Compact a BlindedPath's introduction node

Add a method to BlindedPath that given a network graph will compact the
IntroductionNode as the DirectedShortChannelId variant. Call this method
from DefaultMessageRouter so that Offer paths use the compact
representation (along with reply paths). This leaves payment paths in
Bolt12Invoice using the NodeId variant, as the compact representation
isn't as useful there.

Author: jkczyz

lightning/src/blinded_path/mod.rs

@@ -21,6 +21,7 @@ use crate::offers::invoice::BlindedPayInfo;
 use crate::routing::gossip::{NodeId, ReadOnlyNetworkGraph};
 use crate::sign::EntropySource;
 use crate::util::ser::{Readable, Writeable, Writer};
+use crate::util::scid_utils;
 
 use crate::io;
 use crate::prelude::*;
@@ -217,6 +218,35 @@ impl BlindedPath {
 			},
 		}
 	}
+
+	/// Attempts to a use a compact representation for the [`IntroductionNode`] by using a directed
+	/// short channel id from a channel in `network_graph` leading to the introduction node.
+	///
+	/// While this may result in a smaller encoding, there is a trade off in that the path may
+	/// become invalid if the channel is closed or hasn't been propagated via gossip. Therefore,
+	/// calling this may not be suitable for long-lived blinded paths.
+	pub fn use_compact_introduction_node(&mut self, network_graph: &ReadOnlyNetworkGraph) {
+		if let IntroductionNode::NodeId(pubkey) = &self.introduction_node {
+			let node_id = NodeId::from_pubkey(pubkey);
+			if let Some(node_info) = network_graph.node(&node_id) {
+				if let Some((scid, channel_info)) = node_info
+					.channels
+					.iter()
+					.filter_map(|scid| network_graph.channel(*scid).map(|info| (*scid, info)))
+					.min_by_key(|(scid, _)| scid_utils::block_from_scid(*scid))
+				{
+					let direction = if node_id == channel_info.node_one {
+						Direction::NodeOne
+					} else {
+						debug_assert_eq!(node_id, channel_info.node_two);
+						Direction::NodeTwo
+					};
+					self.introduction_node =
+						IntroductionNode::DirectedShortChannelId(direction, scid);
+				}
+			}
+		}
+	}
 }
 
 impl Writeable for BlindedPath {

lightning/src/ln/offers_tests.rs

@@ -41,6 +41,7 @@
 //! blinded paths are used.
 
 use bitcoin::network::constants::Network;
+use bitcoin::secp256k1::PublicKey;
 use core::time::Duration;
 use crate::blinded_path::{BlindedPath, IntroductionNode};
 use crate::blinded_path::payment::{Bolt12OfferContext, Bolt12RefundContext, PaymentContext};
@@ -133,6 +134,12 @@ fn announce_node_address<'a, 'b, 'c>(
 	}
 }
 
+fn resolve_introduction_node<'a, 'b, 'c>(node: &Node<'a, 'b, 'c>, path: &BlindedPath) -> PublicKey {
+	path.public_introduction_node_id(&node.network_graph.read_only())
+		.and_then(|node_id| node_id.as_pubkey().ok())
+		.unwrap()
+}
+
 fn route_bolt12_payment<'a, 'b, 'c>(
 	node: &Node<'a, 'b, 'c>, path: &[&Node<'a, 'b, 'c>], invoice: &Bolt12Invoice
 ) {
@@ -273,8 +280,9 @@ fn prefers_non_tor_nodes_in_blinded_paths() {
 	assert_ne!(offer.signing_pubkey(), Some(bob_id));
 	assert!(!offer.paths().is_empty());
 	for path in offer.paths() {
-		assert_ne!(path.introduction_node, IntroductionNode::NodeId(bob_id));
-		assert_ne!(path.introduction_node, IntroductionNode::NodeId(charlie_id));
+		let introduction_node_id = resolve_introduction_node(david, &path);
+		assert_ne!(introduction_node_id, bob_id);
+		assert_ne!(introduction_node_id, charlie_id);
 	}
 
 	// Use a one-hop blinded path when Bob is announced and all his peers are Tor-only.
@@ -288,7 +296,8 @@ fn prefers_non_tor_nodes_in_blinded_paths() {
 	assert_ne!(offer.signing_pubkey(), Some(bob_id));
 	assert!(!offer.paths().is_empty());
 	for path in offer.paths() {
-		assert_eq!(path.introduction_node, IntroductionNode::NodeId(bob_id));
+		let introduction_node_id = resolve_introduction_node(david, &path);
+		assert_eq!(introduction_node_id, bob_id);
 	}
 }
 
@@ -338,7 +347,8 @@ fn prefers_more_connected_nodes_in_blinded_paths() {
 	assert_ne!(offer.signing_pubkey(), Some(bob_id));
 	assert!(!offer.paths().is_empty());
 	for path in offer.paths() {
-		assert_eq!(path.introduction_node, IntroductionNode::NodeId(nodes[4].node.get_our_node_id()));
+		let introduction_node_id = resolve_introduction_node(david, &path);
+		assert_eq!(introduction_node_id, nodes[4].node.get_our_node_id());
 	}
 }
 
@@ -388,7 +398,9 @@ fn creates_and_pays_for_offer_using_two_hop_blinded_path() {
 	assert_ne!(offer.signing_pubkey(), Some(alice_id));
 	assert!(!offer.paths().is_empty());
 	for path in offer.paths() {
-		assert_eq!(path.introduction_node, IntroductionNode::NodeId(bob_id));
+		let introduction_node_id = resolve_introduction_node(david, &path);
+		assert_eq!(introduction_node_id, bob_id);
+		assert!(matches!(path.introduction_node, IntroductionNode::DirectedShortChannelId(..)));
 	}
 
 	let payment_id = PaymentId([1; 32]);
@@ -415,9 +427,11 @@ fn creates_and_pays_for_offer_using_two_hop_blinded_path() {
 			payer_note_truncated: None,
 		},
 	});
+	let introduction_node_id = resolve_introduction_node(alice, &reply_path);
 	assert_eq!(invoice_request.amount_msats(), None);
 	assert_ne!(invoice_request.payer_id(), david_id);
-	assert_eq!(reply_path.introduction_node, IntroductionNode::NodeId(charlie_id));
+	assert_eq!(introduction_node_id, charlie_id);
+	assert!(matches!(reply_path.introduction_node, IntroductionNode::DirectedShortChannelId(..)));
 
 	let onion_message = alice.onion_messenger.next_onion_message_for_peer(charlie_id).unwrap();
 	charlie.onion_messenger.handle_onion_message(&alice_id, &onion_message);
@@ -489,7 +503,9 @@ fn creates_and_pays_for_refund_using_two_hop_blinded_path() {
 	assert_ne!(refund.payer_id(), david_id);
 	assert!(!refund.paths().is_empty());
 	for path in refund.paths() {
-		assert_eq!(path.introduction_node, IntroductionNode::NodeId(charlie_id));
+		let introduction_node_id = resolve_introduction_node(alice, &path);
+		assert_eq!(introduction_node_id, charlie_id);
+		assert!(matches!(path.introduction_node, IntroductionNode::DirectedShortChannelId(..)));
 	}
 	expect_recent_payment!(david, RecentPaymentDetails::AwaitingInvoice, payment_id);
 
@@ -545,7 +561,9 @@ fn creates_and_pays_for_offer_using_one_hop_blinded_path() {
 	assert_ne!(offer.signing_pubkey(), Some(alice_id));
 	assert!(!offer.paths().is_empty());
 	for path in offer.paths() {
-		assert_eq!(path.introduction_node, IntroductionNode::NodeId(alice_id));
+		let introduction_node_id = resolve_introduction_node(bob, &path);
+		assert_eq!(introduction_node_id, alice_id);
+		assert!(matches!(path.introduction_node, IntroductionNode::DirectedShortChannelId(..)));
 	}
 
 	let payment_id = PaymentId([1; 32]);
@@ -564,9 +582,11 @@ fn creates_and_pays_for_offer_using_one_hop_blinded_path() {
 			payer_note_truncated: None,
 		},
 	});
+	let introduction_node_id = resolve_introduction_node(alice, &reply_path);
 	assert_eq!(invoice_request.amount_msats(), None);
 	assert_ne!(invoice_request.payer_id(), bob_id);
-	assert_eq!(reply_path.introduction_node, IntroductionNode::NodeId(bob_id));
+	assert_eq!(introduction_node_id, bob_id);
+	assert!(matches!(reply_path.introduction_node, IntroductionNode::DirectedShortChannelId(..)));
 
 	let onion_message = alice.onion_messenger.next_onion_message_for_peer(bob_id).unwrap();
 	bob.onion_messenger.handle_onion_message(&alice_id, &onion_message);
@@ -614,7 +634,9 @@ fn creates_and_pays_for_refund_using_one_hop_blinded_path() {
 	assert_ne!(refund.payer_id(), bob_id);
 	assert!(!refund.paths().is_empty());
 	for path in refund.paths() {
-		assert_eq!(path.introduction_node, IntroductionNode::NodeId(bob_id));
+		let introduction_node_id = resolve_introduction_node(alice, &path);
+		assert_eq!(introduction_node_id, bob_id);
+		assert!(matches!(path.introduction_node, IntroductionNode::DirectedShortChannelId(..)));
 	}
 	expect_recent_payment!(bob, RecentPaymentDetails::AwaitingInvoice, payment_id);
 

lightning/src/onion_message/messenger.rs

@@ -445,7 +445,7 @@ where
 			.take(MAX_PATHS)
 			.collect::<Result<Vec<_>, _>>();
 
-		match paths {
+		let mut paths = match paths {
 			Ok(paths) if !paths.is_empty() => Ok(paths),
 			_ => {
 				if is_recipient_announced {
@@ -455,7 +455,12 @@ where
 					Err(())
 				}
 			},
+		}?;
+		for path in &mut paths {
+			path.use_compact_introduction_node(&network_graph);
 		}
+
+		Ok(paths)
 	}
 }