When the controller receives the responder's (KEM-less) offer it replies with
its own KEM offer instead of answering, so the only transaction that brings the
tunnel up is the one that also carries the PSK. Guard that reply with
ShouldSendBootstrapOffer so it fires only when no exchange is in flight: without
it, every responder offer triggered another offer (an offer-per-offer runaway).
The whole behaviour is isolated to the KEM path (config.PQ != nil); non-PQ
connections answer as before.
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.
OnRekeyFailed now re-runs the KEM bootstrap over Signal (conn.RequestReoffer ->
handshaker.SendOffer) instead of only logging: a fresh signalling offer starts a new
exchange that overwrites the stalled PSK on both sides, resyncing after a persistent
data-path desync. Chosen over a responder-side awaitingAck revert (which fights the
confirm-less ack timing) and a full tunnel teardown (heavier). The tunnel stays up on
the previous PSK meanwhile since Signal is independent of the broken data path.
- Add a trace slog level (NB_PQ_MLKEM_LOG_LEVEL=trace) and move the verbose
per-exchange lifecycle logs (offer/answer/PSK/ack/rotation) to it, so debug
stays quiet and troubleshooting is opt-in.
- Stop logging the raw preshared key; drop the temporary pqkem-dbg OnRemoteOffer/
OnRemoteAnswer probes.
- Demote the per-handshake conn log to trace.
Source OnDataPathRekeyed from the WGWatcher's per-handshake callback
(onWGCheckSuccess), which fires only on a fresh handshake, and OnDataPathDown
from the handshake-timeout path. A fresh handshake clocks the next chained
KEM exchange pushed over the data-path UDP transport.
Learn the peer's data-path endpoint from the signalling offer/answer: its WG
overlay IP combined with the advertised pq UDP port (SetRemotePort -> AddPeer).
Registering here is safe before the tunnel is up because sends only ever fire
once it is (clocked by OnDataPathRekeyed). RemovePeer is wired at peer teardown
(engine.removePeer), not on transient disconnect.
riccardom