From db068a47b968766b67ff0226a68cd5de1f9ee16f Mon Sep 17 00:00:00 2001
From: Florian Duros <florian.duros@ormaz.fr>
Date: Thu, 7 Nov 2024 15:11:10 +0100
Subject: [PATCH] Get backup info from a specific version

---
 spec/integ/crypto/megolm-backup.spec.ts |  4 ++++
 src/rust-crypto/backup.ts               | 23 ++++++++++++++++++-----
 src/rust-crypto/rust-crypto.ts          | 16 +++++++++++++++-
 3 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/spec/integ/crypto/megolm-backup.spec.ts b/spec/integ/crypto/megolm-backup.spec.ts
index 94d98b4cef..e0a69e94bc 100644
--- a/spec/integ/crypto/megolm-backup.spec.ts
+++ b/spec/integ/crypto/megolm-backup.spec.ts
@@ -317,6 +317,10 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("megolm-keys backup (%s)", (backe
 
         beforeEach(async () => {
             fetchMock.get("path:/_matrix/client/v3/room_keys/version", testData.SIGNED_BACKUP_DATA);
+            fetchMock.get(
+                `path:/_matrix/client/v3/room_keys/version/${testData.SIGNED_BACKUP_DATA.version}`,
+                testData.SIGNED_BACKUP_DATA,
+            );
 
             aliceClient = await initTestClient();
             aliceCrypto = aliceClient.getCrypto()!;
diff --git a/src/rust-crypto/backup.ts b/src/rust-crypto/backup.ts
index 35b6d491ab..2781f42551 100644
--- a/src/rust-crypto/backup.ts
+++ b/src/rust-crypto/backup.ts
@@ -502,12 +502,14 @@ export class RustBackupManager extends TypedEventEmitter<RustBackupCryptoEvents,
     }
 
     /**
-     * Get information about the current key backup from the server
-     *
+     * Get information about a key backup from the server
+     * - If version is provided, get information about that backup version.
+     * - If no version is provided, get information about the latest backup.
+     * @param version -
      * @returns Information object from API or null if there is no active backup.
      */
-    private async requestKeyBackupVersion(): Promise<KeyBackupInfo | null> {
-        return await requestKeyBackupVersion(this.http);
+    public async requestKeyBackupVersion(version?: string): Promise<KeyBackupInfo | null> {
+        return await requestKeyBackupVersion(this.http, version);
     }
 
     /**
@@ -798,11 +800,22 @@ export class RustBackupDecryptor implements BackupDecryptor {
     }
 }
 
+/**
+ * Fetch a key backup info from the server.
+ * - If `version` is provided call GET /room_keys/version/$version and get the backup info for that version.
+ * https://spec.matrix.org/latest/client-server-api/#get_matrixclientv3room_keysversionversion
+ * - If not, call GET /room_keys/version and get the latest backup info.
+ * https://spec.matrix.org/latest/client-server-api/#get_matrixclientv3room_keysversion
+ * @param http
+ * @param version - the specific version of the backup info to fetch
+ */
 export async function requestKeyBackupVersion(
     http: MatrixHttpApi<IHttpOpts & { onlyData: true }>,
+    version?: string,
 ): Promise<KeyBackupInfo | null> {
     try {
-        return await http.authedRequest<KeyBackupInfo>(Method.Get, "/room_keys/version", undefined, undefined, {
+        const path = version ? encodeUri("/room_keys/version/$version", { $version: version }) : "/room_keys/version";
+        return await http.authedRequest<KeyBackupInfo>(Method.Get, path, undefined, undefined, {
             prefix: ClientPrefix.V3,
         });
     } catch (e) {
diff --git a/src/rust-crypto/rust-crypto.ts b/src/rust-crypto/rust-crypto.ts
index 6725e44feb..aadd7f9397 100644
--- a/src/rust-crypto/rust-crypto.ts
+++ b/src/rust-crypto/rust-crypto.ts
@@ -1181,6 +1181,15 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, CryptoEventH
         return Buffer.from(backupKeys.decryptionKey.toBase64(), "base64");
     }
 
+    /**
+     * Fetch the backup version we have saved in our store.
+     * @returns the backup version, if any, or null
+     */
+    private async getSessionBackupVersion(): Promise<string | null> {
+        const backupKeys: RustSdkCryptoJs.BackupKeys = await this.olmMachine.getBackupKeys();
+        return backupKeys.backupVersion || null;
+    }
+
     /**
      * Store the backup decryption key.
      *
@@ -1325,10 +1334,15 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, CryptoEventH
      * Implementation of {@link CryptoApi#restoreKeyBackup}.
      */
     public async restoreKeyBackup(opts?: KeyBackupRestoreOpts): Promise<KeyBackupRestoreResult> {
+        // Get the decryption key from the cache
         const privateKeyFromCache = await this.getSessionBackupPrivateKey();
         if (!privateKeyFromCache) throw new Error("No decryption key found in cache");
 
-        const backupInfo = await this.backupManager.getServerBackupInfo();
+        // Get the backup version from the cache
+        const backupVersion = await this.getSessionBackupVersion();
+        if (!backupVersion) throw new Error("No backup version found in cache");
+
+        const backupInfo = await this.backupManager.requestKeyBackupVersion(backupVersion);
         if (!backupInfo?.version) throw new Error("Missing version in backup info");
 
         const backupDecryptor = await this.getBackupDecryptor(backupInfo, privateKeyFromCache);