Implement CryptoApi.isKeyBackupTrusted (#3632)

* Implement `CryptoApi.isKeyBackupTrusted`

Fixes element-hq/crypto-internal#110

* Bump matrix-sdk-crypto-wasm to v1.2.0

* Back out some changes

These are unneeded, and break backwards compat

package.json

@@ -55,7 +55,7 @@
     ],
     "dependencies": {
         "@babel/runtime": "^7.12.5",
-        "@matrix-org/matrix-sdk-crypto-wasm": "^1.1.0",
+        "@matrix-org/matrix-sdk-crypto-wasm": "^1.2.0",
         "another-json": "^0.2.0",
         "bs58": "^5.0.0",
         "content-type": "^1.0.4",

spec/integ/crypto/megolm-backup.spec.ts

@@ -16,6 +16,7 @@ limitations under the License.
 
 import fetchMock from "fetch-mock-jest";
 import "fake-indexeddb/auto";
+import { IDBFactory } from "fake-indexeddb";
 
 import { IKeyBackupSession } from "../../../src/crypto/keybackup";
 import { createClient, CryptoEvent, ICreateClientOpts, IEvent, MatrixClient } from "../../../src";
@@ -25,6 +26,7 @@ import { E2EKeyResponder } from "../../test-utils/E2EKeyResponder";
 import { mockInitialApiRequests } from "../../test-utils/mockEndpoints";
 import { awaitDecryption, CRYPTO_BACKENDS, InitCrypto, syncPromise } from "../../test-utils/test-utils";
 import * as testData from "../../test-utils/test-data";
+import { KeyBackupInfo } from "../../../src/crypto-api/keybackup";
 
 const ROOM_ID = "!ROOM:ID";
 
@@ -74,6 +76,13 @@ const CURVE25519_KEY_BACKUP_DATA: IKeyBackupSession = {
 const TEST_USER_ID = "@alice:localhost";
 const TEST_DEVICE_ID = "xzcvb";
 
+afterEach(() => {
+    // reset fake-indexeddb after each test, to make sure we don't leak connections
+    // cf https://github.com/dumbmatter/fakeIndexedDB#wipingresetting-the-indexeddb-for-a-fresh-state
+    // eslint-disable-next-line no-global-assign
+    indexedDB = new IDBFactory();
+});
+
 describe.each(Object.entries(CRYPTO_BACKENDS))("megolm-keys backup (%s)", (backend: string, initCrypto: InitCrypto) => {
     // oldBackendOnly is an alternative to `it` or `test` which will skip the test if we are running against the
     // Rust backend. Once we have full support in the rust sdk, it will go away.
@@ -204,7 +213,7 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("megolm-keys backup (%s)", (backe
             overwriteRoutes: true,
         });
 
-        // check that signaling is working
+        // check that signalling is working
         const backupPromise = new Promise<void>((resolve, reject) => {
             aliceClient.on(CryptoEvent.KeyBackupStatus, (enabled) => {
                 if (enabled) {
@@ -222,6 +231,61 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("megolm-keys backup (%s)", (backe
         expect(backupStatus).toStrictEqual(testData.SIGNED_BACKUP_DATA.version);
     });
 
+    describe("isKeyBackupTrusted", () => {
+        it("does not trust a backup signed by an untrusted device", async () => {
+            aliceClient = await initTestClient();
+            const aliceCrypto = aliceClient.getCrypto()!;
+
+            // download the device list, to match the trusted case
+            await aliceClient.startClient();
+            await waitForDeviceList();
+
+            const result = await aliceCrypto.isKeyBackupTrusted(testData.SIGNED_BACKUP_DATA);
+            expect(result).toEqual({ trusted: false, matchesDecryptionKey: false });
+        });
+
+        it("trusts a backup signed by a trusted device", async () => {
+            aliceClient = await initTestClient();
+            const aliceCrypto = aliceClient.getCrypto()!;
+
+            // tell Alice to trust the dummy device that signed the backup
+            await aliceClient.startClient();
+            await waitForDeviceList();
+            await aliceCrypto.setDeviceVerified(testData.TEST_USER_ID, testData.TEST_DEVICE_ID);
+
+            const result = await aliceCrypto.isKeyBackupTrusted(testData.SIGNED_BACKUP_DATA);
+            expect(result).toEqual({ trusted: true, matchesDecryptionKey: false });
+        });
+
+        it("recognises a backup which matches the decryption key", async () => {
+            aliceClient = await initTestClient();
+            const aliceCrypto = aliceClient.getCrypto()!;
+
+            await aliceClient.startClient();
+            await aliceCrypto.storeSessionBackupPrivateKey(
+                Buffer.from(testData.BACKUP_DECRYPTION_KEY_BASE64, "base64"),
+            );
+
+            const result = await aliceCrypto.isKeyBackupTrusted(testData.SIGNED_BACKUP_DATA);
+            expect(result).toEqual({ trusted: false, matchesDecryptionKey: true });
+        });
+
+        it("is not fooled by a backup which matches the decryption key but uses a different algorithm", async () => {
+            aliceClient = await initTestClient();
+            const aliceCrypto = aliceClient.getCrypto()!;
+
+            await aliceClient.startClient();
+            await aliceCrypto.storeSessionBackupPrivateKey(
+                Buffer.from(testData.BACKUP_DECRYPTION_KEY_BASE64, "base64"),
+            );
+
+            const backup: KeyBackupInfo = JSON.parse(JSON.stringify(testData.SIGNED_BACKUP_DATA));
+            backup.algorithm = "m.megolm_backup.v1.aes-hmac-sha2";
+            const result = await aliceCrypto.isKeyBackupTrusted(backup);
+            expect(result).toEqual({ trusted: false, matchesDecryptionKey: false });
+        });
+    });
+
     /** make sure that the client knows about the dummy device */
     async function waitForDeviceList(): Promise<void> {
         // Completing the initial sync will make the device list download outdated device lists (of which our own

src/client.ts

@@ -3282,6 +3282,8 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
 
     /**
      * @param info - key backup info dict from getKeyBackupVersion()
+     *
+     * @deprecated Prefer {@link CryptoApi.isKeyBackupTrusted | `CryptoApi.isKeyBackupTrusted`}.
      */
     public isKeyBackupTrusted(info: IKeyBackupInfo): Promise<TrustInfo> {
         if (!this.crypto) {

src/crypto-api.ts

@@ -20,7 +20,7 @@ import { DeviceMap } from "./models/device";
 import { UIAuthCallback } from "./interactive-auth";
 import { AddSecretStorageKeyOpts, SecretStorageCallbacks, SecretStorageKeyDescription } from "./secret-storage";
 import { VerificationRequest } from "./crypto-api/verification";
-import { KeyBackupInfo } from "./crypto-api/keybackup";
+import { BackupTrustInfo, KeyBackupInfo } from "./crypto-api/keybackup";
 import { ISignatures } from "./@types/signed";
 
 /**
@@ -332,6 +332,13 @@ export interface CryptoApi {
      * @returns If automatic key backups are enabled, the `version` of the active backup. Otherwise, `null`.
      */
     getActiveSessionBackupVersion(): Promise<string | null>;
+
+    /**
+     * Determine if a key backup can be trusted.
+     *
+     * @param info - key backup info dict from {@link MatrixClient#getKeyBackupVersion}.
+     */
+    isKeyBackupTrusted(info: KeyBackupInfo): Promise<BackupTrustInfo>;
 }
 
 /**

src/crypto-api/keybackup.ts

@@ -31,7 +31,10 @@ export interface Aes256AuthData {
 }
 
 /**
- * Extra info of a recovery key
+ * Information about a server-side key backup.
+ *
+ * Returned by [`GET /_matrix/client/v3/room_keys/version`](https://spec.matrix.org/v1.7/client-server-api/#get_matrixclientv3room_keysversion)
+ * and hence {@link MatrixClient#getKeyBackupVersion}.
  */
 export interface KeyBackupInfo {
     algorithm: string;
@@ -40,3 +43,20 @@ export interface KeyBackupInfo {
     etag?: string;
     version?: string; // number contained within
 }
+
+/**
+ * Information on whether a given server-side backup is trusted.
+ */
+export interface BackupTrustInfo {
+    /**
+     * Is this backup trusted?
+     *
+     * True if, and only if, there is a valid signature on the backup from a trusted device.
+     */
+    readonly trusted: boolean;
+
+    /**
+     * True if this backup matches the stored decryption key.
+     */
+    readonly matchesDecryptionKey: boolean;
+}

src/crypto/backup.ts

@@ -40,6 +40,7 @@ import { UnstableValue } from "../NamespacedValue";
 import { CryptoEvent } from "./index";
 import { crypto } from "./crypto";
 import { HTTPError, MatrixError } from "../http-api";
+import { BackupTrustInfo } from "../crypto-api/keybackup";
 
 const KEY_BACKUP_KEYS_PER_REQUEST = 200;
 const KEY_BACKUP_CHECK_RATE_LIMIT = 5000; // ms
@@ -54,6 +55,7 @@ type SigInfo = {
     deviceTrust?: DeviceTrustLevel;
 };
 
+/** @deprecated Prefer {@link BackupTrustInfo} */
 export type TrustInfo = {
     usable: boolean; // is the backup trusted, true iff there is a sig that is valid & from a trusted device
     sigs: SigInfo[];
@@ -829,3 +831,15 @@ export const algorithmsByName: Record<string, BackupAlgorithmClass> = {
 };
 
 export const DefaultAlgorithm: BackupAlgorithmClass = Curve25519;
+
+/**
+ * Map a legacy {@link TrustInfo} into a new-style {@link BackupTrustInfo}.
+ *
+ * @param trustInfo - trustInfo to convert
+ */
+export function backupTrustInfoFromLegacyTrustInfo(trustInfo: TrustInfo): BackupTrustInfo {
+    return {
+        trusted: trustInfo.usable,
+        matchesDecryptionKey: trustInfo.trusted_locally ?? false,
+    };
+}

src/crypto/deviceinfo.ts

@@ -28,6 +28,8 @@ export interface IDevice {
 
 /**
  * Information about a user's device
+ *
+ * Superceded by {@link Device}.
  */
 export class DeviceInfo {
     /**

src/crypto/index.ts

@@ -50,7 +50,7 @@ import { IllegalMethod } from "./verification/IllegalMethod";
 import { KeySignatureUploadError } from "../errors";
 import { calculateKeyCheck, decryptAES, encryptAES, IEncryptedPayload } from "./aes";
 import { DehydrationManager } from "./dehydration";
-import { BackupManager } from "./backup";
+import { BackupManager, backupTrustInfoFromLegacyTrustInfo } from "./backup";
 import { IStore } from "../store";
 import { Room, RoomEvent } from "../models/room";
 import { RoomMember, RoomMemberEvent } from "../models/room-member";
@@ -87,10 +87,12 @@ import {
 } from "../secret-storage";
 import { ISecretRequest } from "./SecretSharing";
 import {
+    BackupTrustInfo,
     BootstrapCrossSigningOpts,
     CrossSigningStatus,
     DeviceVerificationStatus,
     ImportRoomKeysOpts,
+    KeyBackupInfo,
     VerificationRequest as CryptoApiVerificationRequest,
 } from "../crypto-api";
 import { Device, DeviceMap } from "../models/device";
@@ -1292,6 +1294,16 @@ export class Crypto extends TypedEventEmitter<CryptoEvent, CryptoEventHandlerMap
         return null;
     }
 
+    /**
+     * Determine if a key backup can be trusted.
+     *
+     * Implementation of {@link Crypto.CryptoApi.isKeyBackupTrusted}.
+     */
+    public async isKeyBackupTrusted(info: KeyBackupInfo): Promise<BackupTrustInfo> {
+        const trustInfo = await this.backupManager.isKeyBackupTrusted(info);
+        return backupTrustInfoFromLegacyTrustInfo(trustInfo);
+    }
+
     /**
      * Checks that a given cross-signing private key matches a given public key.
      * This can be used by the getCrossSigningKey callback to verify that the

src/rust-crypto/backup.ts

@@ -14,12 +14,42 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import { OlmMachine, SignatureVerification } from "@matrix-org/matrix-sdk-crypto-wasm";
+import * as RustSdkCryptoJs from "@matrix-org/matrix-sdk-crypto-wasm";
+
+import { BackupTrustInfo, Curve25519AuthData, KeyBackupInfo } from "../crypto-api/keybackup";
+
+/**
+ * @internal
+ */
 export class RustBackupManager {
+    public constructor(private readonly olmMachine: OlmMachine) {}
+
     /**
      * Get the backup version we are currently backing up to, if any
      */
     public async getActiveBackupVersion(): Promise<string | null> {
         // TODO stub
         return null;
     }
+
+    /**
+     * Determine if a key backup can be trusted.
+     *
+     * @param info - key backup info dict from {@link MatrixClient#getKeyBackupVersion}.
+     */
+    public async isKeyBackupTrusted(info: KeyBackupInfo): Promise<BackupTrustInfo> {
+        const signatureVerification: SignatureVerification = await this.olmMachine.verifyBackup(info);
+
+        const backupKeys: RustSdkCryptoJs.BackupKeys = await this.olmMachine.getBackupKeys();
+        const pubKeyForSavedPrivateKey = backupKeys?.decryptionKey?.megolmV1PublicKey;
+        const backupMatchesSavedPrivateKey =
+            info.algorithm === pubKeyForSavedPrivateKey?.algorithm &&
+            (info.auth_data as Curve25519AuthData)?.public_key === pubKeyForSavedPrivateKey.publicKeyBase64;
+
+        return {
+            matchesDecryptionKey: backupMatchesSavedPrivateKey,
+            trusted: signatureVerification.trusted(),
+        };
+    }
 }

src/rust-crypto/rust-crypto.ts

@@ -31,6 +31,7 @@ import { OutgoingRequest, OutgoingRequestProcessor } from "./OutgoingRequestProc
 import { KeyClaimManager } from "./KeyClaimManager";
 import { MapWithDefault } from "../utils";
 import {
+    BackupTrustInfo,
     BootstrapCrossSigningOpts,
     CreateSecretStorageOpts,
     CrossSigningKey,
@@ -40,6 +41,7 @@ import {
     GeneratedSecretStorageKey,
     ImportRoomKeyProgressData,
     ImportRoomKeysOpts,
+    KeyBackupInfo,
     VerificationRequest,
     CrossSigningKeyInfo,
 } from "../crypto-api";
@@ -112,7 +114,7 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, RustCryptoEv
         this.outgoingRequestProcessor = new OutgoingRequestProcessor(olmMachine, http);
         this.keyClaimManager = new KeyClaimManager(olmMachine, this.outgoingRequestProcessor);
         this.eventDecryptor = new EventDecryptor(olmMachine);
-        this.backupManager = new RustBackupManager();
+        this.backupManager = new RustBackupManager(olmMachine);
 
         // Fire if the cross signing keys are imported from the secret storage
         const onCrossSigningKeysImport = (): void => {
@@ -766,8 +768,8 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, RustCryptoEv
      */
     public async getSessionBackupPrivateKey(): Promise<Uint8Array | null> {
         const backupKeys: RustSdkCryptoJs.BackupKeys = await this.olmMachine.getBackupKeys();
-        if (!backupKeys.decryptionKeyBase64) return null;
-        return Buffer.from(backupKeys.decryptionKeyBase64, "base64");
+        if (!backupKeys.decryptionKey) return null;
+        return Buffer.from(backupKeys.decryptionKey.toBase64(), "base64");
     }
 
     /**
@@ -793,6 +795,15 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, RustCryptoEv
         return await this.backupManager.getActiveBackupVersion();
     }
 
+    /**
+     * Determine if a key backup can be trusted.
+     *
+     * Implementation of {@link Crypto.CryptoApi.isKeyBackupTrusted}.
+     */
+    public async isKeyBackupTrusted(info: KeyBackupInfo): Promise<BackupTrustInfo> {
+        return await this.backupManager.isKeyBackupTrusted(info);
+    }
+
     ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////
     //
     // SyncCryptoCallbacks implementation

yarn.lock

@@ -1482,10 +1482,10 @@
     "@jridgewell/resolve-uri" "3.1.0"
     "@jridgewell/sourcemap-codec" "1.4.14"
 
-"@matrix-org/matrix-sdk-crypto-wasm@^1.1.0":
-  version "1.1.0"
-  resolved "https://registry.yarnpkg.com/@matrix-org/matrix-sdk-crypto-wasm/-/matrix-sdk-crypto-wasm-1.1.0.tgz#43996a2c5fc8786999eeaaf6df51007244f6b3c4"
-  integrity sha512-BSMYqXRgQOHG3N18z8b05x3UQcdLL3XDrxjtjjA88t9PadZ7RwNowLm1Sx3ESzdzRX+r1SEVAWs2JnTTs0rv3Q==
+"@matrix-org/matrix-sdk-crypto-wasm@^1.2.0":
+  version "1.2.0"
+  resolved "https://registry.yarnpkg.com/@matrix-org/matrix-sdk-crypto-wasm/-/matrix-sdk-crypto-wasm-1.2.0.tgz#115cd21cb2bba3c8166cf09e7d61da0902aa8973"
+  integrity sha512-vmpbtXYFzfBSFjeAx/PNRjy7zyH+Xx2HVXNKdApgo3+hSALewcXwdOTJy5pKq+poumM2TjjKDhG2s6/zSDNUYg==
 
 "@matrix-org/olm@https://gitlab.matrix.org/api/v4/projects/27/packages/npm/@matrix-org/olm/-/@matrix-org/olm-3.2.14.tgz":
   version "3.2.14"