Ensure attachments are re-encryptable to same digest

ts/util/trimPadding.ts

@@ -2,25 +2,51 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 
 import { Transform } from 'node:stream';
+import { strictAssert } from './assert';
 
 /**
- * Truncates the stream to the target size.
+ * Truncates the stream to the target size and analyzes padding type.
  */
-export function trimPadding(size: number): Transform {
+export function trimPadding(
+  size: number,
+  onPaddingAnalyzed: ({
+    isPaddingAllZeros,
+  }: {
+    isPaddingAllZeros: boolean;
+  }) => void
+): Transform {
   let total = 0;
+  let seenNonZeroPadding = false;
   return new Transform({
     transform(chunk, _encoding, callback) {
+      strictAssert(chunk instanceof Uint8Array, 'chunk must be Uint8Array');
       const chunkSize = chunk.byteLength;
       const sizeLeft = size - total;
+      let paddingInThisChunk: Uint8Array | undefined;
       if (sizeLeft >= chunkSize) {
         total += chunkSize;
         callback(null, chunk);
       } else if (sizeLeft > 0) {
         total += sizeLeft;
-        callback(null, chunk.subarray(0, sizeLeft));
+        const data = chunk.subarray(0, sizeLeft);
+        paddingInThisChunk = chunk.subarray(sizeLeft);
+        callback(null, data);
       } else {
+        paddingInThisChunk = chunk;
         callback(null, null);
       }
+
+      if (
+        paddingInThisChunk &&
+        !seenNonZeroPadding &&
+        !paddingInThisChunk.every(el => el === 0)
+      ) {
+        seenNonZeroPadding = true;
+      }
+    },
+    flush(callback) {
+      onPaddingAnalyzed({ isPaddingAllZeros: !seenNonZeroPadding });
+      callback();
     },
   });
 }