Introduces a forced WG handshake on initial MLKEM bootstrap.

To ensure two peers agree on a key, we need asymmetry. one peer is
the controller ("initiator") the other is the "responder".

Otherwise imagine two offers in parallel driving two answers at the same time

   A                   B
   | <----B-OFFER----- |
   | -----A-OFFER----> |
   |                   |
   |                   |
   ---------------------------------
  |****** ICE + WG Handshake ****** |
   ---------------------------------
   |                   |
   | <----B-ANSWER---- |
   | -----A-ANSWER---> |

PSK is derived on receive of offer, so A and B derive different PSKs.
When WG handshake takes place it picks misaligned PSKs.

So we impair the two nodes and only the offer of one of the two (the controller/initiator)
carries the KEM material.

This means that if the responder OFFER/ANSWER comes first, when the controller/initiator's one
completes (and the genuine PSK is shared between A and B, we need to force a new WG handshake with
the proper keys.

client/internal/engine.go

@@ -670,6 +670,14 @@ func (e *Engine) Start(netbirdConfig *mgmProto.NetbirdConfig, mgmtURL *url.URL)
 						conn.RequestReoffer()
 					}
 				},
+				// On a freshly bootstrapped PSK, force the peer's WireGuard session to
+				// re-handshake so it adopts the PSK even if the tunnel already came up on
+				// a pre-PQ key (KEM-vs-endpoint-config race).
+				rehandshake: func(remoteKey string) {
+					if conn, ok := e.peerStore.PeerConn(remoteKey); ok {
+						conn.ForcePQRehandshake()
+					}
+				},
 			}
 			e.pqkemManager = pqkem.NewManager(pqkem.LocalID(publicKey.String()), cbHandler, pqkem.NewLogger())
 			e.pqkemManager.Start(tr)

client/internal/peer/conn.go

@@ -736,6 +736,35 @@ func (conn *Conn) RequestReoffer() {
 	}
 }
 
+// ForcePQRehandshake makes WireGuard adopt a freshly bootstrapped post-quantum PSK on
+// an already-up session. The KEM exchange can complete after the WG endpoint was
+// configured (a race between the KEM and the relay/ICE connection coming up), so the
+// live session may be keyed with a pre-PQ key (the strict sentinel, or the ordinary
+// key in permissive mode); SetPresharedKey only updates config, not the running
+// session. Removing and re-adding the peer forces a fresh handshake that uses the real
+// PSK. No-op if not connected yet (the upcoming endpoint config will pull the PSK) or
+// if no PSK exists (a non-PQ peer stays fail-closed).
+func (conn *Conn) ForcePQRehandshake() {
+	conn.mu.Lock()
+	defer conn.mu.Unlock()
+
+	if conn.ctx.Err() != nil || conn.config.PQ == nil {
+		return
+	}
+	psk, ok := conn.config.PQ.PSK(conn.config.Key)
+	if !ok {
+		return
+	}
+	if conn.currentConnPriority == conntype.None {
+		return
+	}
+	conn.Log.Debugf("pqkem: bootstrap PSK ready, forcing WireGuard re-handshake to adopt it")
+	wgPsk := wgtypes.Key(psk)
+	if err := conn.endpointUpdater.ForceRehandshake(&wgPsk); err != nil {
+		conn.Log.Warnf("pqkem: force re-handshake failed: %v", err)
+	}
+}
+
 func (conn *Conn) onWGDisconnected(watcherCtx context.Context) {
 	conn.mu.Lock()
 	defer conn.mu.Unlock()

client/internal/peer/endpoint.go

@@ -20,10 +20,13 @@ type EndpointUpdater struct {
 	wgConfig  WgConfig
 	initiator bool
 
-	// mu protects cancelFunc
+	// mu protects cancelFunc and lastEndpoint
 	mu         sync.Mutex
 	cancelFunc func()
 	updateWg   sync.WaitGroup
+	// lastEndpoint is the most recent non-nil endpoint applied to the peer, used to
+	// re-add it on a forced re-handshake (ForceRehandshake).
+	lastEndpoint *net.UDPAddr
 }
 
 func NewEndpointUpdater(log *logrus.Entry, wgConfig WgConfig, initiator bool) *EndpointUpdater {
@@ -38,6 +41,10 @@ func (e *EndpointUpdater) ConfigureWGEndpoint(addr *net.UDPAddr, presharedKey *w
 	e.mu.Lock()
 	defer e.mu.Unlock()
 
+	if addr != nil {
+		e.lastEndpoint = addr
+	}
+
 	if e.initiator {
 		e.log.Debugf("configure up WireGuard as initiator")
 		return e.configureAsInitiator(addr, presharedKey)
@@ -51,12 +58,36 @@ func (e *EndpointUpdater) SwitchWGEndpoint(addr *net.UDPAddr, presharedKey *wgty
 	e.mu.Lock()
 	defer e.mu.Unlock()
 
+	if addr != nil {
+		e.lastEndpoint = addr
+	}
+
 	// prevent to run new update while cancel the previous update
 	e.waitForCloseTheDelayedUpdate()
 
 	return e.updateWireGuardPeer(addr, presharedKey)
 }
 
+// ForceRehandshake removes and re-adds the peer so WireGuard drops the current session
+// and negotiates a fresh one with the given PSK. Used when a post-quantum PSK is
+// bootstrapped after the session already came up on a pre-PQ key. No-op if no endpoint
+// has been applied yet (the pending config will pull the PSK itself).
+func (e *EndpointUpdater) ForceRehandshake(presharedKey *wgtypes.Key) error {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	if e.lastEndpoint == nil {
+		return nil
+	}
+	// Cancel any pending delayed responder update: it carries the stale pre-PQ key and
+	// would otherwise re-poison the session after we reset it.
+	e.waitForCloseTheDelayedUpdate()
+	if err := e.wgConfig.WgInterface.RemovePeer(e.wgConfig.RemoteKey); err != nil {
+		return err
+	}
+	return e.updateWireGuardPeer(e.lastEndpoint, presharedKey)
+}
+
 func (e *EndpointUpdater) RemoveWgPeer() error {
 	e.mu.Lock()
 	defer e.mu.Unlock()

client/internal/pqkem/callbacks.go

@@ -9,7 +9,13 @@ type CallbackHandler interface {
 	// and must be programmed into the consumer's secure channel. It is invoked at
 	// the commit point of each side: the initiator on receiving the answer, the
 	// responder on receiving the confirm.
-	OnNewPSKReady(remoteID RemoteID, psk PSK) error
+	//
+	// bootstrap is true when the PSK came from a signalling exchange (initial setup or
+	// a wake re-negotiation) rather than a data-path rotation. On bootstrap the consumer
+	// must ensure the live channel actually adopts the PSK now (e.g. force a WireGuard
+	// re-handshake) — the channel may already be up on a pre-PQ key. A rotation
+	// (bootstrap=false) is adopted at the channel's next natural rekey.
+	OnNewPSKReady(remoteID RemoteID, psk PSK, bootstrap bool) error
 
 	// OnRekeyFailed fires when an exchange fails to converge within the allotted
 	// time. The host should tear the peer connection down so it re-establishes, and

client/internal/pqkem/convergence.go

@@ -67,7 +67,7 @@ func (m *Manager) startExchange(remoteID RemoteID, viaSignal bool, ackID Exchang
 // (that offer riding the data path under the freshly adopted key proves it worked),
 // then derives the PSK for the new offer, commits it optimistically, and returns the
 // framed answer. A duplicate offer returns the cached answer without re-deriving.
-func (m *Manager) processOffer(remoteID RemoteID, o *OfferMsg) ([]byte, error) {
+func (m *Manager) processOffer(remoteID RemoteID, o *OfferMsg, viaSignal bool) ([]byte, error) {
 	m.trace("pqkem: offer received", "peer", remoteID, "exchange", idHex(o.ExchangeID), "acks", idHex(o.AckID))
 
 	if o.AckID != (ExchangeID{}) {
@@ -114,7 +114,7 @@ func (m *Manager) processOffer(remoteID RemoteID, o *OfferMsg) ([]byte, error) {
 	m.trace("pqkem: new PSK derived", "peer", remoteID, "exchange", idHex(o.ExchangeID), "role", "responder", "psk_fp", pskFingerprint(psk))
 
 	// Commit optimistically so our data path can rekey to the new PSK.
-	if err := m.cbHandler.OnNewPSKReady(remoteID, psk); err != nil {
+	if err := m.cbHandler.OnNewPSKReady(remoteID, psk, viaSignal); err != nil {
 		return nil, err
 	}
 	m.trace("pqkem: answer sent", "peer", remoteID, "exchange", idHex(o.ExchangeID))
@@ -125,7 +125,7 @@ func (m *Manager) processOffer(remoteID RemoteID, o *OfferMsg) ([]byte, error) {
 // stateAwaitingRekey; the next offer (chained from OnDataPathRekeyed) will acknowledge
 // this exchange. Only valid in stateAwaitingAnswer; advancing the state under the
 // lock makes a concurrent/duplicate answer bail.
-func (m *Manager) processAnswer(remoteID RemoteID, a *AnswerMsg) error {
+func (m *Manager) processAnswer(remoteID RemoteID, a *AnswerMsg, viaSignal bool) error {
 	m.mu.Lock()
 	ex := m.exchanges[remoteID]
 	if ex == nil || ex.id != a.ExchangeID || ex.state != stateAwaitingAnswer {
@@ -159,7 +159,7 @@ func (m *Manager) processAnswer(remoteID RemoteID, a *AnswerMsg) error {
 
 	m.trace("pqkem: new PSK derived", "peer", remoteID, "exchange", idHex(a.ExchangeID), "role", "initiator", "psk_fp", pskFingerprint(psk))
 
-	return m.cbHandler.OnNewPSKReady(remoteID, psk)
+	return m.cbHandler.OnNewPSKReady(remoteID, psk, viaSignal)
 }
 
 // ackConverged (responder) records convergence of the exchange named by ackID: a

client/internal/pqkem/manager.go

@@ -305,7 +305,7 @@ func (m *Manager) SignalOnOffer(remoteID RemoteID, offer []byte) ([]byte, error)
 	if typ != MsgOffer {
 		return nil, fmt.Errorf("expected offer from %s, got type %d", remoteID, typ)
 	}
-	return m.processOffer(remoteID, msg.(*OfferMsg))
+	return m.processOffer(remoteID, msg.(*OfferMsg), true)
 }
 
 // SignalOnAnswer processes a KEM answer the host extracted from an incoming answer.
@@ -318,7 +318,7 @@ func (m *Manager) SignalOnAnswer(remoteID RemoteID, answer []byte) error {
 	if typ != MsgAnswer {
 		return fmt.Errorf("expected answer from %s, got type %d", remoteID, typ)
 	}
-	return m.processAnswer(remoteID, msg.(*AnswerMsg))
+	return m.processAnswer(remoteID, msg.(*AnswerMsg), true)
 }
 
 // ---- Data path ----
@@ -346,7 +346,7 @@ func (m *Manager) OnDataPathMessage(remoteID RemoteID, raw []byte) error {
 	}
 	switch typ {
 	case MsgOffer:
-		answer, err := m.processOffer(remoteID, msg.(*OfferMsg))
+		answer, err := m.processOffer(remoteID, msg.(*OfferMsg), false)
 		if err != nil {
 			return err
 		}
@@ -355,7 +355,7 @@ func (m *Manager) OnDataPathMessage(remoteID RemoteID, raw []byte) error {
 		}
 		return m.pushDataPath(remoteID, answer)
 	case MsgAnswer:
-		return m.processAnswer(remoteID, msg.(*AnswerMsg))
+		return m.processAnswer(remoteID, msg.(*AnswerMsg), false)
 	default:
 		return fmt.Errorf("unhandled data-path message type %d from %s", typ, remoteID)
 	}

client/internal/pqkem/manager_test.go

@@ -65,7 +65,7 @@ type fakeWG struct {
 
 func newFakeWG() *fakeWG { return &fakeWG{psks: map[RemoteID]PSK{}} }
 
-func (f *fakeWG) OnNewPSKReady(remoteID RemoteID, psk PSK) error {
+func (f *fakeWG) OnNewPSKReady(remoteID RemoteID, psk PSK, bootstrap bool) error {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 	f.psks[remoteID] = psk

client/internal/pqkem_adapter.go

@@ -23,16 +23,30 @@ type pqCallbackHandler struct {
 	// reoffer re-bootstraps the KEM over Signal for a peer (a fresh signalling offer)
 	// to recover from a persistent data-path rekey failure. Nil disables recovery.
 	reoffer func(remoteKey string)
+	// rehandshake forces the peer's WireGuard session to adopt a freshly bootstrapped
+	// PSK (the live session may be up on a pre-PQ key). Nil disables it.
+	rehandshake func(remoteKey string)
 }
 
 // OnNewPSKReady programs the freshly derived PSK for the peer (updateOnly: a no-op
 // if the peer is not present, mirroring Rosenpass).
-func (h pqCallbackHandler) OnNewPSKReady(remoteID pqkem.RemoteID, psk pqkem.PSK) error {
+func (h pqCallbackHandler) OnNewPSKReady(remoteID pqkem.RemoteID, psk pqkem.PSK, bootstrap bool) error {
 	// updateOnly: applies to an already-configured peer (rotation). At bootstrap the
 	// peer is not configured yet, so this is a no-op there and the PSK is instead
 	// pulled at peer-config time (pqHandshaker.PSK / conn.presharedKey).
 	log.Tracef("pqkem: programming PSK for peer %s", remoteID)
-	return h.wg.SetPresharedKey(string(remoteID), wgtypes.Key(psk), true)
+	if err := h.wg.SetPresharedKey(string(remoteID), wgtypes.Key(psk), true); err != nil {
+		return err
+	}
+	// A bootstrap PSK (initial setup or wake re-negotiation) may arrive after the
+	// WireGuard session already came up on a pre-PQ key (a race between the KEM
+	// exchange and the endpoint config). SetPresharedKey only updates config, not the
+	// live session, so force a re-handshake to actually adopt the PSK now. Rotations
+	// are left to WireGuard's next natural rekey.
+	if bootstrap && h.rehandshake != nil {
+		h.rehandshake(string(remoteID))
+	}
+	return nil
 }
 
 // OnRekeyFailed reports a failed PQ (re)key convergence and re-bootstraps the KEM over

client/internal/pqkem_adapter_test.go

@@ -10,8 +10,8 @@ import (
 
 type pqNoopHandler struct{}
 
-func (pqNoopHandler) OnNewPSKReady(pqkem.RemoteID, pqkem.PSK) error { return nil }
+func (pqNoopHandler) OnNewPSKReady(pqkem.RemoteID, pqkem.PSK, bool) error { return nil }
-func (pqNoopHandler) OnRekeyFailed(pqkem.RemoteID) error            { return nil }
+func (pqNoopHandler) OnRekeyFailed(pqkem.RemoteID) error                  { return nil }
 
 // TestPQAdapter_CapabilityRoleAware locks the role-aware capability signal: the KEM
 // payload only flows initiator-offer -> responder-answer, so an empty message in the