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diff --git a/packages/meshbay-hub/src/meshbay_hub/static/crypto.js b/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
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+/**
+ * MeshBay Browser Crypto — AES-256-GCM private group decryption.
+ * Uses WebCrypto SubtleCrypto API (available in all modern browsers).
+ *
+ * Handles groups with cipher="aes-256-gcm" (browser-accessible groups).
+ * ChaCha20-Poly1305 groups (cipher="chacha20-poly1305") require the
+ * native client (node) for decryption — not supported in browser.
+ *
+ * Usage:
+ *   const gek = await importGEK(gekB64);
+ *   const plaintext = await decryptChunk(gek, fileHashHex, chunkIndex, nonceB64, ctB64);
+ */
+
+const CIPHER_INFO_PREFIX = new TextEncoder().encode('file:');
+const CIPHER_INFO_SUFFIX_AES = new TextEncoder().encode(':aes');
+
+
+// ── Key derivation ────────────────────────────────────────────────────────────
+
+/**
+ * Import a raw GEK (base64) as a WebCrypto key for HKDF.
+ * @param {string} gekB64 - base64-encoded GEK (32 bytes)
+ * @returns {Promise<CryptoKey>}
+ */
+async function importGEK(gekB64) {
+  const raw = b64decode(gekB64);
+  return crypto.subtle.importKey('raw', raw, 'HKDF', false, ['deriveKey', 'deriveBits']);
+}
+
+/**
+ * Derive a per-chunk AES-256-GCM key from the GEK.
+ * Mirrors meshbay_common/webcrypto.py::chunk_key_aes().
+ *
+ * @param {CryptoKey} gek - HKDF key from importGEK()
+ * @param {string} fileHashHex - blake3 hash of file (hex, 64 chars)
+ * @param {number} chunkIndex
+ * @returns {Promise<CryptoKey>}
+ */
+async function deriveChunkKey(gek, fileHashHex, chunkIndex) {
+  // Build HKDF info: "file:" + file_hash_bytes + ":chunk:" + uint32be + ":aes"
+  const fileHashBytes = hexToBytes(fileHashHex);
+  const chunkIdxBytes = new Uint8Array(4);
+  new DataView(chunkIdxBytes.buffer).setUint32(0, chunkIndex, false);   // big-endian
+
+  const infoParts = [
+    new TextEncoder().encode('file:'),
+    fileHashBytes,
+    new TextEncoder().encode(':chunk:'),
+    chunkIdxBytes,
+    new TextEncoder().encode(':aes'),
+  ];
+  const info = concatBuffers(infoParts);
+
+  return crypto.subtle.deriveKey(
+    { name: 'HKDF', hash: 'SHA-256', salt: new Uint8Array(0), info },
+    gek,
+    { name: 'AES-GCM', length: 256 },
+    false,
+    ['decrypt'],
+  );
+}
+
+
+// ── 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);
+}
+
+
+// ── Helpers ───────────────────────────────────────────────────────────────────
+
+function b64decode(b64) {
+  const binary = atob(b64);
+  const bytes = new Uint8Array(binary.length);
+  for (let i = 0; i < binary.length; i++) bytes[i] = binary.charCodeAt(i);
+  return bytes;
+}
+
+function hexToBytes(hex) {
+  const bytes = new Uint8Array(hex.length / 2);
+  for (let i = 0; i < hex.length; i += 2)
+    bytes[i / 2] = parseInt(hex.substring(i, 2), 16);
+  return bytes;
+}
+
+function concatBuffers(arrays) {
+  const total = arrays.reduce((s, a) => s + a.byteLength, 0);
+  const result = new Uint8Array(total);
+  let offset = 0;
+  for (const arr of arrays) {
+    result.set(new Uint8Array(arr.buffer || arr), offset);
+    offset += arr.byteLength;
+  }
+  return result;
+}
+
+// Export for use in app.js
+window.MeshBayCrypto = { importGEK, deriveChunkKey, decryptChunk, decryptFile };