From e88bf72c734c986f07f417c557d3e0f1bb676ebb Mon Sep 17 00:00:00 2001
From: trevor-signal <131492920+trevor-signal@users.noreply.github.com>
Date: Thu, 2 Apr 2026 12:41:37 -0400
Subject: [PATCH] Fix local backup backupId iv and counter mechanism

---
 ts/services/backups/constants.std.ts         |  2 +-
 ts/services/backups/util/localBackup.node.ts | 20 ++++++++++++++++++--
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/ts/services/backups/constants.std.ts b/ts/services/backups/constants.std.ts
index 9d47df6ccd..94f39b41e9 100644
--- a/ts/services/backups/constants.std.ts
+++ b/ts/services/backups/constants.std.ts
@@ -8,7 +8,7 @@ export const BACKUP_VERSION = 1n;
 
 export const LOCAL_BACKUP_VERSION = 1;
 
-export const LOCAL_BACKUP_BACKUP_ID_IV_LENGTH = 16;
+export const LOCAL_BACKUP_BACKUP_ID_IV_LENGTH = 12;
 
 const { WallpaperPreset } = Backups.ChatStyle;
 
diff --git a/ts/services/backups/util/localBackup.node.ts b/ts/services/backups/util/localBackup.node.ts
index c87fbebc5a..cdaa0c7b92 100644
--- a/ts/services/backups/util/localBackup.node.ts
+++ b/ts/services/backups/util/localBackup.node.ts
@@ -212,13 +212,25 @@ export function getAttachmentLocalBackupPathFromSnapshotDir(
   );
 }
 
+function getBackupIdIvAndCounter({
+  iv,
+}: {
+  iv: Uint8Array<ArrayBuffer>;
+}): Uint8Array<ArrayBuffer> {
+  return Buffer.concat([iv, Buffer.alloc(4)]);
+}
+
 export async function writeLocalBackupMetadata({
   snapshotDir,
   backupId,
   metadataKey,
 }: LocalBackupMetadataVerificationType): Promise<void> {
   const iv = randomBytes(LOCAL_BACKUP_BACKUP_ID_IV_LENGTH);
-  const encryptedId = encryptAesCtr(metadataKey, backupId, iv);
+  const encryptedId = encryptAesCtr(
+    metadataKey,
+    backupId,
+    getBackupIdIvAndCounter({ iv })
+  );
 
   const metadataSerialized = Signal.backup.local.Metadata.encode({
     backupId: {
@@ -257,7 +269,11 @@ export async function verifyLocalBackupMetadata({
     'verifyLocalBackupMetadata: Must have backupId.encryptedId'
   );
 
-  const localBackupBackupId = decryptAesCtr(metadataKey, encryptedId, iv);
+  const localBackupBackupId = decryptAesCtr(
+    metadataKey,
+    encryptedId,
+    getBackupIdIvAndCounter({ iv })
+  );
   strictAssert(
     Bytes.areEqual(backupId, localBackupBackupId),
     'verifyLocalBackupMetadata: backupId must match the local backup backupId'