PoC Onion messaging using `msgmux` #9868

gijswijs · 2025-05-26T14:18:23Z

In the spirit of share early, share often and also in the spirit of relatively small and incremental PRs, I'm sharing my first stab at onion messaging using the new message multiplexer package msgmux. It's derived from the custom message handling, but adjusted as needed to conform to the msgmux constraint.

Scope of this PR:

Receive onion messages by registering a msgmux endpoint in brontide
'Handle' onion messages by dispatching the message to subscribers
Sending onion messages to a specified peer (The peer is passed through the endpoint; not derived from the onion message itself)
itest testing the above behavior

Out-of-scope (for now)

Onion message payload
Path finding
Blinded paths
Registering endpoint in brontide based on
- onion messaging support from peers
- active channel with peers
announcing onion message support through featurebit 38/39
rate limiting/backpressure to mitigate DoS (see queue: add new BackpressureQueue[T] variant #9838)
BOLT12 offer and invoice request

coderabbitai · 2025-05-26T14:18:30Z

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ziggie1984

Nice start of the onion message saga 🎉, I think the msgmux is the way to go here. I am not sure if we should use one server for all peers but if we use it we should buffer the update channel of the onionserver.

Let's now decrypt the onion message in the next PR ?

ziggie1984 · 2025-06-07T09:04:33Z

lnwire/onion_message.go

+	}
+
+	onionLen := len(o.OnionBlob)
+	if err := WriteElement(w, uint16(onionLen)); err != nil {


Why not use varint ?

I think WriteUint16 is maybe even a better choice. WriteElement is about to be decommissioned so I changed the other references as well.

lnrpc/lightning.proto

ziggie1984 · 2025-06-07T09:06:33Z

lnrpc/lightning.proto

+    // onion message.
+    string blinding_point = 2;
+
+    // Onion is the raw serialized mix header used to relay messages in a


Q: what do you mean by mix header I mean this is the body of the message why do you call it header ?

"mix header" is the term used in the original Sphinx paper. The "mix header" is the part that contains the per-hop encrypted instructions for each node along the route. I find the usage here confusing as well. (It's copy-pasta from Carla's code, but I'll take responsibility for any confusion 😄).

I'll update it to this:

// Onion is the raw serialized Sphinx onion packet containing the // per-hop encrypted routing information used to relay this message // through the network in a privacy-preserving manner.

msgmux/onion_endpoint.go

ziggie1984 · 2025-06-07T09:10:50Z

msgmux/onion_endpoint.go

+
+// SendMessage processes the incoming onion message.
+// It returns true if the message was successfully processed.
+func (o *OnionEndpoint) SendMessage(ctx context.Context, msg PeerMsg) bool {


I find the name confusing it is called SendMessage but handles Incoming onion messages maybe call it forwardToListener or something ?

Tell me about it! But this is what the msgmux interface enforces:
https://github.com/gijswijs/lnd/blob/dea09c824f91bd7a08b901ad54caab610204a4b0/msgmux/msg_router.go#L37-L51

server.go

ziggie1984 · 2025-06-07T09:17:27Z

server.go

@@ -5314,7 +5314,7 @@ func (s *server) applyChannelUpdate(update *lnwire.ChannelUpdate1,
 func (s *server) SendCustomMessage(peerPub [33]byte, msgType lnwire.MessageType,
 	data []byte) error {

-	peer, err := s.FindPeerByPubStr(hex.EncodeToString(peerPub[:]))
+	peer, err := s.FindPeerByPubStr(string(peerPub[:]))


we should fix this in the FindPeerByPubStr method, we should use a proper hex-encoded string.

ziggie1984 · 2025-06-07T09:47:50Z

peer/brontide.go

@@ -886,6 +890,21 @@ func (p *Brontide) Start() error {
 		return fmt.Errorf("unable to load channels: %w", err)
 	}

+	onionMessageEndpoint := msgmux.NewOnionEndpoint(
+		p.cfg.OnionMessageServer,


I am not sure if this is the right approach, we use 1 server-level Server with an update channel which is not buffered to funnel in all onion message from all peers, seem not really efficient ?

this is the update channel of the server:

lnd/subscribe/subscribe.go

Lines 154 to 161 in c6d6d4c

func (s *Server) SendUpdate(update interface{}) error {

select {

case s.updates <- update:

return nil

case <-s.quit:

return ErrServerShuttingDown

}

Not sure if I completely follow you critique.

We have a single subscription endpoint for onion messages (similar to custom messages) which allows for "listening in" on onion messages received by this node. We pass along the subscription server here to the msgmux endpoint so that when we handle the onion message, we can notify a subscriber of an incoming message.

Buffering or rate limiting the actual onion message itself should happen upstream of the notification server, right?

I think we talked about this via other means and agreed upon the fact that we need to differentiate incoming messages so that onion-messages will not hinder channel msges to be received in time.

ellemouton

removing request for review in the mean time :) feel free to re-request when ready!

This message type is a message that carries an onion-encrypted payload used for BOLT12 messages.

This commit creates the necessary endpoints for onion messages. Specifically, it adds the following: - `SendOnionMessage` endpoint to send onion messages. - `SubscribeOnionMessages` endpoint to subscribe to incoming onion messages. It uses the `msgmux` package to handle the onion messages.

mohamedawnallah

Thanks, @gijswijs, for putting this together! I've added some comments. Overall, I like the design and how it's seamlessly integrated with the LND system/subsystems. I can form a clear mental picture of the changes. Nice work!

mohamedawnallah · 2025-08-04T15:15:05Z

lnwire/onion_message.go

+		return err
+	}
+
+	o.OnionBlob = make([]byte, onionLen)


nit: Perhaps adding a basic sanity check about onion message length (e.g. empty one) and the onion blob size (we can utilize lnwire/lnwire.MaxMsgBody) makes the size boundary more clear

mohamedawnallah · 2025-08-04T15:30:39Z

onion_message/onion_endpoint.go

+
+// A compile-time check to ensure OnionEndpoint implements the Endpoint
+// interface.
+var _ msgmux.Endpoint = (*OnionEndpoint)(nil)


nit: The interface check var _ msgmux.Endpoint = (*OnionEndpoint)(nil) is separated from the OnionEndpoint struct making it harder to see what interfaces are implemented, considering move it immediately after the OnionEndpoint definition (and perhaps considering moving OnionMessageUpdate before OnionEndpoint for better organization). With this in place perhaps this comment wouldn't have posted #9868 (comment)

mohamedawnallah · 2025-08-04T15:43:08Z

onion_message/onion_endpoint.go

+type OnionEndpoint struct {
+	// subscribe.Server is used for subscriptions to onion messages.
+	onionMessageServer *subscribe.Server
+}


Q: This is for my education. It seems there is no rate limiting exists on incoming onion messages per peer which allows peers to spam unlimited onion messages potentially overwhelming node resources and message questions ?

I've already saw your comment #9868 (comment):

Buffering or rate limiting the actual onion message itself should happen upstream of the notification server, right?

Does that mean we currently get out of the box rate limiting (assuming same priority) for each endpoint we register or it is a TODO?

mohamedawnallah · 2025-08-04T16:14:13Z

server.go

+
+	// Send the message as low-priority. For now we assume that all
+	// application-defined message are low priority.
+	return peer.SendMessageLazy(true, msg)


mohamedawnallah · 2025-08-04T16:15:43Z

onion_message/onion_endpoint.go

+
+	var peerArr [33]byte
+	copy(peerArr[:], peer)
+	err := o.onionMessageServer.SendUpdate(&OnionMessageUpdate{


It appears this sends updates to the subscribers without any priority control which could block subscriber notifications when processing high-volume onion messages but when we send the onion message we assign it low priority as in SendOnionMessage using SendMessageLazy. Should we enforce same/similar priority mechanism constraint here so it wouldn't interfere with probably higher priority updates?

This may overlap with this comment #9868 (comment).

mohamedawnallah · 2025-08-04T16:19:23Z

server.go

@@ -5404,6 +5420,33 @@ func (s *server) SendCustomMessage(peerPub [33]byte, msgType lnwire.MessageType,
 	return peer.SendMessageLazy(true, msg)
 }

+// SendOnionMessage sends a custom message to the peer with the specified
+// pubkey.
+// TODO(gijs): change this message to include path finding.


Perhaps this is the reason why the PR in draft mode?

gijswijs requested review from ziggie1984 and ellemouton and removed request for ziggie1984 May 26, 2025 14:18

saubyk assigned gijswijs May 27, 2025

ziggie1984 reviewed Jun 7, 2025

View reviewed changes

gijswijs force-pushed the onion-messaging branch 2 times, most recently from 5762808 to 9f23971 Compare June 10, 2025 15:49

saubyk added this to the v0.20.0 milestone Jun 11, 2025

saubyk added the onion routing label Jun 11, 2025

saubyk added this to lnd v0.20 Jun 11, 2025

saubyk moved this to In progress in lnd v0.20 Jun 11, 2025

ellemouton reviewed Jul 8, 2025

View reviewed changes

gijswijs force-pushed the onion-messaging branch from 9f23971 to e5726a7 Compare July 9, 2025 13:36

ZZiigguurraatt mentioned this pull request Jul 15, 2025

Onion messaging + bolt 12 offers #9369

Closed

5 tasks

gijswijs added 3 commits July 17, 2025 21:01

lnwire: add onion message type

942a4b3

This message type is a message that carries an onion-encrypted payload used for BOLT12 messages.

lnrpc: SendOnionMessage rpc endpoint

f498067

gijswijs force-pushed the onion-messaging branch from e5726a7 to fefe590 Compare July 17, 2025 19:02

gijswijs mentioned this pull request Jul 17, 2025

Onion message forwarding #10089

Open

mohamedawnallah reviewed Aug 4, 2025

View reviewed changes

	func (s *Server) SendUpdate(update interface{}) error {

	select {
	case s.updates <- update:
	return nil
	case <-s.quit:
	return ErrServerShuttingDown
	}

PoC Onion messaging using msgmux #9868

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PoC Onion messaging using `msgmux` #9868

PoC Onion messaging using `msgmux` #9868

gijswijs commented May 26, 2025 •

edited by saubyk

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