Typescriptify & use service worker for MSC3916 authentication (#27326)

* Typescriptify & use service worker for MSC3916 authentication

* appease the linter

* appease jest

* appease linter

* Get the access token directly

* Add a bit of jitter

* Improve legibility, use factored-out functions for pickling

* Add docs

* Appease the linter

* Document risks of postMessage

* Split service worker post message handling out to function

* Move registration to async function

* Use more early returns

* Thanks(?), WebStorm

* Handle case of no access token for /versions

* Appease linter

* Apply suggestions from code review

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

* Remove spurious try/catch

* Factor out fetch config stuff

* Apply suggestions from code review

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

* Finish applying code review suggestions

---------

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

res/sw.js

@@ -1 +0,0 @@
-self.addEventListener("fetch", () => {});

src/serviceworker/index.ts

@@ -0,0 +1,186 @@
+/*
+Copyright 2024 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { idbLoad } from "matrix-react-sdk/src/utils/StorageAccess";
+import { ACCESS_TOKEN_IV, tryDecryptToken } from "matrix-react-sdk/src/utils/tokens/tokens";
+import { buildAndEncodePickleKey } from "matrix-react-sdk/src/utils/tokens/pickling";
+
+const serverSupportMap: {
+    [serverUrl: string]: {
+        supportsMSC3916: boolean;
+        cacheExpiryTimeMs: number;
+    };
+} = {};
+
+self.addEventListener("install", (event) => {
+    // We skipWaiting() to update the service worker more frequently, particularly in development environments.
+    // @ts-expect-error - service worker types are not available. See 'fetch' event handler.
+    event.waitUntil(skipWaiting());
+});
+
+self.addEventListener("activate", (event) => {
+    // We force all clients to be under our control, immediately. This could be old tabs.
+    // @ts-expect-error - service worker types are not available. See 'fetch' event handler.
+    event.waitUntil(clients.claim());
+});
+
+// @ts-expect-error - the service worker types conflict with the DOM types available through TypeScript. Many hours
+// have been spent trying to convince the type system that there's no actual conflict, but it has yet to work. Instead
+// of trying to make it do the thing, we force-cast to something close enough where we can (and ignore errors otherwise).
+self.addEventListener("fetch", (event: FetchEvent) => {
+    // This is the authenticated media (MSC3916) check, proxying what was unauthenticated to the authenticated variants.
+
+    if (event.request.method !== "GET") {
+        return; // not important to us
+    }
+
+    // Note: ideally we'd keep the request headers etc, but in practice we can't even see those details.
+    // See https://stackoverflow.com/a/59152482
+    let url = event.request.url;
+
+    // We only intercept v3 download and thumbnail requests as presumably everything else is deliberate.
+    // For example, `/_matrix/media/unstable` or `/_matrix/media/v3/preview_url` are something well within
+    // the control of the application, and appear to be choices made at a higher level than us.
+    if (!url.includes("/_matrix/media/v3/download") && !url.includes("/_matrix/media/v3/thumbnail")) {
+        return; // not a URL we care about
+    }
+
+    // We need to call respondWith synchronously, otherwise we may never execute properly. This means
+    // later on we need to proxy the request through if it turns out the server doesn't support authentication.
+    event.respondWith(
+        (async (): Promise<Response> => {
+            let accessToken: string | undefined;
+            try {
+                // Figure out which homeserver we're communicating with
+                const csApi = url.substring(0, url.indexOf("/_matrix/media/v3"));
+
+                // Add jitter to reduce request spam, particularly to `/versions` on initial page load
+                await new Promise<void>((resolve) => setTimeout(() => resolve(), Math.random() * 10));
+
+                // Locate our access token, and populate the fetchConfig with the authentication header.
+                // @ts-expect-error - service worker types are not available. See 'fetch' event handler.
+                const client = await self.clients.get(event.clientId);
+                accessToken = await getAccessToken(client);
+
+                // Update or populate the server support map using a (usually) authenticated `/versions` call.
+                await tryUpdateServerSupportMap(csApi, accessToken);
+
+                // If we have server support (and a means of authentication), rewrite the URL to use MSC3916 endpoints.
+                if (serverSupportMap[csApi].supportsMSC3916 && accessToken) {
+                    // Currently unstable only.
+                    // TODO: Support stable endpoints when available.
+                    url = url.replace(/\/media\/v3\/(.*)\//, "/client/unstable/org.matrix.msc3916/media/$1/");
+                } // else by default we make no changes
+            } catch (err) {
+                console.error("SW: Error in request rewrite.", err);
+            }
+
+            // Add authentication and send the request. We add authentication even if MSC3916 endpoints aren't
+            // being used to ensure patches like this work:
+            // https://github.com/matrix-org/synapse/commit/2390b66bf0ec3ff5ffb0c7333f3c9b239eeb92bb
+            return fetch(url, fetchConfigForToken(accessToken));
+        })(),
+    );
+});
+
+async function tryUpdateServerSupportMap(clientApiUrl: string, accessToken?: string): Promise<void> {
+    // only update if we don't know about it, or if the data is stale
+    if (serverSupportMap[clientApiUrl]?.cacheExpiryTimeMs > new Date().getTime()) {
+        return; // up to date
+    }
+
+    const config = fetchConfigForToken(accessToken);
+    const versions = await (await fetch(`${clientApiUrl}/_matrix/client/versions`, config)).json();
+
+    serverSupportMap[clientApiUrl] = {
+        supportsMSC3916: Boolean(versions?.unstable_features?.["org.matrix.msc3916"]),
+        cacheExpiryTimeMs: new Date().getTime() + 2 * 60 * 60 * 1000, // 2 hours from now
+    };
+}
+
+// Ideally we'd use the `Client` interface for `client`, but since it's not available (see 'fetch' listener), we use
+// unknown for now and force-cast it to something close enough later.
+async function getAccessToken(client: unknown): Promise<string | undefined> {
+    // Access tokens are encrypted at rest, so while we can grab the "access token", we'll need to do work to get the
+    // real thing.
+    const encryptedAccessToken = await idbLoad("account", "mx_access_token");
+
+    // We need to extract a user ID and device ID from localstorage, which means calling WebPlatform for the
+    // read operation. Service workers can't access localstorage.
+    const { userId, deviceId } = await askClientForUserIdParams(client);
+
+    // ... and this is why we need the user ID and device ID: they're index keys for the pickle key table.
+    const pickleKeyData = await idbLoad("pickleKey", [userId, deviceId]);
+    if (pickleKeyData && (!pickleKeyData.encrypted || !pickleKeyData.iv || !pickleKeyData.cryptoKey)) {
+        console.error("SW: Invalid pickle key loaded - ignoring");
+        return undefined;
+    }
+
+    // Finally, try decrypting the thing and return that. This may fail, but that's okay.
+    try {
+        const pickleKey = await buildAndEncodePickleKey(pickleKeyData, userId, deviceId);
+        return tryDecryptToken(pickleKey, encryptedAccessToken, ACCESS_TOKEN_IV);
+    } catch (e) {
+        console.error("SW: Error decrypting access token.", e);
+        return undefined;
+    }
+}
+
+// Ideally we'd use the `Client` interface for `client`, but since it's not available (see 'fetch' listener), we use
+// unknown for now and force-cast it to something close enough inside the function.
+async function askClientForUserIdParams(client: unknown): Promise<{ userId: string; deviceId: string }> {
+    return new Promise((resolve, reject) => {
+        // Dev note: this uses postMessage, which is a highly insecure channel. postMessage is typically visible to other
+        // tabs, windows, browser extensions, etc, making it far from ideal for sharing sensitive information. This is
+        // why our service worker calculates/decrypts the access token manually: we don't want the user's access token
+        // to be available to (potentially) malicious listeners. We do require some information for that decryption to
+        // work though, and request that in the least sensitive way possible.
+        //
+        // We could also potentially use some version of TLS to encrypt postMessage, though that feels way more involved
+        // than just reading IndexedDB ourselves.
+
+        // Avoid stalling the tab in case something goes wrong.
+        const timeoutId = setTimeout(() => reject(new Error("timeout in postMessage")), 1000);
+
+        // We don't need particularly good randomness here - we just use this to generate a request ID, so we know
+        // which postMessage reply is for our active request.
+        const responseKey = Math.random().toString(36);
+
+        // Add the listener first, just in case the tab is *really* fast.
+        const listener = (event: MessageEvent): void => {
+            if (event.data?.responseKey !== responseKey) return; // not for us
+            clearTimeout(timeoutId); // do this as soon as possible, avoiding a race between resolve and reject.
+            resolve(event.data); // "unblock" the remainder of the thread, if that were such a thing in JavaScript.
+            self.removeEventListener("message", listener); // cleanup, since we're not going to do anything else.
+        };
+        self.addEventListener("message", listener);
+
+        // Ask the tab for the information we need. This is handled by WebPlatform.
+        (client as Window).postMessage({ responseKey, type: "userinfo" });
+    });
+}
+
+function fetchConfigForToken(accessToken?: string): RequestInit | undefined {
+    if (!accessToken) {
+        return undefined; // no headers/config to specify
+    }
+
+    return {
+        headers: {
+            Authorization: `Bearer ${accessToken}`,
+        },
+    };
+}

src/vector/platform/WebPlatform.ts

@@ -1,7 +1,7 @@
 /*
 Copyright 2016 Aviral Dasgupta
 Copyright 2016 OpenMarket Ltd
-Copyright 2017-2020 New Vector Ltd
+Copyright 2017-2020, 2024 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -44,9 +44,41 @@ export default class WebPlatform extends VectorBasePlatform {
 
     public constructor() {
         super();
-        // Register service worker if available on this platform
+
-        if ("serviceWorker" in navigator) {
+        // Register the service worker in the background
-            navigator.serviceWorker.register("sw.js");
+        this.tryRegisterServiceWorker().catch((e) => console.error("Error registering/updating service worker:", e));
+    }
+
+    private async tryRegisterServiceWorker(): Promise<void> {
+        if (!("serviceWorker" in navigator)) {
+            return; // not available on this platform - don't try to register the service worker
+        }
+
+        // sw.js is exported by webpack, sourced from `/src/serviceworker/index.ts`
+        const registration = await navigator.serviceWorker.register("sw.js");
+        if (!registration) {
+            // Registration didn't work for some reason - assume failed and ignore.
+            // This typically happens in Jest.
+            return;
+        }
+
+        await registration.update();
+        navigator.serviceWorker.addEventListener("message", this.onServiceWorkerPostMessage.bind(this));
+    }
+
+    private onServiceWorkerPostMessage(event: MessageEvent): void {
+        try {
+            if (event.data?.["type"] === "userinfo" && event.data?.["responseKey"]) {
+                const userId = localStorage.getItem("mx_user_id");
+                const deviceId = localStorage.getItem("mx_device_id");
+                event.source!.postMessage({
+                    responseKey: event.data["responseKey"],
+                    userId,
+                    deviceId,
+                });
+            }
+        } catch (e) {
+            console.error("Error responding to service worker: ", e);
         }
     }
 

webpack.config.js

@@ -153,6 +153,10 @@ module.exports = (env, argv) => {
             mobileguide: "./src/vector/mobile_guide/index.ts",
             jitsi: "./src/vector/jitsi/index.ts",
             usercontent: "./node_modules/matrix-react-sdk/src/usercontent/index.ts",
+            serviceworker: {
+                import: "./src/serviceworker/index.ts",
+                filename: "sw.js", // update WebPlatform if this changes
+            },
             ...(useHMR ? {} : cssThemes),
         },
 
@@ -666,7 +670,7 @@ module.exports = (env, argv) => {
                 // HtmlWebpackPlugin will screw up our formatting like the names
                 // of the themes and which chunks we actually care about.
                 inject: false,
-                excludeChunks: ["mobileguide", "usercontent", "jitsi"],
+                excludeChunks: ["mobileguide", "usercontent", "jitsi", "serviceworker"],
                 minify: false,
                 templateParameters: {
                     og_image_url: ogImageUrl,
@@ -740,7 +744,6 @@ module.exports = (env, argv) => {
                     "res/jitsi_external_api.min.js",
                     "res/jitsi_external_api.min.js.LICENSE.txt",
                     "res/manifest.json",
-                    "res/sw.js",
                     "res/welcome.html",
                     { from: "welcome/**", context: path.resolve(__dirname, "res") },
                     { from: "themes/**", context: path.resolve(__dirname, "res") },