Generalize onion message ForwardTlvs::next_node_id

Allow either using a node id or a short channel id when forwarding an
onion message. This allows for a more compact representation of blinded
paths, which is advantageous for reducing offer QR code size.

Follow-up commits will implement handling the short channel id case as
it requires looking up the destination node id.

Author: jkczyz

lightning/src/blinded_path/message.rs

@@ -19,8 +19,8 @@ use core::ops::Deref;
 /// TLVs to encode in an intermediate onion message packet's hop data. When provided in a blinded
 /// route, they are encoded into [`BlindedHop::encrypted_payload`].
 pub(crate) struct ForwardTlvs {
-	/// The node id of the next hop in the onion message's path.
-	pub(crate) next_node_id: PublicKey,
+	/// The next hop in the onion message's path.
+	pub(crate) next_hop: NextHop,
 	/// Senders to a blinded path use this value to concatenate the route they find to the
 	/// introduction node with the blinded path.
 	pub(crate) next_blinding_override: Option<PublicKey>,
@@ -34,11 +34,25 @@ pub(crate) struct ReceiveTlvs {
 	pub(crate) path_id: Option<[u8; 32]>,
 }
 
+/// The next hop to forward the onion message along its path.
+#[derive(Debug)]
+pub enum NextHop {
+	/// The node id of the next hop.
+	NodeId(PublicKey),
+	/// The short channel id leading to the next hop.
+	ShortChannelId(u64),
+}
+
 impl Writeable for ForwardTlvs {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
+		let (next_node_id, short_channel_id) = match self.next_hop {
+			NextHop::NodeId(pubkey) => (Some(pubkey), None),
+			NextHop::ShortChannelId(scid) => (None, Some(scid)),
+		};
 		// TODO: write padding
 		encode_tlv_stream!(writer, {
-			(4, self.next_node_id, required),
+			(2, short_channel_id, option),
+			(4, next_node_id, option),
 			(8, self.next_blinding_override, option)
 		});
 		Ok(())
@@ -61,9 +75,8 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 ) -> Result<Vec<BlindedHop>, secp256k1::Error> {
 	let blinded_tlvs = unblinded_path.iter()
 		.skip(1) // The first node's TLVs contains the next node's pubkey
-		.map(|pk| {
-			ControlTlvs::Forward(ForwardTlvs { next_node_id: *pk, next_blinding_override: None })
-		})
+		.map(|pk| ForwardTlvs { next_hop: NextHop::NodeId(*pk), next_blinding_override: None })
+		.map(|tlvs| ControlTlvs::Forward(tlvs))
 		.chain(core::iter::once(ControlTlvs::Receive(ReceiveTlvs { path_id: None })));
 
 	utils::construct_blinded_hops(secp_ctx, unblinded_path.iter(), blinded_tlvs, session_priv)
@@ -80,9 +93,13 @@ pub(crate) fn advance_path_by_one<NS: Deref, T: secp256k1::Signing + secp256k1::
 	let mut s = Cursor::new(&encrypted_control_tlvs);
 	let mut reader = FixedLengthReader::new(&mut s, encrypted_control_tlvs.len() as u64);
 	match ChaChaPolyReadAdapter::read(&mut reader, rho) {
-		Ok(ChaChaPolyReadAdapter { readable: ControlTlvs::Forward(ForwardTlvs {
-			mut next_node_id, next_blinding_override,
-		})}) => {
+		Ok(ChaChaPolyReadAdapter {
+			readable: ControlTlvs::Forward(ForwardTlvs { next_hop, next_blinding_override })
+		}) => {
+			let mut next_node_id = match next_hop {
+				NextHop::NodeId(pubkey) => pubkey,
+				NextHop::ShortChannelId(_) => todo!(),
+			};
 			let mut new_blinding_point = match next_blinding_override {
 				Some(blinding_point) => blinding_point,
 				None => {

lightning/src/onion_message/messenger.rs

@@ -16,7 +16,7 @@ use bitcoin::hashes::sha256::Hash as Sha256;
 use bitcoin::secp256k1::{self, PublicKey, Scalar, Secp256k1, SecretKey};
 
 use crate::blinded_path::BlindedPath;
-use crate::blinded_path::message::{advance_path_by_one, ForwardTlvs, ReceiveTlvs};
+use crate::blinded_path::message::{advance_path_by_one, ForwardTlvs, NextHop, ReceiveTlvs};
 use crate::blinded_path::utils;
 use crate::events::{Event, EventHandler, EventsProvider};
 use crate::sign::{EntropySource, NodeSigner, Recipient};
@@ -538,7 +538,7 @@ pub trait CustomOnionMessageHandler {
 #[derive(Debug)]
 pub enum PeeledOnion<T: OnionMessageContents> {
 	/// Forwarded onion, with the next node id and a new onion
-	Forward(PublicKey, OnionMessage),
+	Forward(NextHop, OnionMessage),
 	/// Received onion message, with decrypted contents, path_id, and reply path
 	Receive(ParsedOnionMessageContents<T>, Option<[u8; 32]>, Option<BlindedPath>)
 }
@@ -647,9 +647,9 @@ where
 			Ok(PeeledOnion::Receive(message, path_id, reply_path))
 		},
 		Ok((Payload::Forward(ForwardControlTlvs::Unblinded(ForwardTlvs {
-			next_node_id, next_blinding_override
+			next_hop, next_blinding_override
 		})), Some((next_hop_hmac, new_packet_bytes)))) => {
-			// TODO: we need to check whether `next_node_id` is our node, in which case this is a dummy
+			// TODO: we need to check whether `next_hop` is our node, in which case this is a dummy
 			// blinded hop and this onion message is destined for us. In this situation, we should keep
 			// unwrapping the onion layers to get to the final payload. Since we don't have the option
 			// of creating blinded paths with dummy hops currently, we should be ok to not handle this
@@ -685,7 +685,7 @@ where
 				onion_routing_packet: outgoing_packet,
 			};
 
-			Ok(PeeledOnion::Forward(next_node_id, onion_message))
+			Ok(PeeledOnion::Forward(next_hop, onion_message))
 		},
 		Err(e) => {
 			log_trace!(logger, "Errored decoding onion message packet: {:?}", e);
@@ -961,7 +961,12 @@ where
 					},
 				}
 			},
-			Ok(PeeledOnion::Forward(next_node_id, onion_message)) => {
+			Ok(PeeledOnion::Forward(next_hop, onion_message)) => {
+				let next_node_id = match next_hop {
+					NextHop::NodeId(pubkey) => pubkey,
+					NextHop::ShortChannelId(_) => todo!(),
+				};
+
 				let mut message_recipients = self.message_recipients.lock().unwrap();
 				if outbound_buffer_full(&next_node_id, &message_recipients) {
 					log_trace!(
@@ -1146,7 +1151,7 @@ fn packet_payloads_and_keys<T: OnionMessageContents, S: secp256k1::Signing + sec
 				if let Some(ss) = prev_control_tlvs_ss.take() {
 					payloads.push((Payload::Forward(ForwardControlTlvs::Unblinded(
 						ForwardTlvs {
-							next_node_id: unblinded_pk_opt.unwrap(),
+							next_hop: NextHop::NodeId(unblinded_pk_opt.unwrap()),
 							next_blinding_override: None,
 						}
 					)), ss));
@@ -1156,7 +1161,7 @@ fn packet_payloads_and_keys<T: OnionMessageContents, S: secp256k1::Signing + sec
 			} else if let Some((intro_node_id, blinding_pt)) = intro_node_id_blinding_pt.take() {
 				if let Some(control_tlvs_ss) = prev_control_tlvs_ss.take() {
 					payloads.push((Payload::Forward(ForwardControlTlvs::Unblinded(ForwardTlvs {
-						next_node_id: intro_node_id,
+						next_hop: NextHop::NodeId(intro_node_id),
 						next_blinding_override: Some(blinding_pt),
 					})), control_tlvs_ss));
 				}

lightning/src/onion_message/packet.rs

@@ -13,7 +13,7 @@ use bitcoin::secp256k1::PublicKey;
 use bitcoin::secp256k1::ecdh::SharedSecret;
 
 use crate::blinded_path::BlindedPath;
-use crate::blinded_path::message::{ForwardTlvs, ReceiveTlvs};
+use crate::blinded_path::message::{ForwardTlvs, NextHop, ReceiveTlvs};
 use crate::blinded_path::utils::Padding;
 use crate::ln::msgs::DecodeError;
 use crate::ln::onion_utils;
@@ -284,20 +284,26 @@ impl Readable for ControlTlvs {
 	fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
 		_init_and_read_tlv_stream!(r, {
 			(1, _padding, option),
-			(2, _short_channel_id, option),
+			(2, short_channel_id, option),
 			(4, next_node_id, option),
 			(6, path_id, option),
 			(8, next_blinding_override, option),
 		});
 		let _padding: Option<Padding> = _padding;
-		let _short_channel_id: Option<u64> = _short_channel_id;
 
-		let valid_fwd_fmt  = next_node_id.is_some() && path_id.is_none();
-		let valid_recv_fmt = next_node_id.is_none() && next_blinding_override.is_none();
+		let next_hop = match (short_channel_id, next_node_id) {
+			(Some(_), Some(_)) => return Err(DecodeError::InvalidValue),
+			(Some(scid), None) => Some(NextHop::ShortChannelId(scid)),
+			(None, Some(pubkey)) => Some(NextHop::NodeId(pubkey)),
+			(None, None) => None,
+		};
+
+		let valid_fwd_fmt = next_hop.is_some() && path_id.is_none();
+		let valid_recv_fmt = next_hop.is_none() && next_blinding_override.is_none();
 
 		let payload_fmt = if valid_fwd_fmt {
 			ControlTlvs::Forward(ForwardTlvs {
-				next_node_id: next_node_id.unwrap(),
+				next_hop: next_hop.unwrap(),
 				next_blinding_override,
 			})
 		} else if valid_recv_fmt {