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

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, 19 insertions, 5 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/nodes.py b/packages/meshbay-hub/src/meshbay_hub/api/nodes.py
index 6205180..403f450 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/nodes.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/nodes.py
@@ -21,10 +21,18 @@ from meshbay_hub.db.models import GroupMember, IPLog, Node, User
 router = APIRouter(prefix="/v1/nodes", tags=["nodes"])
 
 
+class MnpTokenRequest(BaseModel):
+    # The base64 Ed25519 key of the node this token is for. The token is bound
+    # to it (E10), so it cannot be replayed to another node. The client knows it
+    # from `/v1/groups/{id}/nodes` before it connects.
+    node_pk: str = ""
+
+
 @router.post("/mnp-token")
 @limiter.limit("60/minute")
 async def mnp_token(
     request: Request,
+    body: MnpTokenRequest | None = None,
     current_user: User = Depends(require_user_scope),
     db: AsyncSession = Depends(get_db),
 ):
@@ -32,16 +40,22 @@ async def mnp_token(
 
     Asked for with the member's own session token (require_user_scope, so a node
     daemon token cannot mint one). The result carries the member's current group
-    membership and `aud=MNP_AUD`, so it authorises the member to a node and is
-    refused by the hub API. Short-lived on purpose; the client refetches it for a
-    new connection or a reconnect, and it is checked only at the handshake, so a
-    film already playing is never interrupted by its expiry.
+    membership, `aud=MNP_AUD` and the target node's key, so it authorises the
+    member to **that** node only and is refused by the hub API and by any other
+    node. Short-lived on purpose; the client refetches it for a new connection or
+    a reconnect, and it is checked only at the handshake, so a film already
+    playing is never interrupted by its expiry.
+
+    The hub does not verify the node key it is handed — binding the token to it
+    only *restricts* the token to whatever node holds that key, which is the one
+    the client is connecting to; a wrong key yields a token no node will accept.
     """
     rows = await db.execute(
         select(GroupMember.group_id).where(GroupMember.user_id == current_user.id))
     group_ids = [gid for (gid,) in rows.all()]
     return {
-        "mnp_token": issue_mnp_token(current_user.id, groups=group_ids),
+        "mnp_token": issue_mnp_token(current_user.id, groups=group_ids,
+                                     node_pk=(body.node_pk if body else "")),
         "expires_in": 900,
     }