ElementR: fix emoji verification stalling when both ends hit start at the same time (#4004)

* Rust crypto: handle the SAS verifier being replaced

* lint

* make changes from review

* apply changes from code review

* remove useless assertions

* wrap acceptance inside a try-catch, and factor out acceptance into a function

* fix bugs

* we don't actually need the .accept variable

* move setInner to inside SAS class, and rename to replaceInner

* use defer to avoid using a closure

* lint

* prettier

* use the right name

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* combine onChangeCallback with onChange

* apply changes from review

* add test for QR code verification, and try changing order in onChange

* lint

---------

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

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

@@ -17,8 +17,19 @@ limitations under the License.
 import * as RustSdkCryptoJs from "@matrix-org/matrix-sdk-crypto-wasm";
 import { Mocked } from "jest-mock";
 
-import { isVerificationEvent, RustVerificationRequest } from "../../../src/rust-crypto/verification";
+import {
-import { OutgoingRequestProcessor } from "../../../src/rust-crypto/OutgoingRequestProcessor";
+    isVerificationEvent,
+    RustVerificationRequest,
+    verificationMethodIdentifierToMethod,
+} from "../../../src/rust-crypto/verification";
+import {
+    ShowSasCallbacks,
+    VerificationRequestEvent,
+    Verifier,
+    VerifierEvent,
+} from "../../../src/crypto-api/verification";
+import { OutgoingRequest, OutgoingRequestProcessor } from "../../../src/rust-crypto/OutgoingRequestProcessor";
+import { IDeviceKeys } from "../../../src/@types/crypto";
 import { EventType, MatrixEvent, MsgType } from "../../../src";
 
 describe("VerificationRequest", () => {
@@ -91,6 +102,354 @@ describe("VerificationRequest", () => {
             );
         });
     });
+
+    it("can verify with SAS", async () => {
+        const aliceUserId = "@alice:example.org";
+        const aliceDeviceId = "ABCDEFG";
+        const bobUserId = "@bob:example.org";
+        const bobDeviceId = "HIJKLMN";
+        const [aliceOlmMachine, aliceDeviceKeys, aliceCrossSigningKeys] = await initOlmMachineAndKeys(
+            aliceUserId,
+            aliceDeviceId,
+        );
+        const [bobOlmMachine, bobDeviceKeys, bobCrossSigningKeys] = await initOlmMachineAndKeys(bobUserId, bobDeviceId);
+
+        const aliceRequestLoop = makeRequestLoop(
+            aliceOlmMachine,
+            aliceDeviceKeys,
+            aliceCrossSigningKeys,
+            bobOlmMachine,
+            bobDeviceKeys,
+            bobCrossSigningKeys,
+        );
+        const bobRequestLoop = makeRequestLoop(
+            bobOlmMachine,
+            bobDeviceKeys,
+            bobCrossSigningKeys,
+            aliceOlmMachine,
+            aliceDeviceKeys,
+            aliceCrossSigningKeys,
+        );
+
+        try {
+            await aliceOlmMachine.updateTrackedUsers([new RustSdkCryptoJs.UserId(bobUserId)]);
+            await bobOlmMachine.updateTrackedUsers([new RustSdkCryptoJs.UserId(aliceUserId)]);
+
+            // Alice requests verification
+            const bobUserIdentity = await aliceOlmMachine.getIdentity(new RustSdkCryptoJs.UserId(bobUserId));
+
+            const roomId = new RustSdkCryptoJs.RoomId("!roomId:example.org");
+            const methods = [verificationMethodIdentifierToMethod("m.sas.v1")];
+            const innerVerificationRequest = await bobUserIdentity.requestVerification(
+                roomId,
+                new RustSdkCryptoJs.EventId("$m.key.verification.request"),
+                methods,
+            );
+            const aliceVerificationRequest = new RustVerificationRequest(
+                aliceOlmMachine,
+                innerVerificationRequest,
+                aliceRequestLoop as unknown as OutgoingRequestProcessor,
+                ["m.sas.v1"],
+            );
+
+            const verificationRequestContent = JSON.parse(await bobUserIdentity.verificationRequestContent(methods));
+            await bobOlmMachine.receiveVerificationEvent(
+                JSON.stringify({
+                    type: "m.room.message",
+                    sender: aliceUserId,
+                    event_id: "$m.key.verification.request",
+                    content: verificationRequestContent,
+                    origin_server_ts: Date.now(),
+                    unsigned: {
+                        age: 0,
+                    },
+                }),
+                roomId,
+            );
+
+            // Bob accepts
+            const bobInnerVerificationRequest = bobOlmMachine.getVerificationRequest(
+                new RustSdkCryptoJs.UserId(aliceUserId),
+                "$m.key.verification.request",
+            )!;
+            const bobVerificationRequest = new RustVerificationRequest(
+                bobOlmMachine,
+                bobInnerVerificationRequest,
+                bobRequestLoop as unknown as OutgoingRequestProcessor,
+                ["m.sas.v1"],
+            );
+
+            await bobVerificationRequest.accept();
+
+            // Alice starts the verification
+            const bobVerifierPromise: Promise<Verifier> = new Promise((resolve, reject) => {
+                bobVerificationRequest.on(VerificationRequestEvent.Change, () => {
+                    const verifier = bobVerificationRequest.verifier;
+                    if (verifier) {
+                        resolve(verifier);
+                    }
+                });
+            });
+            const aliceVerifier = await aliceVerificationRequest.startVerification("m.sas.v1");
+            const bobVerifier = await bobVerifierPromise;
+
+            // create a function to compare the SAS, and then let the verification run
+            let otherCallbacks: ShowSasCallbacks | undefined;
+            const compareSas = (callbacks: ShowSasCallbacks): void => {
+                if (otherCallbacks) {
+                    const ourDecimal = callbacks.sas.decimal!;
+                    const theirDecimal = otherCallbacks.sas.decimal!;
+                    if (ourDecimal.every((el, idx) => el == theirDecimal[idx])) {
+                        otherCallbacks.confirm();
+                        callbacks.confirm();
+                    } else {
+                        otherCallbacks.mismatch();
+                        callbacks.mismatch();
+                    }
+                } else {
+                    otherCallbacks = callbacks;
+                }
+            };
+            aliceVerifier.on(VerifierEvent.ShowSas, compareSas);
+            bobVerifier.on(VerifierEvent.ShowSas, compareSas);
+
+            await Promise.all([aliceVerifier.verify(), await bobVerifier.verify()]);
+        } finally {
+            await aliceRequestLoop.stop();
+            await bobRequestLoop.stop();
+        }
+    });
+
+    it("can handle simultaneous starts in SAS", async () => {
+        const aliceUserId = "@alice:example.org";
+        const aliceDeviceId = "ABCDEFG";
+        const bobUserId = "@bob:example.org";
+        const bobDeviceId = "HIJKLMN";
+        const [aliceOlmMachine, aliceDeviceKeys, aliceCrossSigningKeys] = await initOlmMachineAndKeys(
+            aliceUserId,
+            aliceDeviceId,
+        );
+        const [bobOlmMachine, bobDeviceKeys, bobCrossSigningKeys] = await initOlmMachineAndKeys(bobUserId, bobDeviceId);
+
+        let aliceStartRequest: RustSdkCryptoJs.RoomMessageRequest | undefined;
+        const aliceRequestLoop = makeRequestLoop(
+            aliceOlmMachine,
+            aliceDeviceKeys,
+            aliceCrossSigningKeys,
+            bobOlmMachine,
+            bobDeviceKeys,
+            bobCrossSigningKeys,
+            async (request): Promise<any> => {
+                // If the request is sending the m.key.verification.start
+                // event, we delay sending it until after Bob has also started
+                // a verification
+                if (
+                    !aliceStartRequest &&
+                    request instanceof RustSdkCryptoJs.RoomMessageRequest &&
+                    request.event_type == "m.key.verification.start"
+                ) {
+                    aliceStartRequest = request;
+                    return { event_id: "$m.key.verification.start" };
+                }
+            },
+        );
+        const bobRequestLoop = makeRequestLoop(
+            bobOlmMachine,
+            bobDeviceKeys,
+            bobCrossSigningKeys,
+            aliceOlmMachine,
+            aliceDeviceKeys,
+            aliceCrossSigningKeys,
+        );
+
+        try {
+            await aliceOlmMachine.updateTrackedUsers([new RustSdkCryptoJs.UserId(bobUserId)]);
+            await bobOlmMachine.updateTrackedUsers([new RustSdkCryptoJs.UserId(aliceUserId)]);
+
+            // Alice requests verification
+            const bobUserIdentity = await aliceOlmMachine.getIdentity(new RustSdkCryptoJs.UserId(bobUserId));
+
+            const roomId = new RustSdkCryptoJs.RoomId("!roomId:example.org");
+            const methods = [verificationMethodIdentifierToMethod("m.sas.v1")];
+            const innerVerificationRequest = await bobUserIdentity.requestVerification(
+                roomId,
+                new RustSdkCryptoJs.EventId("$m.key.verification.request"),
+                methods,
+            );
+            const aliceVerificationRequest = new RustVerificationRequest(
+                aliceOlmMachine,
+                innerVerificationRequest,
+                aliceRequestLoop as unknown as OutgoingRequestProcessor,
+                ["m.sas.v1"],
+            );
+
+            const verificationRequestContent = JSON.parse(await bobUserIdentity.verificationRequestContent(methods));
+            await bobOlmMachine.receiveVerificationEvent(
+                JSON.stringify({
+                    type: "m.room.message",
+                    sender: aliceUserId,
+                    event_id: "$m.key.verification.request",
+                    content: verificationRequestContent,
+                    origin_server_ts: Date.now(),
+                    unsigned: {
+                        age: 0,
+                    },
+                }),
+                roomId,
+            );
+
+            // Bob accepts
+            const bobInnerVerificationRequest = bobOlmMachine.getVerificationRequest(
+                new RustSdkCryptoJs.UserId(aliceUserId),
+                "$m.key.verification.request",
+            )!;
+            const bobVerificationRequest = new RustVerificationRequest(
+                bobOlmMachine,
+                bobInnerVerificationRequest,
+                bobRequestLoop as unknown as OutgoingRequestProcessor,
+                ["m.sas.v1"],
+            );
+
+            await bobVerificationRequest.accept();
+
+            // Alice and Bob both start the verification
+            const aliceVerifier = await aliceVerificationRequest.startVerification("m.sas.v1");
+            const bobVerifier = await bobVerificationRequest.startVerification("m.sas.v1");
+            // We can now send Alice's start message to Bob
+            await aliceRequestLoop.makeOutgoingRequest(aliceStartRequest!);
+
+            // create a function to compare the SAS, and then let the verification run
+            let otherCallbacks: ShowSasCallbacks | undefined;
+            const compareSas = (callbacks: ShowSasCallbacks) => {
+                if (otherCallbacks) {
+                    const ourDecimal = callbacks.sas.decimal!;
+                    const theirDecimal = otherCallbacks.sas.decimal!;
+                    if (ourDecimal.every((el, idx) => el == theirDecimal[idx])) {
+                        otherCallbacks.confirm();
+                        callbacks.confirm();
+                    } else {
+                        otherCallbacks.mismatch();
+                        callbacks.mismatch();
+                    }
+                } else {
+                    otherCallbacks = callbacks;
+                }
+            };
+            aliceVerifier.on(VerifierEvent.ShowSas, compareSas);
+            bobVerifier.on(VerifierEvent.ShowSas, compareSas);
+
+            await Promise.all([aliceVerifier.verify(), await bobVerifier.verify()]);
+        } finally {
+            await aliceRequestLoop.stop();
+            await bobRequestLoop.stop();
+        }
+    });
+
+    it("can verify by QR code", async () => {
+        const aliceUserId = "@alice:example.org";
+        const aliceDeviceId = "ABCDEFG";
+        const bobUserId = "@bob:example.org";
+        const bobDeviceId = "HIJKLMN";
+        const [aliceOlmMachine, aliceDeviceKeys, aliceCrossSigningKeys] = await initOlmMachineAndKeys(
+            aliceUserId,
+            aliceDeviceId,
+        );
+        const [bobOlmMachine, bobDeviceKeys, bobCrossSigningKeys] = await initOlmMachineAndKeys(bobUserId, bobDeviceId);
+
+        const aliceRequestLoop = makeRequestLoop(
+            aliceOlmMachine,
+            aliceDeviceKeys,
+            aliceCrossSigningKeys,
+            bobOlmMachine,
+            bobDeviceKeys,
+            bobCrossSigningKeys,
+        );
+        const bobRequestLoop = makeRequestLoop(
+            bobOlmMachine,
+            bobDeviceKeys,
+            bobCrossSigningKeys,
+            aliceOlmMachine,
+            aliceDeviceKeys,
+            aliceCrossSigningKeys,
+        );
+
+        try {
+            await aliceOlmMachine.updateTrackedUsers([new RustSdkCryptoJs.UserId(bobUserId)]);
+            await bobOlmMachine.updateTrackedUsers([new RustSdkCryptoJs.UserId(aliceUserId)]);
+
+            // Alice requests verification
+            const bobUserIdentity = await aliceOlmMachine.getIdentity(new RustSdkCryptoJs.UserId(bobUserId));
+
+            const roomId = new RustSdkCryptoJs.RoomId("!roomId:example.org");
+            const methods = [
+                verificationMethodIdentifierToMethod("m.reciprocate.v1"),
+                verificationMethodIdentifierToMethod("m.qr_code.show.v1"),
+            ];
+            const innerVerificationRequest = await bobUserIdentity.requestVerification(
+                roomId,
+                new RustSdkCryptoJs.EventId("$m.key.verification.request"),
+                methods,
+            );
+            const aliceVerificationRequest = new RustVerificationRequest(
+                aliceOlmMachine,
+                innerVerificationRequest,
+                aliceRequestLoop as unknown as OutgoingRequestProcessor,
+                ["m.reciprocate.v1", "m.qr_code.show.v1"],
+            );
+
+            const verificationRequestContent = JSON.parse(await bobUserIdentity.verificationRequestContent(methods));
+            await bobOlmMachine.receiveVerificationEvent(
+                JSON.stringify({
+                    type: "m.room.message",
+                    sender: aliceUserId,
+                    event_id: "$m.key.verification.request",
+                    content: verificationRequestContent,
+                    origin_server_ts: Date.now(),
+                    unsigned: {
+                        age: 0,
+                    },
+                }),
+                roomId,
+            );
+
+            // Bob accepts
+            const bobInnerVerificationRequest = bobOlmMachine.getVerificationRequest(
+                new RustSdkCryptoJs.UserId(aliceUserId),
+                "$m.key.verification.request",
+            )!;
+            const bobVerificationRequest = new RustVerificationRequest(
+                bobOlmMachine,
+                bobInnerVerificationRequest,
+                bobRequestLoop as unknown as OutgoingRequestProcessor,
+                ["m.reciprocate.v1", "m.qr_code.show.v1", "m.qr_code.scan.v1"],
+            );
+
+            await bobVerificationRequest.accept();
+
+            // Bob scans
+            const qrCode = await aliceVerificationRequest.generateQRCode();
+
+            const aliceVerifierPromise: Promise<Verifier> = new Promise((resolve, reject) => {
+                aliceVerificationRequest.on(VerificationRequestEvent.Change, () => {
+                    const verifier = aliceVerificationRequest.verifier;
+                    if (verifier) {
+                        resolve(verifier);
+                    }
+                });
+            });
+            const bobVerifier = await bobVerificationRequest.scanQRCode(qrCode!);
+
+            const aliceVerifier = await aliceVerifierPromise;
+            aliceVerifier.on(VerifierEvent.ShowReciprocateQr, (showQrCodeCallbacks) => {
+                showQrCodeCallbacks.confirm();
+            });
+
+            await Promise.all([aliceVerifier.verify(), await bobVerifier.verify()]);
+        } finally {
+            await aliceRequestLoop.stop();
+            await bobRequestLoop.stop();
+        }
+    });
 });
 
 describe("isVerificationEvent", () => {
@@ -152,3 +511,148 @@ function makeMockedInner(): Mocked<RustSdkCryptoJs.VerificationRequest> {
         },
     } as unknown as Mocked<RustSdkCryptoJs.VerificationRequest>;
 }
+
+interface CrossSigningKeys {
+    master_key: any;
+    self_signing_key: any;
+    user_signing_key: any;
+}
+
+/** create an Olm machine and device/cross-signing keys for a user */
+async function initOlmMachineAndKeys(
+    userId: string,
+    deviceId: string,
+): Promise<[RustSdkCryptoJs.OlmMachine, IDeviceKeys, CrossSigningKeys]> {
+    const olmMachine = await RustSdkCryptoJs.OlmMachine.initialize(
+        new RustSdkCryptoJs.UserId(userId),
+        new RustSdkCryptoJs.DeviceId(deviceId),
+        undefined,
+        undefined,
+    );
+    const { uploadKeysRequest, uploadSignaturesRequest, uploadSigningKeysRequest } =
+        await olmMachine.bootstrapCrossSigning(true);
+    const deviceKeys = JSON.parse(uploadKeysRequest.body).device_keys;
+    await olmMachine.markRequestAsSent(
+        uploadKeysRequest.id,
+        uploadKeysRequest.type,
+        '{"one_time_key_counts":{"signed_curve25519":100}}',
+    );
+    const crossSigningSignatures = JSON.parse(uploadSignaturesRequest.body);
+    for (const [keyId, signature] of Object.entries(crossSigningSignatures[userId][deviceId]["signatures"][userId])) {
+        deviceKeys["signatures"][userId][keyId] = signature;
+    }
+    const crossSigningKeys = JSON.parse(uploadSigningKeysRequest.body);
+    // note: the upload signatures request and upload signing keys requests
+    // don't need to be marked as sent in the Olm machine
+
+    return [olmMachine, deviceKeys, crossSigningKeys];
+}
+
+type CustomRequestHandler = (request: OutgoingRequest | RustSdkCryptoJs.UploadSigningKeysRequest) => Promise<any>;
+
+/** Loop for handling outgoing requests from an Olm machine.
+ *
+ * Simulates a server with two users: "us" and "them".  Handles key query
+ * requests, querying either our keys or the other user's keys.  Room messages
+ * are sent as incoming verification events to the other user.  A custom
+ * handler can be added to override default request processing (the handler
+ * should return a response body to inhibit default processing).
+ *
+ * Can also be used as an OutgoingRequestProcessor. */
+function makeRequestLoop(
+    ourOlmMachine: RustSdkCryptoJs.OlmMachine,
+    ourDeviceKeys: IDeviceKeys,
+    ourCrossSigningKeys: CrossSigningKeys,
+    theirOlmMachine: RustSdkCryptoJs.OlmMachine,
+    theirDeviceKeys: IDeviceKeys,
+    theirCrossSigningKeys: CrossSigningKeys,
+    customHandler?: CustomRequestHandler,
+) {
+    let stopRequestLoop = false;
+    const ourUserId = ourOlmMachine.userId.toString();
+    const ourDeviceId = ourOlmMachine.deviceId.toString();
+    const theirUserId = theirOlmMachine.userId.toString();
+    const theirDeviceId = theirOlmMachine.deviceId.toString();
+
+    function defaultHandler(request: OutgoingRequest | RustSdkCryptoJs.UploadSigningKeysRequest): any {
+        if (request instanceof RustSdkCryptoJs.KeysQueryRequest) {
+            const resp: Record<string, any> = {
+                device_keys: {},
+            };
+            const body = JSON.parse(request.body);
+            const query = body.device_keys;
+            const masterKeys: Record<string, any> = {};
+            const selfSigningKeys: Record<string, any> = {};
+            if (ourUserId in query) {
+                resp.device_keys[ourUserId] = { [ourDeviceId]: ourDeviceKeys };
+                masterKeys[ourUserId] = ourCrossSigningKeys.master_key;
+                selfSigningKeys[ourUserId] = ourCrossSigningKeys.self_signing_key;
+                resp.user_signing_keys = {
+                    [ourUserId]: ourCrossSigningKeys.user_signing_key,
+                };
+            }
+            if (theirUserId in query) {
+                resp.device_keys[theirUserId] = {
+                    [theirDeviceId]: theirDeviceKeys,
+                };
+                masterKeys[theirUserId] = theirCrossSigningKeys.master_key;
+                selfSigningKeys[theirUserId] = theirCrossSigningKeys.self_signing_key;
+            }
+            if (Object.keys(masterKeys).length) {
+                resp.master_keys = masterKeys;
+            }
+            if (Object.keys(selfSigningKeys).length) {
+                resp.self_signing_keys = selfSigningKeys;
+            }
+            return resp;
+        } else if (request instanceof RustSdkCryptoJs.RoomMessageRequest) {
+            theirOlmMachine.receiveVerificationEvent(
+                JSON.stringify({
+                    type: request.event_type,
+                    sender: ourUserId,
+                    event_id: "$" + request.event_type,
+                    content: JSON.parse(request.body),
+                    origin_server_ts: Date.now(),
+                    unsigned: {
+                        age: 0,
+                    },
+                }),
+                new RustSdkCryptoJs.RoomId(request.room_id),
+            );
+            return { event_id: "$" + request.event_type };
+        } else if (request instanceof RustSdkCryptoJs.SignatureUploadRequest) {
+            // this only gets called at the end after the verification
+            // succeeds, so we don't actually have to do anything.
+            return { failures: {} };
+        }
+        return {};
+    }
+
+    async function makeOutgoingRequest(
+        request: OutgoingRequest | RustSdkCryptoJs.UploadSigningKeysRequest,
+    ): Promise<any> {
+        const resp = (await customHandler?.(request)) ?? defaultHandler(request);
+        if (!(request instanceof RustSdkCryptoJs.UploadSigningKeysRequest) && request.id) {
+            await ourOlmMachine.markRequestAsSent(request.id!, request.type, JSON.stringify(resp));
+        }
+    }
+
+    async function runLoop() {
+        while (!stopRequestLoop) {
+            const requests = await ourOlmMachine.outgoingRequests();
+            for (const request of requests) {
+                await makeOutgoingRequest(request);
+            }
+        }
+    }
+
+    const loopCompletedPromise = runLoop();
+
+    return {
+        makeOutgoingRequest,
+        stop: async () => {
+            stopRequestLoop = true;
+            await loopCompletedPromise;
+        },
+    };
+}

src/rust-crypto/verification.ts

@@ -34,6 +34,7 @@ import { OutgoingRequest, OutgoingRequestProcessor } from "./OutgoingRequestProc
 import { TypedReEmitter } from "../ReEmitter";
 import { MatrixEvent } from "../models/event";
 import { EventType, MsgType } from "../@types/event";
+import { defer, IDeferred } from "../utils";
 
 /**
  * An incoming, or outgoing, request to verify a user or a device via cross-signing.
@@ -76,11 +77,16 @@ export class RustVerificationRequest
         const onChange = async (): Promise<void> => {
             const verification: RustSdkCryptoJs.Qr | RustSdkCryptoJs.Sas | undefined = this.inner.getVerification();
 
-            // If we now have a `Verification` where we lacked one before, or we have transitioned from QR to SAS,
+            // Set the _verifier object (wrapping the rust `Verification` as a js-sdk Verifier) if:
-            // wrap the new rust Verification as a js-sdk Verifier.
+            // - we now have a `Verification` where we lacked one before
+            // - we have transitioned from QR to SAS
+            // - we are verifying with SAS, but we need to replace our verifier with a new one because both parties
+            //   tried to start verification at the same time, and we lost the tie breaking
             if (verification instanceof RustSdkCryptoJs.Sas) {
                 if (this._verifier === undefined || this._verifier instanceof RustQrCodeVerifier) {
                     this.setVerifier(new RustSASVerifier(verification, this, outgoingRequestProcessor));
+                } else if (this._verifier instanceof RustSASVerifier) {
+                    this._verifier.replaceInner(verification);
                 }
             } else if (verification instanceof RustSdkCryptoJs.Qr && this._verifier === undefined) {
                 this.setVerifier(new RustQrCodeVerifier(verification, outgoingRequestProcessor));
@@ -456,46 +462,45 @@ abstract class BaseRustVerifer<InnerType extends RustSdkCryptoJs.Qr | RustSdkCry
     VerifierEvent | VerificationRequestEvent,
     VerifierEventHandlerMap & VerificationRequestEventHandlerMap
 > {
-    /** A promise which completes when the verification completes (or rejects when it is cancelled/fails) */
+    /** A deferred which completes when the verification completes (or rejects when it is cancelled/fails) */
-    protected readonly completionPromise: Promise<void>;
+    protected readonly completionDeferred: IDeferred<void>;
 
     public constructor(
-        protected readonly inner: InnerType,
+        protected inner: InnerType,
         protected readonly outgoingRequestProcessor: OutgoingRequestProcessor,
     ) {
         super();
 
-        this.completionPromise = new Promise<void>((resolve, reject) => {
+        this.completionDeferred = defer();
-            const onChange = async (): Promise<void> => {
+        inner.registerChangesCallback(async () => {
-                this.onChange();
+            this.onChange();
-
-                if (this.inner.isDone()) {
-                    resolve(undefined);
-                } else if (this.inner.isCancelled()) {
-                    const cancelInfo = this.inner.cancelInfo()!;
-                    reject(
-                        new Error(
-                            `Verification cancelled by ${
-                                cancelInfo.cancelledbyUs() ? "us" : "them"
-                            } with code ${cancelInfo.cancelCode()}: ${cancelInfo.reason()}`,
-                        ),
-                    );
-                }
-
-                this.emit(VerificationRequestEvent.Change);
-            };
-            inner.registerChangesCallback(onChange);
         });
         // stop the runtime complaining if nobody catches a failure
-        this.completionPromise.catch(() => null);
+        this.completionDeferred.promise.catch(() => null);
     }
 
     /**
      * Hook which is called when the underlying rust class notifies us that there has been a change.
      *
-     * Can be overridden by subclasses to see if we can notify the application about an update.
+     * Can be overridden by subclasses to see if we can notify the application about an update. The overriding method
+     * must call `super.onChange()`.
      */
-    protected onChange(): void {}
+    protected onChange(): void {
+        if (this.inner.isDone()) {
+            this.completionDeferred.resolve(undefined);
+        } else if (this.inner.isCancelled()) {
+            const cancelInfo = this.inner.cancelInfo()!;
+            this.completionDeferred.reject(
+                new Error(
+                    `Verification cancelled by ${
+                        cancelInfo.cancelledbyUs() ? "us" : "them"
+                    } with code ${cancelInfo.cancelCode()}: ${cancelInfo.reason()}`,
+                ),
+            );
+        }
+
+        this.emit(VerificationRequestEvent.Change);
+    }
 
     /**
      * Returns true if the verification has been cancelled, either by us or the other side.
@@ -565,6 +570,8 @@ export class RustQrCodeVerifier extends BaseRustVerifer<RustSdkCryptoJs.Qr> impl
                 cancel: () => this.cancel(),
             };
         }
+
+        super.onChange();
     }
 
     /**
@@ -580,7 +587,7 @@ export class RustQrCodeVerifier extends BaseRustVerifer<RustSdkCryptoJs.Qr> impl
             this.emit(VerifierEvent.ShowReciprocateQr, this.callbacks);
         }
         // Nothing to do here but wait.
-        await this.completionPromise;
+        await this.completionDeferred.promise;
     }
 
     /**
@@ -657,15 +664,24 @@ export class RustSASVerifier extends BaseRustVerifer<RustSdkCryptoJs.Sas> implem
      *    or times out.
      */
     public async verify(): Promise<void> {
+        await this.sendAccept();
+        await this.completionDeferred.promise;
+    }
+
+    /**
+     * Send the accept or start event, if it hasn't already been sent
+     */
+    private async sendAccept(): Promise<void> {
         const req: undefined | OutgoingRequest = this.inner.accept();
         if (req) {
             await this.outgoingRequestProcessor.makeOutgoingRequest(req);
         }
-        await this.completionPromise;
     }
 
     /** if we can now show the callbacks, do so */
     protected onChange(): void {
+        super.onChange();
+
         if (this.callbacks === null) {
             const emoji = this.inner.emoji();
             const decimal = this.inner.decimals();
@@ -717,6 +733,25 @@ export class RustSASVerifier extends BaseRustVerifer<RustSdkCryptoJs.Sas> implem
     public getShowSasCallbacks(): ShowSasCallbacks | null {
         return this.callbacks;
     }
+
+    /**
+     * Replace the inner Rust verifier with a different one.
+     *
+     * @param inner - the new Rust verifier
+     * @internal
+     */
+    public replaceInner(inner: RustSdkCryptoJs.Sas): void {
+        if (this.inner != inner) {
+            this.inner = inner;
+            inner.registerChangesCallback(async () => {
+                this.onChange();
+            });
+            // replaceInner will only get called if we started the verification at the same time as the other side, and we lost
+            // the tie breaker.  So we need to re-accept their verification.
+            this.sendAccept();
+            this.onChange();
+        }
+    }
 }
 
 /** For each specced verification method, the rust-side `VerificationMethod` corresponding to it */