New attachment storage system

ts/util/getMacAndUpdateHmac.ts

@@ -0,0 +1,52 @@
+// Copyright 2024 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { Buffer } from 'node:buffer';
+import { type Hmac } from 'node:crypto';
+import { Transform } from 'node:stream';
+
+import { MAC_LENGTH } from '../types/Crypto';
+
+/**
+ * Updates an hmac with the stream except for the last MAC_LENGTH
+ * bytes. The last MAC_LENGTH bytes are passed to the callback.
+ */
+export function getMacAndUpdateHmac(
+  hmac: Hmac,
+  onTheirMac: (theirMac: Uint8Array) => void
+): Transform {
+  // Because we don't have a view of the entire stream, we don't know when we're
+  // at the end. We need to omit the last MAC_LENGTH bytes from
+  // `hmac.update` so we only push what we know is not the mac.
+  let maybeMacBytes = Buffer.alloc(0);
+
+  function updateWithKnownNonMacBytes() {
+    let knownNonMacBytes = null;
+    if (maybeMacBytes.byteLength > MAC_LENGTH) {
+      knownNonMacBytes = maybeMacBytes.subarray(0, -MAC_LENGTH);
+      maybeMacBytes = maybeMacBytes.subarray(-MAC_LENGTH);
+      hmac.update(knownNonMacBytes);
+    }
+    return knownNonMacBytes;
+  }
+
+  return new Transform({
+    transform(chunk, _encoding, callback) {
+      try {
+        maybeMacBytes = Buffer.concat([maybeMacBytes, chunk]);
+        const knownNonMac = updateWithKnownNonMacBytes();
+        callback(null, knownNonMac);
+      } catch (error) {
+        callback(error);
+      }
+    },
+    flush(callback) {
+      try {
+        onTheirMac(maybeMacBytes);
+        callback(null, null);
+      } catch (error) {
+        callback(error);
+      }
+    },
+  });
+}