fix: bound pending admin challenges and sign every value an op acts on

Any member could make a node hold unbounded challenge requests; a
connection now keeps at most 8, 64 KiB each. root_add, group_attach,
invite_create and tmdb_config signed less than they did; their subjects
are now canonical JSON of every value (the TMDB token by SHA-256).
MNP 5.0, floor kept at 4.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 13 insertions, 6 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/admission.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/admission.py
index b02e59a..e678a13 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/admission.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/admission.py
@@ -8,7 +8,12 @@ import time
 
 from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey
 from meshbay_common import MNP_VERSION
-from meshbay_common.adminop import OP_INVITE_CANCEL, OP_INVITE_CREATE, OP_INVITE_LINK_CREATE
+from meshbay_common.adminop import (
+    OP_INVITE_CANCEL,
+    OP_INVITE_CREATE,
+    OP_INVITE_LINK_CREATE,
+    invite_create_subject,
+)
 from meshbay_common.crypto import wrap_gek_aes
 from meshbay_common.device import (
     DEVICE_TTL,
@@ -71,11 +76,13 @@ class AdmissionMixin:
             })
             return
 
-        self._issue_admin_challenge(OP_INVITE_CREATE, invitee_id, {
-            "group_id": group_id,
-            "user_id": invitee_id,
-            "username": str(msg.get("username", ""))[:64],
-        })
+        username = str(msg.get("username", ""))[:64]
+        self._issue_admin_challenge(
+            OP_INVITE_CREATE, invite_create_subject(invitee_id, username), {
+                "group_id": group_id,
+                "user_id": invitee_id,
+                "username": username,
+            })
 
     def _do_invite_link_create(self, msg: dict) -> None:
         """