refactor(hub): drop the base64 chunk fallback and the dead HTTP file client

With MNP 0.15 no node can emit a base64 `file_chunk`, so the browser's fallback
for that shape is unreachable. Three things go with it:

- `file-utils.js` kept a third branch below the fallback that base64-decoded
  `chunkMsg.ct_b64 || chunkMsg.data_b64` when neither was present, i.e. decoded
  `undefined` and wrote the result into the file the user was saving. A chunk we
  cannot decrypt now stops the download with an error naming the file and
  suggesting the node is older than the page. Deliberately not in
  `_isRetryableTransportError`: this is a version mismatch, not a bad moment on
  the link.
- `crypto.js` `decryptChunk` (base64) was the real path until Phase 9.15 and has
  had no caller since.
- `crypto.js` `decryptFile` was never called in any commit. It fetched
  `${nodeUrl}/file/${id}/${chunk}?token=` in a loop — the node's unauthenticated
  HTTP file API, which is finding C1 and was deleted in Phase 11.5. A client for
  an endpoint that no longer exists, kept alive by being exported.

`decryptChunkBin` — every file download and every video segment — is untouched.

`packages/meshbay-client/ui/` was resynchronised with `npm run sync-ui`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

2 files changed, 29 insertions, 78 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/crypto.js b/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
index 1ddaa50..69b8c7a 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
@@ -8,7 +8,7 @@
  *
  * Usage:
  *   const gek = await importGEK(gekB64);
- *   const plaintext = await decryptChunk(gek, fileHashHex, chunkIndex, nonceB64, ctB64);
+ *   const plaintext = await decryptChunkBin(gek, fileHashHex, chunkIndex, nonce, ct);
  */
 
 const CIPHER_INFO_PREFIX = new TextEncoder().encode('file:');
@@ -63,65 +63,16 @@ async function deriveChunkKey(gek, fileHashHex, chunkIndex) {
 
 // ── Decryption ────────────────────────────────────────────────────────────────
 
-/**
- * Decrypt one chunk of a private group file.
- * @param {CryptoKey} gek - from importGEK()
- * @param {string} fileHashHex
- * @param {number} chunkIndex
- * @param {string} nonceB64 - 12-byte nonce, base64
- * @param {string} ctB64 - ciphertext + GCM tag, base64
- * @returns {Promise<Uint8Array>} plaintext
- */
-async function decryptChunk(gek, fileHashHex, chunkIndex, nonceB64, ctB64) {
-  const chunkKey = await deriveChunkKey(gek, fileHashHex, chunkIndex);
-  const nonce = b64decode(nonceB64);
-  const ct    = b64decode(ctB64);
-  const plaintext = await crypto.subtle.decrypt(
-    { name: 'AES-GCM', iv: nonce },
-    chunkKey,
-    ct,
-  );
-  return new Uint8Array(plaintext);
-}
-
-/**
- * Decrypt a full file by fetching and decrypting all chunks in order.
- * @param {CryptoKey} gek
- * @param {string} nodeUrl - base URL of node HTTP API
- * @param {string} fileId - blake3 hex hash (= file_id in index)
- * @param {string} fileHashHex - same as fileId (blake3 of file content)
- * @param {string} jwtToken
- * @returns {Promise<Blob>} decrypted file as Blob
- */
-async function decryptFile(gek, nodeUrl, fileId, fileHashHex, jwtToken) {
-  const chunks = [];
-  let chunkIdx = 0;
-
-  while (true) {
-    const sep = nodeUrl.includes('?') ? '&' : '?';
-    const url = `${nodeUrl}/file/${fileId}/${chunkIdx}${sep}token=${jwtToken}`;
-    const resp = await fetch(url);
-    if (!resp.ok) break;
-
-    const data = await resp.json();
-    if (data.encrypted === false) {
-      // Public group: data_b64 is plaintext
-      chunks.push(b64decode(data.data_b64));
-    } else {
-      // Private group with AES-256-GCM
-      const plain = await decryptChunk(
-        gek, fileHashHex, chunkIdx,
-        data.nonce_b64, data.ct_b64,
-      );
-      chunks.push(plain);
-    }
-
-    if (data.plaintext_size < 1024 * 1024) break;   // last chunk (< 1MB)
-    chunkIdx++;
-  }
-
-  return new Blob(chunks);
-}
+// `decryptChunk` (base64) and `decryptFile` were removed on 2026-09-03.
+//
+// `decryptChunk` was the real path until Phase 9.15 moved chunks to a binary wire
+// format; `decryptChunkBin` below replaced it, and MNP 0.15 removed the last node
+// that could still emit the base64 shape (the QUIC encoder, left behind by 9.15).
+//
+// `decryptFile` was never called by anything, in any commit: it fetched
+// `${nodeUrl}/file/${id}/${chunk}?token=` in a loop — the node's unauthenticated
+// HTTP file API, which is finding C1 and was deleted in Phase 11.5. A client for an
+// endpoint that no longer exists, kept alive only by being exported.
 
 
 // ── Helpers ───────────────────────────────────────────────────────────────────
@@ -420,7 +371,7 @@ async function verifyNodeSignature(nodePkB64, sigB64, transcript) {
 
 // Export for use in app.js
 window.MeshBayCrypto = {
-  importGEK, deriveChunkKey, decryptChunk, decryptChunkBin, decryptFile,
+  importGEK, deriveChunkKey, decryptChunkBin,
   generateGEK, wrapGEK, unwrapGEK, encryptChunk, b64encode, b64decode,
   adminTranscript, handshakeTranscript, handshakeProof, webrtcBinding,
   joinTranscript, verifyNodeSignature, constantTimeEqual,
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/file-utils.js b/packages/meshbay-hub/src/meshbay_hub/static/file-utils.js
index ba76ac9..09ee6c9 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/file-utils.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/file-utils.js
@@ -110,13 +110,6 @@ function _saveBlob(blob, filename) {
   URL.revokeObjectURL(url);
 }
 
-function _b64ToU8(b64) {
-  const bin = atob(b64);
-  const arr = new Uint8Array(bin.length);
-  for (let i = 0; i < bin.length; i++) arr[i] = bin.charCodeAt(i);
-  return arr;
-}
-
 // A dead transport (screen-lock WebRTC failure, see transport.js's
 // _reconnectLoop) surfaces here as a rejected fetchChunk — TransportLostError
 // when the pending request was killed outright, a plain timeout if it was
@@ -173,16 +166,23 @@ async function pipelinedDownload(transport, gekKey, fileId, totalChunks, onChunk
       throw err;
     }
     const chunkMsg = await inflight[nextRecv];
-    let plaintext;
-    if (gekKey && chunkMsg.ct) {
-      plaintext = await window.MeshBayCrypto.decryptChunkBin(
-        gekKey, fileId, nextRecv, chunkMsg.nonce, chunkMsg.ct);
-    } else if (gekKey && chunkMsg.ct_b64) {
-      plaintext = await window.MeshBayCrypto.decryptChunk(
-        gekKey, fileId, nextRecv, chunkMsg.nonce_b64, chunkMsg.ct_b64);
-    } else {
-      plaintext = _b64ToU8(chunkMsg.ct_b64 || chunkMsg.data_b64);
+    // One shape, and a refusal for anything else. There used to be two fallbacks
+    // below this: a base64 `ct_b64` chunk, which was the real wire format until
+    // the binary switch in Phase 9.15 and which no node has sent since, and a
+    // plaintext branch that base64-decoded `undefined` when neither field was
+    // there. That last one is why this now throws: a chunk we cannot decrypt has
+    // to stop the download, not write whatever it decoded into the file the user
+    // is saving. MNP 0.15 removed the last producer of the base64 shape (it was
+    // still emitted on the QUIC transport), so a node that sends one is a node
+    // older than the SPA serving this page.
+    if (!gekKey || !chunkMsg.ct) {
+      throw new Error(
+        `Chunk ${nextRecv} of ${fileId.slice(0, 8)} cannot be decrypted `
+        + `(${gekKey ? 'unexpected chunk format' : 'no group key'}) — `
+        + 'the node may be running an older version.');
     }
+    const plaintext = await window.MeshBayCrypto.decryptChunkBin(
+      gekKey, fileId, nextRecv, chunkMsg.nonce, chunkMsg.ct);
     if (writable) {
       await writable.write(plaintext);
     } else {
@@ -328,6 +328,6 @@ async function downloadDirectory(transfers, transport, gek, entries, dir, { setE
 export {
   FILE_ICONS,
   formatSize, formatDate, PREVIEWABLE_TEXT, canPreview, CHUNK_SIZE,
-  _openDownloadTarget, _saveBlob, _b64ToU8, pipelinedDownload, downloadEntry,
+  _openDownloadTarget, _saveBlob, pipelinedDownload, downloadEntry,
   downloadDirectory,
 };