fix: refuse an unsigned handshake challenge

Every node the 4.0 floor admits signs its challenge, and one without a
channel binding could not complete the proof anyway, so a missing
signature is refused like a wrong one (browser and QUIC client).

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

1 files changed, 13 insertions, 10 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/quic_client.py b/packages/meshbay-node/src/meshbay_node/transport/quic_client.py
index 8f3fb74..d548103 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/quic_client.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/quic_client.py
@@ -212,16 +212,19 @@ class QuicChunkClient:
                 "session — refusing to handshake without channel binding")
         binding = quic_binding(self._peer_cert_der)
 
-        # MNP 3.4: a signed challenge proves the node key before we send anything
-        # else. A wrong signature is refused; an absent one is an older node.
-        if reply.get("sig"):
-            try:
-                Ed25519PublicKey.from_public_bytes(
-                    base64.b64decode(reply.get("node_pk", ""))
-                ).verify(base64.b64decode(reply["sig"]), challenge_transcript(
-                    self._group_id, nonce_c, nonce_s, binding))
-            except Exception as exc:
-                raise ConnectionError(f"Node challenge signature invalid: {exc}") from exc
+        # A signed challenge proves the node key before we send anything else.
+        # Every node we can reach signs (the floor is 4.0, signing is 3.4), and
+        # one without a certificate to bind could not complete the proof anyway,
+        # so a missing signature is refused like a wrong one.
+        if not reply.get("sig"):
+            raise ConnectionError("Node challenge is not signed")
+        try:
+            Ed25519PublicKey.from_public_bytes(
+                base64.b64decode(reply.get("node_pk", ""))
+            ).verify(base64.b64decode(reply["sig"]), challenge_transcript(
+                self._group_id, nonce_c, nonce_s, binding))
+        except Exception as exc:
+            raise ConnectionError(f"Node challenge signature invalid: {exc}") from exc
 
         self._proto._send(self._ctrl_stream, {
             "type":  MNP.HANDSHAKE_RESPONSE,