeyVerificationRequest) {
-                const sender = message.sender;
-                const transactionId = message.content.transaction_id;
+            const parsedMessage: IToDeviceEvent = JSON.parse(message.rawEvent);
+
+            // look for interesting to-device messages
+            if (parsedMessage.type === EventType.KeyVerificationRequest) {
+                const sender = parsedMessage.sender;
+                const transactionId = parsedMessage.content.transaction_id;
                 if (transactionId && sender) {
                     this.onIncomingKeyVerificationRequest(sender, transactionId);
                 }
             }
+
+            switch (message.type) {
+                case RustSdkCryptoJs.ProcessedToDeviceEventType.Decrypted: {
+                    const encryptionInfo = (message as RustSdkCryptoJs.DecryptedToDeviceEvent).encryptionInfo;
+                    received.push({
+                        message: parsedMessage,
+                        encryptionInfo: {
+                            sender: encryptionInfo.sender.toString(),
+                            senderDevice: encryptionInfo.senderDevice?.toString(),
+                            senderCurve25519KeyBase64: encryptionInfo.senderCurve25519Key,
+                            senderVerified: encryptionInfo.isSenderVerified(),
+                        },
+                    });
+                    break;
+                }
+                case RustSdkCryptoJs.ProcessedToDeviceEventType.PlainText: {
+                    received.push({
+                        message: parsedMessage,
+                        encryptionInfo: null,
+                    });
+                    break;
+                }
+                case RustSdkCryptoJs.ProcessedToDeviceEventType.UnableToDecrypt:
+                    // ignore messages we cannot decrypt
+                    break;
+                case RustSdkCryptoJs.ProcessedToDeviceEventType.Invalid:
+                    // ignore invalid messages
+                    break;
+            }
         }
-        return processed;
+
+        return received;
     }
 
     /** called by the sync loop to process one time key counts and unused fallback keys

src/sliding-sync-sdk.ts

@@ -37,6 +37,7 @@ import {
     type IStateEvent,
     type IStrippedState,
     type ISyncResponse,
+    type ReceivedToDeviceMessage,
 } from "./sync-accumulator.ts";
 import { MatrixError } from "./http-api/index.ts";
 import {
@@ -150,11 +151,20 @@ class ExtensionToDevice implements Extension<ExtensionToDeviceRequest, Extension
     }
 
     public async onResponse(data: ExtensionToDeviceResponse): Promise<void> {
-        let events = data["events"] || [];
-        if (events.length > 0 && this.cryptoCallbacks) {
-            events = await this.cryptoCallbacks.preprocessToDeviceMessages(events);
+        const events = data["events"] || [];
+        let receivedToDeviceMessages: ReceivedToDeviceMessage[];
+        if (this.cryptoCallbacks) {
+            receivedToDeviceMessages = await this.cryptoCallbacks.preprocessToDeviceMessages(events);
+        } else {
+            receivedToDeviceMessages = events.map((rawEvent) =>
+                // Crypto is not enabled, so we just return the events.
+                ({
+                    message: rawEvent,
+                    encryptionInfo: null,
+                }),
+            );
         }
-        processToDeviceMessages(events, this.client);
+        processToDeviceMessages(receivedToDeviceMessages, this.client);
 
         this.nextBatch = data.next_batch;
     }

src/sync-accumulator.ts

@@ -25,6 +25,7 @@ import { type IRoomSummary } from "./models/room-summary.ts";
 import { type EventType } from "./@types/event.ts";
 import { UNREAD_THREAD_NOTIFICATIONS } from "./@types/sync.ts";
 import { ReceiptAccumulator } from "./receipt-accumulator.ts";
+import { type OlmEncryptionInfo } from "./crypto-api/index.ts";
 
 interface IOpts {
     /**
@@ -134,12 +135,31 @@ interface IAccountData {
     events: IMinimalEvent[];
 }
 
+/** A to-device message as received from the sync. */
 export interface IToDeviceEvent {
     content: IContent;
     sender: string;
     type: string;
 }
 
+/**
+ * A (possibly decrypted) to-device message after it has been successfully processed by the sdk.
+ *
+ * If the message was encrypted, the `encryptionInfo` field will contain the encryption information.
+ * If the message was sent in clear, this field will be null.
+ *
+ * The `message` field contains the message `type`, `content`, and `sender` as if the message was sent in clear.
+ */
+export interface ReceivedToDeviceMessage {
+    /** The message type, content, and sender as if the message was sent in clear. */
+    message: IToDeviceEvent;
+    /**
+     * Information about the encryption of the message.
+     * Will be null if the message was sent in clear
+     */
+    encryptionInfo: OlmEncryptionInfo | null;
+}
+
 interface IToDevice {
     events: IToDeviceEvent[];
 }

src/sync.ts

@@ -44,8 +44,8 @@ import {
     type IInvitedRoom,
     type IInviteState,
     type IJoinedRoom,
-    type ILeftRoom,
     type IKnockedRoom,
+    type ILeftRoom,
     type IMinimalEvent,
     type IRoomEvent,
     type IStateEvent,
@@ -53,13 +53,14 @@ import {
     type ISyncResponse,
     type ITimeline,
     type IToDeviceEvent,
+    type ReceivedToDeviceMessage,
 } from "./sync-accumulator.ts";
-import { MatrixEvent, type IEvent } from "./models/event.ts";
+import { MatrixEvent } from "./models/event.ts";
 import { type MatrixError, Method } from "./http-api/index.ts";
 import { type ISavedSync } from "./store/index.ts";
 import { EventType } from "./@types/event.ts";
 import { type IPushRules } from "./@types/PushRules.ts";
-import { RoomStateEvent, type IMarkerFoundOptions } from "./models/room-state.ts";
+import { type IMarkerFoundOptions, RoomStateEvent } from "./models/room-state.ts";
 import { RoomMemberEvent } from "./models/room-member.ts";
 import { BeaconEvent } from "./models/beacon.ts";
 import { type IEventsResponse } from "./@types/requests.ts";
@@ -1144,13 +1145,23 @@ export class SyncApi {
 
         // handle to-device events
         if (data.to_device && Array.isArray(data.to_device.events) && data.to_device.events.length > 0) {
-            let toDeviceMessages: IToDeviceEvent[] = data.to_device.events.filter(noUnsafeEventProps);
+            const toDeviceMessages: IToDeviceEvent[] = data.to_device.events.filter(noUnsafeEventProps);
 
+            let receivedToDeviceMessages: ReceivedToDeviceMessage[];
             if (this.syncOpts.cryptoCallbacks) {
-                toDeviceMessages = await this.syncOpts.cryptoCallbacks.preprocessToDeviceMessages(toDeviceMessages);
+                receivedToDeviceMessages =
+                    await this.syncOpts.cryptoCallbacks.preprocessToDeviceMessages(toDeviceMessages);
+            } else {
+                receivedToDeviceMessages = toDeviceMessages.map((rawEvent) =>
+                    // Crypto is not enabled, so we just return the events.
+                    ({
+                        message: rawEvent,
+                        encryptionInfo: null,
+                    }),
+                );
             }
 
-            processToDeviceMessages(toDeviceMessages, client);
+            processToDeviceMessages(receivedToDeviceMessages, client);
         } else {
             // no more to-device events: we can stop polling with a short timeout.
             this.catchingUp = false;
@@ -1909,21 +1920,21 @@ export function _createAndReEmitRoom(client: MatrixClient, roomId: string, opts:
 /**
  * Process a list of (decrypted, where possible) received to-device events.
  *
- * Converts the events into `MatrixEvent`s, and emits appropriate {@link ClientEvent.ToDeviceEvent} events.
+ * Emits the appropriate {@link ClientEvent.ReceivedToDeviceMessage} event.
+ * Also converts the events into `MatrixEvent`s, and emits the now deprecated {@link ClientEvent.ToDeviceEvent} events for compatibility.
  * */
-export function processToDeviceMessages(toDeviceMessages: IToDeviceEvent[], client: MatrixClient): void {
+export function processToDeviceMessages(toDeviceMessages: ReceivedToDeviceMessage[], client: MatrixClient): void {
     const cancelledKeyVerificationTxns: string[] = [];
     toDeviceMessages
-        .map(mapToDeviceEvent)
-        .map((toDeviceEvent) => {
+        .map((processedMessage) => {
             // map is a cheap inline forEach
             // We want to flag m.key.verification.start events as cancelled
             // if there's an accompanying m.key.verification.cancel event, so
             // we pull out the transaction IDs from the cancellation events
             // so we can flag the verification events as cancelled in the loop
             // below.
-            if (toDeviceEvent.getType() === "m.key.verification.cancel") {
-                const txnId: string = toDeviceEvent.getContent()["transaction_id"];
+            if (processedMessage.message.type === "m.key.verification.cancel") {
+                const txnId: string = processedMessage.message.content["transaction_id"];
                 if (txnId) {
                     cancelledKeyVerificationTxns.push(txnId);
                 }
@@ -1931,32 +1942,40 @@ export function processToDeviceMessages(toDeviceMessages: IToDeviceEvent[], clie
 
             // as mentioned above, .map is a cheap inline forEach, so return
             // the unmodified event.
-            return toDeviceEvent;
+            return processedMessage;
         })
-        .forEach(function (toDeviceEvent) {
-            const content = toDeviceEvent.getContent();
-            if (toDeviceEvent.getType() == "m.room.message" && content.msgtype == "m.bad.encrypted") {
-                // the mapper already logged a warning.
-                logger.log("Ignoring undecryptable to-device event from " + toDeviceEvent.getSender());
-                return;
-            }
-
-            if (
-                toDeviceEvent.getType() === "m.key.verification.start" ||
-                toDeviceEvent.getType() === "m.key.verification.request"
-            ) {
-                const txnId = content["transaction_id"];
-                if (cancelledKeyVerificationTxns.includes(txnId)) {
-                    toDeviceEvent.flagCancelled();
+        .forEach(function (processedEvent) {
+            // For backwards compatibility, we also emit the event as a `MatrixEvent` using `ClientEvent.ToDeviceEvent`.
+            {
+                const toDeviceEvent = processedEvent.message;
+                const content = toDeviceEvent.content;
+                // The message is cloned before being passed to the MatrixEvent constructor, because
+                // the `makeEncrypted` method will mutate the type and content properties of the original message and will interfere
+                // with the emitted event for `ReceivedToDeviceMessage`.
+                const deprecatedCompatibilityEvent = new MatrixEvent(Object.assign({}, toDeviceEvent));
+                if (
+                    toDeviceEvent.type === "m.key.verification.start" ||
+                    toDeviceEvent.type === "m.key.verification.request"
+                ) {
+                    const txnId = content["transaction_id"];
+                    if (cancelledKeyVerificationTxns.includes(txnId)) {
+                        deprecatedCompatibilityEvent.flagCancelled();
+                    }
                 }
+                if (processedEvent.encryptionInfo) {
+                    // Restore partially the legacy behavior to detect encrypted messages.
+                    // Now `event.isEncrypted()` will return true.
+                    deprecatedCompatibilityEvent.makeEncrypted(
+                        EventType.RoomMessageEncrypted,
+                        { ciphertext: "" },
+                        processedEvent.encryptionInfo.senderCurve25519KeyBase64,
+                        "",
+                    );
+                }
+
+                client.emit(ClientEvent.ToDeviceEvent, deprecatedCompatibilityEvent);
             }
 
-            client.emit(ClientEvent.ToDeviceEvent, toDeviceEvent);
+            client.emit(ClientEvent.ReceivedToDeviceMessage, processedEvent);
         });
 }
-
-function mapToDeviceEvent(plainOldJsObject: Partial<IEvent>): MatrixEvent {
-    // to-device events should not have a `room_id` property, but let's be sure
-    delete plainOldJsObject.room_id;
-    return new MatrixEvent(plainOldJsObject);
-}