Backup encryption and improvements

ts/test-both/util/prependStream_test.ts

@@ -0,0 +1,21 @@
+// Copyright 2024 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { assert } from 'chai';
+
+import { prependStream } from '../../util/prependStream';
+
+describe('prependStream', () => {
+  it('should prepend stream with a prefix', async () => {
+    const stream = prependStream(Buffer.from('prefix:'));
+    stream.end('hello');
+
+    const chunks = new Array<Buffer>();
+    for await (const chunk of stream) {
+      chunks.push(chunk);
+    }
+
+    const buf = Buffer.concat(chunks);
+    assert.strictEqual(buf.toString(), 'prefix:hello');
+  });
+});