crypto/machine: add function to download and store latest key backup

Signed-off-by: Sumner Evans <sumner@beeper.com>

crypto/machine.go

@@ -688,3 +688,43 @@ func (mach *OlmMachine) ExpiredKeyDeleteLoop(ctx context.Context) {
 		}
 	}
 }
+
+func (mach *OlmMachine) DownloadAndStoreLatestKeyBackup(ctx context.Context) error {
+	log := mach.machOrContextLog(ctx).With().
+		Str("action", "download and store latest key backup").
+		Logger()
+	versionInfo, err := mach.Client.GetKeyBackupLatestVersion(ctx)
+	if err != nil {
+		return err
+	}
+
+	if versionInfo.Algorithm != id.KeyBackupAlgorithmCurve25519AESSHA2 {
+		return fmt.Errorf("unsupported key backup algorithm: %s", versionInfo.Algorithm)
+	}
+
+	log = log.With().
+		Int("count", versionInfo.Count).
+		Str("etag", versionInfo.ETag).
+		Str("key_backup_version", versionInfo.Version).
+		Logger()
+
+	if versionInfo.Count == 0 {
+		log.Debug().Msg("No keys found in key backup")
+		return nil
+	}
+
+	keys, err := mach.Client.GetKeyBackup(ctx, versionInfo.Version)
+	if err != nil {
+		return err
+	}
+
+	for roomID, backup := range keys.Rooms {
+		fmt.Printf("Sessions for %s\n", roomID)
+		for sessionID, keyBackupData := range backup.Sessions {
+			// TODO
+			fmt.Printf("\t%s: %v\n", sessionID, keyBackupData)
+		}
+	}
+
+	return nil
+}