feat(protocol): bind the MNP token to the node it is for (E10)HEADmain

The audience split stopped a member's node credential from opening the hub API.
It did not stop the credential being *replayed to another node*: the MNP token
carried the member's whole group set and named no node, so a token handed to
node A's operator could be presented to node B the member also belongs to. That
does not read content on B — the handshake still requires proving node B's group
key, which the operator lacks — but it reaches B's pre-proof window and fetches
the member's *encrypted* keypair bundle for B (offline-attackable, bounded,
audited): a disclosure §2.4 says should not follow from hosting a member on A.

The token now names the node it is minted for (a `node` claim = that node's
Ed25519 key), and authorize_token refuses one that names a different key. The
client already knows the target node's key (from /v1/groups/{id}/nodes) and asks
for a token bound to it: POST /v1/nodes/mnp-token takes node_pk, and
transport.connect threads it (group-page, the connection pool and rewrap pass
n.pk_node; reconnect preserves it). A token that names no node is still
accepted, because the hub only mints one for the authenticated requester, so an
unbound token grants nothing across accounts — which also keeps non-binding
callers working with no churn.

Done before deploy, so it folds into the MNP 4.0 flag day rather than needing
its own. Docs: §5.2, register E10, MESHBAY_NODE_PROTOCOL.md §6.3.
test_handshake.py and test_mnp_token.py hold the binding (a token for node A is
refused by node B, accepted by node A; an unbound token still works); red
before, green after. common/node/hub suites green.

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

1 files changed, 21 insertions, 0 deletions

diff --git a/packages/meshbay-hub/tests/test_mnp_token.py b/packages/meshbay-hub/tests/test_mnp_token.py
index 7b483ff..3aa3115 100644
--- a/packages/meshbay-hub/tests/test_mnp_token.py
+++ b/packages/meshbay-hub/tests/test_mnp_token.py
@@ -69,3 +69,24 @@ async def test_a_session_token_is_refused_by_a_node_but_the_mnp_token_is_not(cli
     # The MNP token authorises the member to the node.
     peer = authorize_token(mnp, pk, group_id=gid)
     assert peer.group_id == gid
+
+
+@pytest.mark.asyncio
+async def test_the_mnp_token_is_bound_to_the_node_it_names(client):
+    """E10: a token minted for node A is refused by node B, so an operator who
+    captures a member's token cannot replay it to another of the member's nodes."""
+    from meshbay_hub.auth import hub_public_key_pem
+
+    tok = await _session_token(client, "mnp_bind_test")
+    H = {"Authorization": f"Bearer {tok}"}
+    gid = (await client.post("/v1/groups", headers=H, json={
+        "name": "g", "visibility": "private", "join_policy": "invite"})).json()["group_id"]
+    # A token bound to node A's key.
+    mnp = (await client.post("/v1/nodes/mnp-token", headers=H,
+                             json={"node_pk": "node-A-pk"})).json()["mnp_token"]
+    pk = hub_public_key_pem()
+    # Node B refuses it; node A accepts it.
+    with pytest.raises(HandshakeError, match="this node"):
+        authorize_token(mnp, pk, group_id=gid, node_pk_b64="node-B-pk")
+    peer = authorize_token(mnp, pk, group_id=gid, node_pk_b64="node-A-pk")
+    assert peer.group_id == gid