Save the key backup key to 4S during bootstrapCrossSigning

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

@@ -705,6 +705,47 @@
         expect(resetKeyBackup.mock.calls).toHaveLength(2);
     });
 
+    it("bootstrapSecretStorage saves megolm backup key if already cached", async () => {
+        const secretStorageCallbacks = {
+            getSecretStorageKey: async (keys: any, name: string) => {
+                return [[...Object.keys(keys.keys)][0], new Uint8Array(32)];
+            },
+        } as SecretStorageCallbacks;
+        const secretStorage = new ServerSideSecretStorageImpl(new DummyAccountDataClient(), secretStorageCallbacks);
+
+        const rustCrypto = await makeTestRustCrypto(
+            {
+                authedRequest: jest.fn().mockImplementation((method, query) => {
+                    if (query === "/room_keys/version") {
+                        return Promise.resolve({ version: 1, algorithm: "dummy", auth_data: {} });
+                    }
+                    return Promise.resolve({});
+                }),
+            } as unknown as MatrixHttpApi<any>,
+            testData.TEST_USER_ID,
+            undefined,
+            secretStorage,
+        );
+
+        async function createSecretStorageKey() {
+            return {
+                keyInfo: {} as AddSecretStorageKeyOpts,
+                privateKey: new Uint8Array(32),
+            };
+        }
+
+        jest.spyOn(rustCrypto, "getSessionBackupPrivateKey").mockResolvedValue(new Uint8Array(32));
+        const storeSpy = jest.spyOn(secretStorage, "store");
+
+        await rustCrypto.bootstrapSecretStorage({
+            createSecretStorageKey,
+            setupNewSecretStorage: true,
+            setupNewKeyBackup: false,
+        });
+
+        expect(storeSpy).toHaveBeenCalledWith("m.megolm_backup.v1", expect.anything());
+    });
+
     it("isSecretStorageReady", async () => {
         const mockSecretStorage = {
             getDefaultKeyId: jest.fn().mockResolvedValue(null),

src/rust-crypto/rust-crypto.ts

@@ -841,7 +841,20 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, CryptoEventH
             await this.secretStorage.store("m.cross_signing.self_signing", crossSigningPrivateKeys.self_signing_key);
         }
 
-        if (setupNewKeyBackup) {
+        // likewise with the key backup key: if we have one, store it in secret storage (if it's not already there)
+        // also don't bother storing it if we're about to set up a new backup
+        if (!setupNewKeyBackup) {
+            const backupKeys: RustSdkCryptoJs.BackupKeys = await this.olmMachine.getBackupKeys();
+            if (backupKeys.decryptionKey) {
+                const keyBackupKeyInStorage = await secretStorageCanAccessSecrets(this.secretStorage, [
+                    "m.megolm_backup.v1",
+                ]);
+
+                if (!keyBackupKeyInStorage) {
+                    await this.secretStorage.store("m.megolm_backup.v1", backupKeys.decryptionKey.toBase64());
+                }
+            }
+        } else {
             await this.resetKeyBackup();
         }
     }