Introduce test for padding

1. Add test to verify blinded message and payment path padding.

lightning/src/blinded_path/utils.rs

@@ -199,4 +199,12 @@ impl<T: Writeable> Writeable for WithPadding<T> {
 
 		self.tlvs.write(writer)
 	}
-}
+}
+
+#[cfg(test)]
+/// Checks if all the packets in the blinded path are properly padded, ensuring they are of equal size.
+pub fn is_properly_padded(path: &BlindedPath) -> bool {
+	let first_hop = path.blinded_hops.first().expect("BlindedPath must have at least one hop");
+	let first_payload_size = first_hop.encrypted_payload.len();
+	path.blinded_hops.iter().all(|hop| hop.encrypted_payload.len() == first_payload_size)
+}

lightning/src/ln/blinded_payment_tests.rs

@@ -8,6 +8,7 @@
 // licenses.
 
 use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};
+use crate::blinded_path::utils::is_properly_padded;
 use crate::blinded_path::BlindedPath;
 use crate::blinded_path::payment::{ForwardNode, ForwardTlvs, PaymentConstraints, PaymentContext, PaymentRelay, ReceiveTlvs};
 use crate::events::{Event, HTLCDestination, MessageSendEvent, MessageSendEventsProvider, PaymentFailureReason};
@@ -1332,3 +1333,39 @@ fn custom_tlvs_to_blinded_path() {
 			.with_custom_tlvs(recipient_onion_fields.custom_tlvs.clone())
 	);
 }
+
+#[test]
+fn blinded_payment_path_padding() {
+	// Make sure that for a blinded payment path, all encrypted payloads are padded to equal lengths.
+	let chanmon_cfgs = create_chanmon_cfgs(5);
+	let node_cfgs = create_node_cfgs(5, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(5, &node_cfgs, &[None, None, None, None, None]);
+	let mut nodes = create_network(5, &node_cfgs, &node_chanmgrs);
+	create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 1_000_000, 0);
+	create_announced_chan_between_nodes_with_value(&nodes, 1, 2, 1_000_000, 0);
+	let chan_upd_2_3 = create_announced_chan_between_nodes_with_value(&nodes, 2, 3, 1_000_000, 0).0.contents;
+	let chan_upd_3_4 = create_announced_chan_between_nodes_with_value(&nodes, 3, 4, 1_000_000, 0).0.contents;
+
+	// Get all our nodes onto the same height so payments don't fail for CLTV violations.
+	connect_blocks(&nodes[0], nodes[4].best_block_info().1 - nodes[0].best_block_info().1);
+	connect_blocks(&nodes[1], nodes[4].best_block_info().1 - nodes[1].best_block_info().1);
+	connect_blocks(&nodes[2], nodes[4].best_block_info().1 - nodes[2].best_block_info().1);
+	assert_eq!(nodes[4].best_block_info().1, nodes[3].best_block_info().1);
+
+	let amt_msat = 5000;
+	let (payment_preimage, payment_hash, payment_secret) = get_payment_preimage_hash(&nodes[4], Some(amt_msat), None);
+
+	let blinded_path = blinded_payment_path(payment_secret, 1, 1_0000_0000,
+		nodes.iter().skip(2).map(|n| n.node.get_our_node_id()).collect(), &[&chan_upd_2_3, &chan_upd_3_4],
+		&chanmon_cfgs[4].keys_manager
+	);
+
+	is_properly_padded(&blinded_path.1);
+
+	let route_params = RouteParameters::from_payment_params_and_value(PaymentParameters::blinded(vec![blinded_path]), amt_msat);
+
+	nodes[0].node.send_payment(payment_hash, RecipientOnionFields::spontaneous_empty(), PaymentId(payment_hash.0), route_params, Retry::Attempts(0)).unwrap();
+	check_added_monitors(&nodes[0], 1);
+	pass_along_route(&nodes[0], &[&[&nodes[1], &nodes[2], &nodes[3], &nodes[4]]], amt_msat, payment_hash, payment_secret);
+	claim_payment(&nodes[0], &[&nodes[1], &nodes[2], &nodes[3], &nodes[4]], payment_preimage);
+}

lightning/src/onion_message/functional_tests.rs

@@ -9,6 +9,7 @@
 
 //! Onion message testing and test utilities live here.
 
+use crate::blinded_path::utils::is_properly_padded;
 use crate::blinded_path::{BlindedPath, EmptyNodeIdLookUp};
 use crate::blinded_path::message::{ForwardNode, MessageContext, OffersContext};
 use crate::events::{Event, EventsProvider};
@@ -539,6 +540,29 @@ fn too_big_packet_error() {
 	assert_eq!(err, SendError::TooBigPacket);
 }
 
+#[test]
+fn blinded_path_padding() {
+	// Make sure that for a blinded path, all encrypted payloads are padded to equal lengths.
+	let nodes = create_nodes(4);
+	let test_msg = TestCustomMessage::Pong;
+
+	let secp_ctx = Secp256k1::new();
+	let intermediate_nodes = [
+		ForwardNode { node_id: nodes[1].node_id, short_channel_id: None },
+		ForwardNode { node_id: nodes[2].node_id, short_channel_id: None },
+	];
+	let context = MessageContext::Custom(Vec::new());
+	let blinded_path = BlindedPath::new_for_message(&intermediate_nodes, nodes[3].node_id, context, &*nodes[3].entropy_source, &secp_ctx).unwrap();
+
+	assert!(is_properly_padded(&blinded_path));
+
+	let destination = Destination::BlindedPath(blinded_path);
+
+	nodes[0].messenger.send_onion_message(test_msg, destination, None).unwrap();
+	nodes[3].custom_message_handler.expect_message(TestCustomMessage::Pong);
+	pass_along_path(&nodes);
+}
+
 #[test]
 fn we_are_intro_node() {
 	// If we are sending straight to a blinded path and we are the introduction node, we need to