Backup encryption and improvements

ts/util/appendMacStream.ts

@@ -0,0 +1,37 @@
+// Copyright 2024 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { createHmac } from 'crypto';
+import { Transform } from 'stream';
+import type { Duplex } from 'stream';
+
+import { HashType } from '../types/Crypto';
+
+export const MAC_KEY_SIZE = 32;
+
+export const MAC_SIZE = 32;
+
+export function appendMacStream(macKey: Uint8Array): Duplex {
+  if (macKey.byteLength !== MAC_KEY_SIZE) {
+    throw new Error('appendMacStream: invalid macKey length');
+  }
+
+  const hmac = createHmac(HashType.size256, macKey);
+  return new Transform({
+    transform(chunk, _encoding, callback) {
+      try {
+        hmac.update(chunk);
+        callback(null, chunk);
+      } catch (error) {
+        callback(error);
+      }
+    },
+    flush(callback) {
+      try {
+        callback(null, hmac.digest());
+      } catch (error) {
+        callback(error);
+      }
+    },
+  });
+}