feat: the node signs its handshake challenge (MNP 3.4)

node_pk in handshake_challenge is now signed over the channel binding
and both nonces, so a client can check the node key before a join
rather than only at the ack. Both transports; the browser and the QUIC
client refuse a wrong signature and treat an absent one as an older node.

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

11 files changed, 345 insertions, 12 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/__init__.py b/packages/meshbay-common/src/meshbay_common/__init__.py
index f10302f..1271c4e 100644
--- a/packages/meshbay-common/src/meshbay_common/__init__.py
+++ b/packages/meshbay-common/src/meshbay_common/__init__.py
@@ -220,5 +220,21 @@ __version__ = "0.15.0"
 # the node's own `stream_init` and from no version number, so the request is
 # never sent to a peer that could not answer it. `MNP_MIN_SUPPORTED` does not
 # move.
-MNP_VERSION = "3.3"
+#
+# **3.4 (2026-09-23): the node signs its challenge.**
+#
+# `handshake_challenge` carries `sig`, Ed25519 by the node key over
+# `meshbay:mnp:challenge:v1` ‖ group_id ‖ nonce_c ‖ nonce_s ‖ binding
+# (`handshake.challenge_transcript`). Until now `node_pk` in the challenge was a
+# claim: the ack proves it, but a join is sent *before* the ack, so an
+# invitation code went to whichever peer answered signaling. With the
+# signature, a client that knows which node it means to reach — an invitation
+# link names it — can refuse to send the code anywhere else.
+#
+# **Additive, and MINOR because nothing is required of an older peer.** A client
+# that ignores `sig` behaves exactly as before; a client that reads it refuses a
+# *wrong* one outright and treats an absent one as "this node cannot prove
+# itself early", which it discovers from the node's answer and never from the
+# version number. `MNP_MIN_SUPPORTED` does not move.
+MNP_VERSION = "3.4"
 MHP_VERSION = "0.1"
diff --git a/packages/meshbay-common/src/meshbay_common/handshake.py b/packages/meshbay-common/src/meshbay_common/handshake.py
index 4d2cac2..73e0b2e 100644
--- a/packages/meshbay-common/src/meshbay_common/handshake.py
+++ b/packages/meshbay-common/src/meshbay_common/handshake.py
@@ -12,7 +12,7 @@ The sequence:
     client → node   handshake            {token, group_id, nonce_c, v, v_min}
     node            check_version()      supported range, both ways
     node            authorize_token()    JWT, scope, denylist, membership, hosting
-    node → client   handshake_challenge  {nonce_s, v, v_min}
+    node → client   handshake_challenge  {nonce_s, v, v_min, node_pk, sig}
     client → node   handshake_response   {proof}
     node            verify client proof  HMAC(GEK, client transcript)
     node → client   handshake_ack        {proof, sig, node_pk, nonce, ct}
@@ -65,6 +65,7 @@ import jwt
 from meshbay_common import MNP_VERSION
 
 HANDSHAKE_PREFIX = b"meshbay:mnp:handshake:v1"
+CHALLENGE_PREFIX = b"meshbay:mnp:challenge:v1"
 
 # The oldest peer this build will talk to.
 #
@@ -157,6 +158,28 @@ def handshake_transcript(
     return bytes(out)
 
 
+def challenge_transcript(
+    group_id: str,
+    nonce_client: bytes,
+    nonce_node: bytes,
+    binding: bytes,
+) -> bytes:
+    """
+    Bytes the node signs in `handshake_challenge` (MNP 3.4).
+
+    The ack proves the node key, but a join is sent before the ack — so without
+    this, `node_pk` in the challenge was an announcement anyone answering
+    signaling could make. `nonce_client` makes the signature fresh and `binding`
+    pins it to this connection, so one cannot be recorded and relayed. No role
+    field: it has its own prefix, and nothing else is signed under it.
+    """
+    out = bytearray(CHALLENGE_PREFIX)
+    for field in (group_id.encode(), nonce_client, nonce_node, binding):
+        out += len(field).to_bytes(4, "big")
+        out += field
+    return bytes(out)
+
+
 def make_proof(
     gek: bytes,
     role: str,
diff --git a/packages/meshbay-common/tests/test_handshake.py b/packages/meshbay-common/tests/test_handshake.py
index 11313aa..ad615a9 100644
--- a/packages/meshbay-common/tests/test_handshake.py
+++ b/packages/meshbay-common/tests/test_handshake.py
@@ -13,6 +13,7 @@ import pytest
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
 from meshbay_common.handshake import (
+    CHALLENGE_PREFIX,
     HANDSHAKE_PREFIX,
     NONCE_LEN,
     ROLE_CLIENT,
@@ -20,6 +21,7 @@ from meshbay_common.handshake import (
     AuthorizedPeer,
     HandshakeError,
     authorize_token,
+    challenge_transcript,
     handshake_transcript,
     make_proof,
     quic_binding,
@@ -131,6 +133,24 @@ def test_transcript_is_domain_separated():
     ).startswith(HANDSHAKE_PREFIX)
 
 
+def test_the_challenge_transcript_is_its_own_domain():
+    """
+    MNP 3.4: the node signs the challenge with the same key that signs the ack.
+    The two must never be interchangeable — a challenge signature passed off as
+    an ack signature would authenticate a node that never proved the GEK.
+    """
+    challenge = challenge_transcript(GROUP, NONCE_C, NONCE_S, BINDING)
+    assert challenge.startswith(CHALLENGE_PREFIX)
+    assert challenge != handshake_transcript(ROLE_NODE, GROUP, NONCE_C, NONCE_S, BINDING)
+    for other in (
+        challenge_transcript("other", NONCE_C, NONCE_S, BINDING),
+        challenge_transcript(GROUP, b"x" * 32, NONCE_S, BINDING),
+        challenge_transcript(GROUP, NONCE_C, b"y" * 32, BINDING),
+        challenge_transcript(GROUP, NONCE_C, NONCE_S, BINDING + b"z"),
+    ):
+        assert other != challenge
+
+
 def test_client_proof_is_not_a_node_proof():
     """
     C3: the node proves itself with the same key over the same connection. Without
diff --git a/packages/meshbay-common/tests/test_js_python_parity.py b/packages/meshbay-common/tests/test_js_python_parity.py
index 3887414..5bf34aa 100644
--- a/packages/meshbay-common/tests/test_js_python_parity.py
+++ b/packages/meshbay-common/tests/test_js_python_parity.py
@@ -22,7 +22,11 @@ from pathlib import Path
 
 import pytest
 from meshbay_common.adminop import admin_transcript
-from meshbay_common.handshake import handshake_transcript, webrtc_binding
+from meshbay_common.handshake import (
+    challenge_transcript,
+    handshake_transcript,
+    webrtc_binding,
+)
 from meshbay_common.join import join_transcript
 
 CRYPTO_JS = (Path(__file__).resolve().parents[2]
@@ -76,7 +80,7 @@ globalThis.crypto = globalThis.crypto || {};
 
 const src = fs.readFileSync(process.argv[2], 'utf8');
 const load = new Function(
-  src + '\nreturn { handshakeTranscript, adminTranscript, joinTranscript, '
+  src + '\nreturn { handshakeTranscript, challengeTranscript, adminTranscript, joinTranscript, '
       + 'webrtcBinding, b64encode };');
 const M = load();
 
@@ -89,7 +93,7 @@ const toHex = (u8) =>
   Array.from(u8).map(b => b.toString(16).padStart(2, '0')).join('');
 
 const input = JSON.parse(fs.readFileSync(process.argv[3], 'utf8'));
-const out = { handshake: [], admin: [], join: [] };
+const out = { handshake: [], challenge: [], admin: [], join: [] };
 
 for (const v of input.handshake) {
   const binding = M.webrtcBinding(hex(v.offer_fp), hex(v.answer_fp));
@@ -97,6 +101,12 @@ for (const v of input.handshake) {
     v.role, v.group_id, hex(v.nonce_c), hex(v.nonce_s), binding)));
 }
 
+for (const v of input.handshake) {
+  const binding = M.webrtcBinding(hex(v.offer_fp), hex(v.answer_fp));
+  out.challenge.push(toHex(M.challengeTranscript(
+    v.group_id, hex(v.nonce_c), hex(v.nonce_s), binding)));
+}
+
 for (const v of input.admin) {
   out.admin.push(toHex(M.adminTranscript(
     v.op, v.node_pk, v.group_id, v.subject, M.b64encode(hex(v.nonce)), v.ts)));
@@ -167,6 +177,20 @@ def test_handshake_transcript_parity(idx, vector, js_output):
     )
 
 
+@pytest.mark.parametrize("idx,vector", list(enumerate(HANDSHAKE_VECTORS)))
+def test_challenge_transcript_parity(idx, vector, js_output):
+    """
+    MNP 3.4. A mismatch means every browser refuses every node that signs its
+    challenge — the signature is checked, and a wrong one is a refusal.
+    """
+    _, group_id, nonce_c, nonce_s, offer_fp, answer_fp = vector
+    expected = challenge_transcript(
+        group_id, bytes.fromhex(nonce_c), bytes.fromhex(nonce_s),
+        webrtc_binding(bytes.fromhex(offer_fp), bytes.fromhex(answer_fp)))
+    assert js_output["challenge"][idx] == expected.hex(), (
+        f"crypto.js and meshbay_common.handshake disagree for group={group_id!r}")
+
+
 @pytest.mark.parametrize("idx,vector", list(enumerate(ADMIN_VECTORS)))
 def test_admin_transcript_parity(idx, vector, js_output):
     """
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/crypto.js b/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
index fd24404..a3680ce 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/crypto.js
@@ -398,6 +398,7 @@ function adminTranscript(op, nodePkB64, groupId, subject, nonceB64, ts) {
 // bound in, so a client proof can never be replayed as a node proof and a missing
 // fingerprint cannot silently degrade the proof to nonce-only (L4).
 const HANDSHAKE_PREFIX = new TextEncoder().encode('meshbay:mnp:handshake:v1');
+const CHALLENGE_PREFIX = new TextEncoder().encode('meshbay:mnp:challenge:v1');
 
 function _lenPrefixed(parts) {
   let total = 0;
@@ -432,6 +433,18 @@ function handshakeTranscript(role, groupId, nonceClient, nonceNode, binding) {
   return out;
 }
 
+// Mirrors meshbay_common/handshake.py challenge_transcript (MNP 3.4): what the
+// node signs in handshake_challenge, so its key can be checked before a join.
+function challengeTranscript(groupId, nonceClient, nonceNode, binding) {
+  const body = _lenPrefixed([
+    new TextEncoder().encode(groupId), nonceClient, nonceNode, binding,
+  ]);
+  const out = new Uint8Array(CHALLENGE_PREFIX.length + body.length);
+  out.set(CHALLENGE_PREFIX, 0);
+  out.set(body, CHALLENGE_PREFIX.length);
+  return out;
+}
+
 async function handshakeProof(gekRaw, role, groupId, nonceClient, nonceNode, binding) {
   const transcript = handshakeTranscript(role, groupId, nonceClient, nonceNode, binding);
   const key = await crypto.subtle.importKey(
@@ -572,7 +585,7 @@ window.MeshBayCrypto = {
   openGroup, sealGroup,
   generateGEK, wrapGEK, unwrapGEK, encryptChunk, b64encode, b64decode,
   adminTranscript, handshakeTranscript, handshakeProof, webrtcBinding,
-  joinTranscript, verifyNodeSignature, constantTimeEqual,
+  challengeTranscript, joinTranscript, verifyNodeSignature, constantTimeEqual,
   deviceRequestTranscript, deviceAddTranscript, deviceHelloTranscript,
   deviceCodeHash,
   sealChat, openChat, chatSigningTranscript, verifyChatSignature,
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/transport.js b/packages/meshbay-hub/src/meshbay_hub/static/transport.js
index 39e0ccf..c43422f 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/transport.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/transport.js
@@ -883,10 +883,19 @@ class MeshBayTransport {
       //
       // nonce_node ties a join to this connection, so one cannot be lifted onto
       // another. node_pk is announced here because a first-time member has no
-      // GEK and so cannot complete the handshake that would prove it; it is
-      // unverified at this point and checked against the ack below.
+      // GEK and so cannot complete the handshake that would prove it. From an
+      // older node it is unverified until the ack below checks it.
       this._nonceNode = window.MeshBayCrypto.b64decode(reply.nonce);
       this.nodePk = reply.node_pk || null;
+      // Since MNP 3.4 the node signs its challenge over this connection, so
+      // node_pk is proved here and not only at the ack — which comes after any
+      // join. A signature that does not verify is a peer lying about which node
+      // it is, and is refused. An absent one is an older node: `nodePkProved`
+      // stays false, and whatever needs the key proved before a code leaves
+      // (an invitation link names its node) reads that — never a version.
+      this.nodePkProved = await _challengeProvesNodeKey(
+        reply, groupId || '', this._nonceClient,
+        this._pc.localDescription.sdp, this._rawAnswerSdp);
 
       // Our identity for THIS node: fetched from it, or created if this is a
       // first join. Keys are per node, so there is nothing to carry between
@@ -3972,6 +3981,29 @@ function _hex(bytes) {
   return [...bytes].map(b => b.toString(16).padStart(2, '0')).join('');
 }
 
+/**
+ * Whether `handshake_challenge` proves the key it announces (MNP 3.4).
+ *
+ * True when it carries a signature that verifies over this connection, false
+ * when it carries none — an older node, which proves its key only at the ack.
+ * A signature that does not verify is a peer lying about which node it is, and
+ * throws: that is a refusal, not a node that merely cannot say.
+ */
+async function _challengeProvesNodeKey(reply, groupId, nonceClient, offerSdp, answerSdp) {
+  if (!reply.sig) return false;
+  const C = window.MeshBayCrypto;
+  let ok = false;
+  try {
+    ok = Boolean(reply.node_pk) && await C.verifyNodeSignature(
+      reply.node_pk, reply.sig,
+      C.challengeTranscript(groupId, nonceClient, C.b64decode(reply.nonce),
+        C.webrtcBinding(_extractDtlsFingerprint(offerSdp),
+                        _extractDtlsFingerprint(answerSdp))));
+  } catch { ok = false; }
+  if (!ok) throw new Error('Node challenge signature invalid — refusing connection');
+  return true;
+}
+
 function _extractDtlsFingerprint(sdp) {
   const match = sdp.match(/a=fingerprint:sha-256 ([0-9A-Fa-f:]+)/);
   if (!match) return new Uint8Array(0);
diff --git a/packages/meshbay-hub/tests/test_challenge_signature_client.py b/packages/meshbay-hub/tests/test_challenge_signature_client.py
new file mode 100644
index 0000000..90aa5f2
--- /dev/null
+++ b/packages/meshbay-hub/tests/test_challenge_signature_client.py
@@ -0,0 +1,100 @@
+"""
+The browser checks the node's challenge signature with the code it ships (MNP 3.4).
+
+The node signs `handshake_challenge` so a client can hold it to the key it
+announces *before* a join, which goes out ahead of the ack. The Python side is
+tested against a real connection in the node suite; this runs the other half —
+the real `_challengeProvesNodeKey` out of `transport.js` over the real
+`crypto.js` — against a challenge Python signed, because the two agreeing on
+paper (the parity test) is not the browser accepting what the node sends.
+"""
+
+import base64
+import json
+import os
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+from meshbay_common.crypto import pk_to_b64
+from meshbay_common.handshake import challenge_transcript, webrtc_binding
+
+STATIC = Path(__file__).resolve().parents[1] / "src" / "meshbay_hub" / "static"
+
+pytestmark = pytest.mark.skipif(shutil.which("node") is None,
+                                reason="node is not available")
+
+GROUP = "g" * 32
+
+_HARNESS = r"""
+const fs = require('fs');
+globalThis.window = globalThis;
+globalThis.addEventListener = () => {};
+globalThis.removeEventListener = () => {};
+globalThis.location = { hash: '' };
+globalThis.document = {
+  addEventListener() {}, removeEventListener() {}, visibilityState: 'visible',
+};
+const STATIC = process.argv[2];
+new Function(fs.readFileSync(`${STATIC}/crypto.js`, 'utf8'))();
+const src = fs.readFileSync(`${STATIC}/transport.js`, 'utf8');
+const { _challengeProvesNodeKey } =
+  new Function(src + '\nreturn { _challengeProvesNodeKey };')();
+
+const cases = JSON.parse(fs.readFileSync(process.argv[3], 'utf8'));
+(async () => {
+  const out = [];
+  for (const c of cases) {
+    const nonceC = Uint8Array.from(Buffer.from(c.nonce_c, 'base64'));
+    try {
+      out.push(String(await _challengeProvesNodeKey(
+        c.reply, c.group_id, nonceC, c.offer_sdp, c.answer_sdp)));
+    } catch (e) {
+      out.push('refused');
+    }
+  }
+  process.stdout.write(JSON.stringify(out));
+})();
+"""
+
+
+def _sdp(fp: bytes) -> str:
+    return "v=0\r\na=fingerprint:sha-256 " + ":".join(f"{b:02X}" for b in fp) + "\r\n"
+
+
+def test_the_browser_holds_the_node_to_its_challenge(tmp_path):
+    sk_node, sk_other = Ed25519PrivateKey.generate(), Ed25519PrivateKey.generate()
+    nonce_c, nonce_s = os.urandom(32), os.urandom(32)
+    offer_fp, answer_fp = os.urandom(32), os.urandom(32)
+    sig = sk_node.sign(challenge_transcript(
+        GROUP, nonce_c, nonce_s, webrtc_binding(offer_fp, answer_fp)))
+
+    def case(*, sig=sig, pk=sk_node, group=GROUP, answer=answer_fp, nonce=nonce_c):
+        reply = {"type": "handshake_challenge", "nonce": base64.b64encode(nonce_s).decode(),
+                 "node_pk": pk_to_b64(pk.public_key())}
+        if sig is not None:
+            reply["sig"] = base64.b64encode(sig).decode()
+        return {"reply": reply, "group_id": group,
+                "nonce_c": base64.b64encode(nonce).decode(),
+                "offer_sdp": _sdp(offer_fp), "answer_sdp": _sdp(answer)}
+
+    cases = {
+        "signed over this connection": (case(), "true"),
+        "an older node, no signature": (case(sig=None), "false"),
+        "another key announced": (case(pk=sk_other), "refused"),
+        "a relay's fingerprint": (case(answer=os.urandom(32)), "refused"),
+        "a replay under another nonce": (case(nonce=os.urandom(32)), "refused"),
+        "another group": (case(group="other"), "refused"),
+        "garbage for a signature": (case(sig=b"\x00" * 64), "refused"),
+    }
+    harness = tmp_path / "harness.js"
+    harness.write_text(_HARNESS)
+    payload = tmp_path / "cases.json"
+    payload.write_text(json.dumps([c for c, _ in cases.values()]))
+    proc = subprocess.run(["node", str(harness), str(STATIC), str(payload)],
+                          capture_output=True, text=True, timeout=60)
+    assert proc.returncode == 0, proc.stderr
+    got = dict(zip(cases, json.loads(proc.stdout)))
+    assert got == {name: want for name, (_, want) in cases.items()}
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 273f225..8f3fb74 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/quic_client.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/quic_client.py
@@ -26,6 +26,7 @@ from meshbay_common.handshake import (
     NONCE_LEN,
     ROLE_CLIENT,
     ROLE_NODE,
+    challenge_transcript,
     check_version,
     handshake_transcript,
     make_proof,
@@ -211,6 +212,17 @@ 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
+
         self._proto._send(self._ctrl_stream, {
             "type":  MNP.HANDSHAKE_RESPONSE,
             "v":     MNP_VERSION,
diff --git a/packages/meshbay-node/src/meshbay_node/transport/quic_server.py b/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
index 036574b..96cd752 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
@@ -41,6 +41,7 @@ from meshbay_common.handshake import (
     ROLE_NODE,
     HandshakeError,
     authorize_token,
+    challenge_transcript,
     check_version,
     handshake_transcript,
     make_proof,
@@ -313,11 +314,23 @@ class _MNPServerProtocol(QuicConnectionProtocol):
         # Decoded but NOT authenticated: authentication is the GEK proof below.
         self._pending = peer
         self._gek_challenge = os.urandom(NONCE_LEN)
+        # The same announcement and signature as WebRTC's challenge (MNP 3.4),
+        # so the two transports stay one handshake. The binding is the node's
+        # certificate, which is known here as it is at the proof.
+        sig = {}
+        cert = self._ctx.get("server_cert_der")
+        if cert:
+            transcript = challenge_transcript(
+                peer.group_id, self._nonce_client, self._gek_challenge,
+                quic_binding(cert))
+            sig = {"sig": base64.b64encode(self._ctx["sk_node"].sign(transcript)).decode()}
         self._send(stream_id, {
-            "type":  MNP.HANDSHAKE_CHALLENGE,
-            "v":     MNP_VERSION,
-            "v_min": MNP_MIN_SUPPORTED,
-            "nonce": base64.b64encode(self._gek_challenge).decode(),
+            "type":    MNP.HANDSHAKE_CHALLENGE,
+            "v":       MNP_VERSION,
+            "v_min":   MNP_MIN_SUPPORTED,
+            "nonce":   base64.b64encode(self._gek_challenge).decode(),
+            "node_pk": pk_to_b64(self._ctx["sk_node"].public_key()),
+            **sig,
         })
 
     def _do_handshake_response_sync(self, stream_id: int, msg: dict) -> None:
diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
index a5e15df..809c5c5 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
@@ -99,6 +99,7 @@ from meshbay_common.handshake import (
     ROLE_NODE,
     HandshakeError,
     authorize_token,
+    challenge_transcript,
     check_version,
     handshake_transcript,
     make_proof,
@@ -962,8 +963,27 @@ class WebRTCPeerSession:
             # the two match, and a wrong value only makes our own verification
             # fail. It is never a substitute for the ack's proof and signature.
             "node_pk": self._node_pk_b64(),
+            # ...except that since 3.4 it is signed, so a client that already
+            # knows which key to expect can check it before it sends a code.
+            **self._challenge_sig(peer.group_id, self._channel_binding()),
         })
 
+    def _challenge_sig(self, group_id: str, binding: bytes) -> dict:
+        """
+        `{"sig": ...}` over the challenge transcript, or nothing (MNP 3.4).
+
+        What makes `node_pk` above more than an announcement: a client about to
+        send an invitation code can check this node holds the key it was told
+        to expect, before the code leaves. No binding means no signature rather
+        than an unbound one — a signature that is not tied to the channel is one
+        somebody can relay, and the handshake proof refuses that case anyway.
+        """
+        if not binding:
+            return {}
+        transcript = challenge_transcript(
+            group_id, self._nonce_client, self._gek_challenge, binding)
+        return {"sig": base64.b64encode(self._ctx["sk_node"].sign(transcript)).decode()}
+
     def _do_handshake_response(self, msg: dict) -> None:
         if not self._gek_challenge or not hasattr(self, "_pending_sub"):
             self._send({"type": "error", "detail": "No pending handshake challenge"})
diff --git a/packages/meshbay-node/tests/test_webrtc_transport.py b/packages/meshbay-node/tests/test_webrtc_transport.py
index 91a6e1c..0d2aa3a 100644
--- a/packages/meshbay-node/tests/test_webrtc_transport.py
+++ b/packages/meshbay-node/tests/test_webrtc_transport.py
@@ -20,6 +20,7 @@ import jwt
 import msgpack
 import pytest
 from aiortc import RTCPeerConnection, RTCSessionDescription
+from cryptography.exceptions import InvalidSignature
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric.ed25519 import (
     Ed25519PrivateKey,
@@ -44,6 +45,7 @@ from meshbay_common.handshake import (
     NONCE_LEN,
     ROLE_CLIENT,
     ROLE_NODE,
+    challenge_transcript,
     handshake_transcript,
     make_proof,
     verify_proof,
@@ -166,6 +168,12 @@ async def _do_mnp_handshake(channel, received, token, gek, pc, group_id):
         _extract_dtls_fp(pc.localDescription.sdp),
         _extract_dtls_fp(pc.remoteDescription.sdp),
     )
+    # MNP 3.4: every challenge in this suite must carry a signature by the key
+    # it announces, over this very connection.
+    Ed25519PublicKey.from_public_bytes(
+        base64.b64decode(msg["node_pk"])
+    ).verify(base64.b64decode(msg["sig"]),
+             challenge_transcript(group_id, nonce_c, nonce_s, binding))
     proof = make_proof(gek, ROLE_CLIENT, group_id, nonce_c, nonce_s, binding)
     channel.send(_pack({
         "type": MNP.HANDSHAKE_RESPONSE, "v": MNP_VERSION,
@@ -909,6 +917,58 @@ async def test_webrtc_dtls_channel_binding_detects_mitm(sk_node, sk_hub, gek, sh
     await transport.close_all()
 
 
+
+@pytest.mark.asyncio
+async def test_the_challenge_signature_is_bound_to_this_connection(
+        sk_node, sk_hub, gek, shared_dir):
+    """
+    MNP 3.4. The node signs its challenge so a client can check `node_pk` before
+    a join — which goes out before the ack that used to be the only proof. That
+    is only worth anything if the signature cannot be carried elsewhere: under a
+    substituted fingerprint (a relay in the middle), another client nonce (a
+    recording replayed) or another group, it must not verify.
+    """
+    indexer = DirectoryIndexer(roots=one_root(shared_dir), group_id="g", sk_node=sk_node, gek=gek)
+    await indexer.initial_scan()
+    transport = WebRTCTransport(
+        sk_node=sk_node, hub_pk_pem=_hub_pk_pem(sk_hub), gek=gek,
+        roots=one_root(shared_dir), index=indexer.index, stun_servers=[],
+    )
+    pc, ch, q = await _open_channel(transport, "peer-sig")
+    try:
+        nonce_c = os.urandom(NONCE_LEN)
+        ch.send(_pack({
+            "type": MNP.HANDSHAKE, "v": MNP_VERSION, "token": _make_jwt(sk_hub),
+            "group_id": TEST_GROUP, "nonce": base64.b64encode(nonce_c).decode(),
+        }))
+        challenge = await asyncio.wait_for(q.get(), timeout=5.0)
+        assert challenge["type"] == MNP.HANDSHAKE_CHALLENGE
+        assert challenge["node_pk"] == pk_to_b64(sk_node.public_key())
+
+        nonce_s = base64.b64decode(challenge["nonce"])
+        offer_fp = _extract_dtls_fp(pc.localDescription.sdp)
+        answer_fp = _extract_dtls_fp(pc.remoteDescription.sdp)
+        pk = Ed25519PublicKey.from_public_bytes(base64.b64decode(challenge["node_pk"]))
+        sig = base64.b64decode(challenge["sig"])
+
+        pk.verify(sig, challenge_transcript(
+            TEST_GROUP, nonce_c, nonce_s, webrtc_binding(offer_fp, answer_fp)))
+        for label, transcript in (
+            ("once relayed", challenge_transcript(
+                TEST_GROUP, nonce_c, nonce_s, webrtc_binding(offer_fp, os.urandom(32)))),
+            ("once replayed", challenge_transcript(
+                TEST_GROUP, os.urandom(NONCE_LEN), nonce_s,
+                webrtc_binding(offer_fp, answer_fp))),
+            ("for another group", challenge_transcript(
+                "other-group", nonce_c, nonce_s, webrtc_binding(offer_fp, answer_fp))),
+        ):
+            with pytest.raises(InvalidSignature):
+                pk.verify(sig, transcript)
+                pytest.fail(f"the challenge signature verified {label}")
+    finally:
+        await pc.close()
+        await transport.close_all()
+
 async def _paired_operator_roster(tmp_path, sk_admin):
     """
     A roster holding one operator, which is the only thing that authorizes an