Fix base64 encoding crash on file attachments over ~1MB

String.fromCharCode(...bytes) spreads every byte as a separate call
argument, exceeding the engine's max-arguments limit on multi-MB files
(RangeError: Maximum call stack size exceeded). Chunk the conversion
instead. Reported by a real submitter trying a 1.5MB file.

1 files changed, 14 insertions, 1 deletions

diff --git a/static/crypto.js b/static/crypto.js
index 04a1400..f1cc6c9 100644
--- a/static/crypto.js
+++ b/static/crypto.js
@@ -8,6 +8,19 @@ const NYMDROP_PUBKEY_HEX = "6aa99a672c48cb8eee65ddd5c5e5f8947a8d52d39a8b8eb4e11b
 // server's --pow-difficulty flag.
 const NYMDROP_POW_DIFFICULTY = 18;
 
+// String.fromCharCode(...bytes) blows the call stack on large files: the
+// spread passes every byte as a separate function argument, and engines cap
+// argument count well below what a multi-MB file needs. Chunking keeps each
+// call under that limit.
+function bytesToBase64(bytes) {
+    const chunkSize = 0x8000;
+    let binary = "";
+    for (let i = 0; i < bytes.length; i += chunkSize) {
+        binary += String.fromCharCode.apply(null, bytes.subarray(i, i + chunkSize));
+    }
+    return btoa(binary);
+}
+
 async function hexToBytes(hex) {
     const bytes = new Uint8Array(hex.length / 2);
     for (let i = 0; i < hex.length; i += 2)
@@ -126,7 +139,7 @@ document.getElementById("drop-form").addEventListener("submit", async (e) => {
         const fileInput = document.getElementById("file");
         if (fileInput.files.length > 0) {
             const fileBytes = await fileInput.files[0].arrayBuffer();
-            const fileB64 = btoa(String.fromCharCode(...new Uint8Array(fileBytes)));
+            const fileB64 = bytesToBase64(new Uint8Array(fileBytes));
             plaintext += "\n---FILE:" + fileInput.files[0].name + "---\n" + fileB64;
         }