feat(mnp): unified handshake with mutual authentication

Phase 11.5.4/5/7/8 — findings C6 (WebRTC half), C3, L4, M1, M9.

New meshbay_common/handshake.py is the single implementation of authorization
and proof: JWT verify, scope, denylist, mandatory group_id, membership, hosting.
The handshake previously existed three times over and only the newest copy
enforced the GEK proof.

C3 — mutual authentication. Authentication ran one way: the client proved
itself, the node proved nothing. handshake_ack.node_pk was never verified
against anything and per-chunk signatures had been dropped in Phase 9.15, so a
peer that had hijacked signaling (C2) or been substituted by the hub could
accept the client's proof, ignore it, and serve a forged index, forged chat
history and a forged is_node_admin flag. The client now sends a nonce; the node
answers with its own GEK proof over that nonce AND an Ed25519 signature over
the transcript; the browser verifies both and refuses otherwise. It also
refuses an unchallenged handshake_ack, which previously let a peer skip proving
anything at all.

L4 — the proof was nonce ‖ offer_fp ‖ answer_fp: bare concatenation, and a
missing fingerprint silently degraded it to nonce-only, dropping MitM detection
(NS5). Every field is now length-prefixed and domain-separated, the role is
bound so a client proof cannot be replayed as a node proof, and an absent
channel binding is refused rather than tolerated.

M1 — group_id was optional; omitting it skipped the membership check entirely
and fell back to the node's first group. Now mandatory.
M9 — node-scoped daemon tokens are refused on the client path.

NOT DONE: quic_server.py still runs its own JWT-only handshake, so C6 remains
open — a forged or stolen token reaches a node over QUIC and can inject chat
without holding the GEK. quic_binding() is written and unit-tested but unwired.
11.5.6 (whether the certificate-hash anchor works with aioquic, or an RFC 5705
exporter is reachable) is unproven. 11.5.8 TOFU pinning of pk_node is not done:
the client verifies the node's signature but does not yet remember which key it
saw last.

Adds packages/meshbay-common/tests/test_handshake.py (18 tests) covering the
properties every transport must inherit. WebRTC test helpers rewritten around
the shared module; _make_jwt now defaults to the test group, since group_id is
mandatory.

Tests: 24 webrtc, 176+ node+common.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/packages/meshbay-node/tests/test_security_regressions.py b/packages/meshbay-node/tests/test_security_regressions.py
index a6d69c6..9299bf4 100644
--- a/packages/meshbay-node/tests/test_security_regressions.py
+++ b/packages/meshbay-node/tests/test_security_regressions.py
@@ -460,10 +460,21 @@ def test_dead_gek_protocol_constants_removed():
 
 
 def test_peer_errors_do_not_leak_internals():
-    """L3: exception text carries filesystem paths and internal state."""
+    """
+    L3: arbitrary exception text carries filesystem paths and internal state, so
+    the catch-all handler must not relay it.
+
+    Deliberately narrow: HandshakeError messages ARE sent to the peer, because a
+    client needs to know why it was refused, and those strings are authored for
+    that purpose. The check targets the generic `except Exception as e` path.
+    """
     source = (Path(__file__).parent.parent / "src" / "meshbay_node"
               / "transport" / "webrtc_server.py").read_text()
-    assert '"detail": str(e)' not in source
+    assert '"detail": str(e)' not in source, (
+        "generic exception text relayed to peer — use a fixed message"
+    )
+    # And the catch-all must still exist, sending something opaque.
+    assert '"detail": "Request failed"' in source
 
 
 def test_pre_handshake_message_budget_is_small():