fix(hub): present a node-audience token in the handshake, not the hub session token

A member authenticated to a node in the MNP handshake with its hub *session*
token — scope=user, valid at the hub API for hours. A node operator is in the
threat model, so this handed them a live hub credential for the member: enough
to enumerate the member's other groups, act as them, and (before the previous
commit closed it) take the account over. The node genuinely needs a hub-signed
membership assertion, so the fix is to make that a separate credential that
opens nothing at the hub API.

Two audiences signed by the one hub key (meshbay_common/tokens.py):
- HUB_API_AUD — session tokens (login, device-auth, node-auth, refresh), used
  for hub calls and signaling. decode_access_token now binds this audience, so
  an MNP token cannot be replayed against the hub API.
- MNP_AUD — a short-lived token a member presents to a node and nothing else,
  from POST /v1/nodes/mnp-token. authorize_token now binds this audience, so a
  session token presented to a node is refused. This closes the disclosure.

The node's own self-decode (hub_client.py) reads its node token with
audience=HUB_API_AUD. The client fetches the MNP token inside transport.connect()
(and on every reconnect) using the session token, so callers are unchanged and
signaling keeps using the session token.

No regression to a long session: the MNP token is checked once, at the
handshake, before any proof — a film already playing is not re-authenticated, so
a 15-minute token does not interrupt a 4-hour film; reconnects refetch a fresh
one. Denylist and membership checks are unchanged (the MNP token carries
sub/jti/groups).

Tests: authorize_token refuses a session/no-audience token and accepts an MNP
token; the hub API refuses an MNP token; POST /v1/nodes/mnp-token is minted only
for a member's own session. Verified red-before/green-after; common, node and
hub suites green (the pre-existing test_cli_golden failure is an argparse/pytest
prog artifact unrelated to this change).

Still to do before deploy (B2): bump the MNP version and client.minimum so a
stale desktop client is told to update rather than getting a handshake refusal,
update docs/MESHBAY_DESIGN.md and MESHBAY_NODE_PROTOCOL.md, and validate against
a real node locally, then deploy hub+node+SPA atomically.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 18 insertions, 4 deletions

diff --git a/packages/meshbay-hub/tests/test_token_hardening.py b/packages/meshbay-hub/tests/test_token_hardening.py
index 0835180..f381f69 100644
--- a/packages/meshbay-hub/tests/test_token_hardening.py
+++ b/packages/meshbay-hub/tests/test_token_hardening.py
@@ -17,6 +17,7 @@ import time
 
 import pytest
 
+from meshbay_common.tokens import HUB_API_AUD
 from meshbay_hub import auth
 
 
@@ -33,7 +34,7 @@ async def test_token_without_exp_is_refused(client):
     import jwt
     assert _sk_pem_loaded()
     # A hub-signed token with a valid scope and sub but NO exp.
-    forged = jwt.encode({"sub": "u", "scope": "user"},
+    forged = jwt.encode({"sub": "u", "scope": "user", "aud": HUB_API_AUD},
                         auth.hub_private_key_pem(), algorithm="EdDSA")
     with pytest.raises(Exception):
         auth.decode_access_token(forged)
@@ -42,7 +43,8 @@ async def test_token_without_exp_is_refused(client):
 @pytest.mark.asyncio
 async def test_expired_token_is_refused(client):
     import jwt
-    forged = jwt.encode({"sub": "u", "scope": "user", "exp": int(time.time()) - 100},
+    forged = jwt.encode({"sub": "u", "scope": "user", "aud": HUB_API_AUD,
+                         "exp": int(time.time()) - 100},
                         auth.hub_private_key_pem(), algorithm="EdDSA")
     with pytest.raises(jwt.ExpiredSignatureError):
         auth.decode_access_token(forged)
@@ -51,7 +53,7 @@ async def test_expired_token_is_refused(client):
 @pytest.mark.asyncio
 async def test_token_without_scope_is_refused(client):
     import jwt
-    forged = jwt.encode({"sub": "u", "exp": int(time.time()) + 3600},
+    forged = jwt.encode({"sub": "u", "exp": int(time.time()) + 3600, "aud": HUB_API_AUD},
                         auth.hub_private_key_pem(), algorithm="EdDSA")
     with pytest.raises(Exception):
         auth.decode_access_token(forged)
@@ -60,13 +62,25 @@ async def test_token_without_scope_is_refused(client):
 @pytest.mark.asyncio
 async def test_unknown_scope_is_refused(client):
     import jwt
-    forged = jwt.encode({"sub": "u", "scope": "root", "exp": int(time.time()) + 3600},
+    forged = jwt.encode({"sub": "u", "scope": "root", "aud": HUB_API_AUD,
+                         "exp": int(time.time()) + 3600},
                         auth.hub_private_key_pem(), algorithm="EdDSA")
     with pytest.raises(jwt.InvalidTokenError):
         auth.decode_access_token(forged)
 
 
 @pytest.mark.asyncio
+async def test_an_mnp_token_is_refused_at_the_hub_api(client):
+    """The MNP token (aud=MNP_AUD) authorises a member to a node; it must not be
+    a session at the hub. A node operator holds one, and this is what stops them
+    replaying it against the hub API."""
+    from meshbay_hub.auth import issue_mnp_token
+    mnp = issue_mnp_token("some-user", groups=[])
+    with pytest.raises(Exception):
+        auth.decode_access_token(mnp)
+
+
+@pytest.mark.asyncio
 async def test_a_revocation_token_is_not_a_session(client):
     """The concrete cross-type case: revocation tokens are hub-signed, carry no
     exp/sub/scope, and are handed to admins and pushed to every node."""