merge: Phase 11.5 security remediation, invite redesign, per-node identity

Brings in the security remediation branch. Three bodies of work, and what they
changed about what this project may claim.

Phase 11.5 closed the gap between the documents and the code: the unauthenticated
node HTTP API and the TCP transport deleted, one handshake shared by the
remaining two transports, mutual authentication, structured admin transcripts,
upload confinement, group isolation, revocation that reaches nodes. Six critical
and seven high findings closed, bounded, or deferred by decision.

The invite redesign closed H3 and M3 — the last open High. The hub was the key
directory: an inviter fetched the invitee's key from it and wrapped the group key
for whatever came back, so a hub answering with its own key was handed the group
key by an honest member following the protocol exactly. That lookup is gone. The
node holds the group key and wraps it itself, for a key its recipient proves
possession of, bound to an account by a one-time code the hub never sees. M3 fell
out of the same work: node authority comes from a local roster, never from the
hub.

Per-node identity cut what remains of C4 down to one operator. A single keypair
used to be copied to every node its owner joined; each node now gets its own, so
cracking the bundle on one machine yields a key that is a stranger everywhere
else — and on that machine, one that unlocks nothing its holder did not already
serve. The bundle KDF moved to Argon2id 128 MB, and the hub stopped storing or
publishing user keys at all.

What this project may now say: the hub cannot read your content unless it ships
you malicious client code. T3 remains, accepted (D1), and is what the native
client removes. C4 is reduced, not closed, until 13.3. Chat is still plaintext at
rest until Phase 15. Draft-v5 §2 states each claim against the adversary it holds
against, which is the convention this branch exists to keep.

Four defects were found by deploying it and using a browser, none by the test
suite: a node going deaf on its hub socket, a token that predated group
membership, a client reading values before they were assigned, and identity keys
a browser held but never re-read. The lessons are recorded in CLAUDE.md.

Tests: 343 across the three packages, plus QE/deploy/e2e.py — register, pair,
invite, join, download, stream, second browser, revoke — run against the live
deployment on a wiped hub and node.

1 files changed, 118 insertions, 9 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/crypto.js b/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
index 5ebf624..21bf05d 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
@@ -230,24 +230,133 @@ function b64encode(bytes) {
   return btoa(String.fromCharCode(...bytes));
 }
 
+// ── Admin operation transcript ───────────────────────────────────────────────
+// Mirrors meshbay_common/adminop.py::admin_transcript(). Both sides build these
+// bytes independently; they are never taken off the wire.
+//
+// Finding H5: the client used to sign 32 raw random bytes chosen by the node — a
+// blind signing oracle. It now reconstructs a domain-separated, length-prefixed
+// transcript naming the operation, subject, node and group, so the UI can show the
+// user what they are authorizing and a signature cannot be reused elsewhere.
+
+const ADMIN_TRANSCRIPT_PREFIX = new TextEncoder().encode('meshbay:admin:v1');
+
+function adminTranscript(op, nodePkB64, groupId, subject, nonceB64, ts) {
+  const enc = new TextEncoder();
+  const fields = [
+    enc.encode(op),
+    enc.encode(nodePkB64),
+    enc.encode(groupId),
+    enc.encode(subject),
+    b64decode(nonceB64),
+    enc.encode(String(ts)),
+  ];
+  let total = ADMIN_TRANSCRIPT_PREFIX.length;
+  for (const f of fields) total += 4 + f.length;
+
+  const out = new Uint8Array(total);
+  out.set(ADMIN_TRANSCRIPT_PREFIX, 0);
+  let off = ADMIN_TRANSCRIPT_PREFIX.length;
+  for (const f of fields) {
+    new DataView(out.buffer).setUint32(off, f.length, false);
+    off += 4;
+    out.set(f, off);
+    off += f.length;
+  }
+  return out;
+}
+
 // ── GEK proof (HMAC-SHA256 for handshake challenge) ─────────────────────────
 
-async function hmacGEK(gekRaw, nonceB64, offerFp, answerFp) {
-  const nonce = b64decode(nonceB64);
-  const data = concatBuffers([
-    nonce,
-    offerFp || new Uint8Array(0),
-    answerFp || new Uint8Array(0),
+// Mirrors meshbay_common/handshake.py. Every field length-prefixed and the role
+// bound in, so a client proof can never be replayed as a node proof and a missing
+// fingerprint cannot silently degrade the proof to nonce-only (L4).
+const HANDSHAKE_PREFIX = new TextEncoder().encode('meshbay:mnp:handshake:v1');
+
+function _lenPrefixed(parts) {
+  let total = 0;
+  for (const p of parts) total += 4 + p.length;
+  const out = new Uint8Array(total);
+  const view = new DataView(out.buffer);
+  let off = 0;
+  for (const p of parts) {
+    view.setUint32(off, p.length, false);
+    off += 4;
+    out.set(p, off);
+    off += p.length;
+  }
+  return out;
+}
+
+function webrtcBinding(offerFp, answerFp) {
+  if (!offerFp || !offerFp.length || !answerFp || !answerFp.length) {
+    throw new Error('Channel binding unavailable — refusing to handshake');
+  }
+  return _lenPrefixed([offerFp, answerFp]);
+}
+
+function handshakeTranscript(role, groupId, nonceClient, nonceNode, binding) {
+  const enc = new TextEncoder();
+  const body = _lenPrefixed([
+    enc.encode(role), enc.encode(groupId), nonceClient, nonceNode, binding,
   ]);
+  const out = new Uint8Array(HANDSHAKE_PREFIX.length + body.length);
+  out.set(HANDSHAKE_PREFIX, 0);
+  out.set(body, HANDSHAKE_PREFIX.length);
+  return out;
+}
+
+async function handshakeProof(gekRaw, role, groupId, nonceClient, nonceNode, binding) {
+  const transcript = handshakeTranscript(role, groupId, nonceClient, nonceNode, binding);
   const key = await crypto.subtle.importKey(
     'raw', gekRaw, { name: 'HMAC', hash: 'SHA-256' }, false, ['sign']);
-  const sig = await crypto.subtle.sign('HMAC', key, data);
-  return b64encode(new Uint8Array(sig));
+  const sig = await crypto.subtle.sign('HMAC', key, transcript);
+  return new Uint8Array(sig);
+}
+
+// ── Join / pairing transcript ───────────────────────────────────────────────
+
+// Mirrors meshbay_common/join.py. Signing both of our public keys together binds
+// the X25519 key to the Ed25519 identity the node pins, so the node can safely
+// wrap the group key for a key that came over the wire instead of one fetched
+// from the hub's directory (H3). nonce_node ties it to this connection.
+const JOIN_PREFIX = new TextEncoder().encode('meshbay:join:v1');
+
+function joinTranscript(nodePkB64, groupId, userId, pkEdB64, pkXB64, nonceNode, ts) {
+  const enc = new TextEncoder();
+  const body = _lenPrefixed([
+    enc.encode(nodePkB64),
+    enc.encode(groupId),
+    enc.encode(userId),
+    enc.encode(pkEdB64),
+    enc.encode(pkXB64),
+    nonceNode,
+    enc.encode(String(ts)),
+  ]);
+  const out = new Uint8Array(JOIN_PREFIX.length + body.length);
+  out.set(JOIN_PREFIX, 0);
+  out.set(body, JOIN_PREFIX.length);
+  return out;
+}
+
+function constantTimeEqual(a, b) {
+  if (a.length !== b.length) return false;
+  let diff = 0;
+  for (let i = 0; i < a.length; i++) diff |= a[i] ^ b[i];
+  return diff === 0;
+}
+
+/** Verify the node's Ed25519 signature over the handshake transcript (C3). */
+async function verifyNodeSignature(nodePkB64, sigB64, transcript) {
+  const raw = b64decode(nodePkB64);
+  const key = await crypto.subtle.importKey('raw', raw, { name: 'Ed25519' }, false, ['verify']);
+  return crypto.subtle.verify('Ed25519', key, b64decode(sigB64), transcript);
 }
 
 // Export for use in app.js
 window.MeshBayCrypto = {
   importGEK, deriveChunkKey, decryptChunk, decryptChunkBin, decryptFile,
   generateGEK, wrapGEK, unwrapGEK, encryptChunk, b64encode, b64decode,
-  hmacGEK,
+  adminTranscript, handshakeTranscript, handshakeProof, webrtcBinding,
+  joinTranscript, verifyNodeSignature, constantTimeEqual,
 };