Add test for dummy hop insertion

Introduces a test to verify correct handling of dummy hops
in constructed blinded paths.
Ensures that the added dummy hops are properly included and
do not interfere with the real path.

Author: shaavan

lightning/src/blinded_path/utils.rs

@@ -281,6 +281,12 @@ pub fn is_padded(hops: &[BlindedHop], padding_round_off: usize) -> bool {
 	let first_hop = hops.first().expect("BlindedPath must have at least one hop");
 	let first_payload_size = first_hop.encrypted_payload.len();
 
+	// If the hops are padded correctly, the first payload size must be at least
+	// the padding round off size.
+	if first_payload_size < padding_round_off {
+		return false;
+	}
+
 	// The unencrypted payload data is padded before getting encrypted.
 	// Assuming the first payload is padded properly, get the extra data length.
 	let extra_length = first_payload_size % padding_round_off;

lightning/src/onion_message/functional_tests.rs

@@ -443,6 +443,34 @@ fn one_blinded_hop() {
 	pass_along_path(&nodes);
 }
 
+#[test]
+fn blinded_path_with_dummy() {
+	let nodes = create_nodes(2);
+	let test_msg = TestCustomMessage::Pong;
+
+	let secp_ctx = Secp256k1::new();
+	let context = MessageContext::Custom(Vec::new());
+	let entropy = &*nodes[1].entropy_source;
+	let receive_key = nodes[1].messenger.node_signer.get_receive_auth_key();
+	let blinded_path = BlindedMessagePath::new_with_dummy_hops(
+		&[],
+		nodes[1].node_id,
+		5,
+		receive_key,
+		context,
+		entropy,
+		&secp_ctx,
+	)
+	.unwrap();
+	// Ensure that dummy hops are added to the blinded path.
+	assert_eq!(blinded_path.blinded_hops().len(), 6);
+	let destination = Destination::BlindedPath(blinded_path);
+	let instructions = MessageSendInstructions::WithoutReplyPath { destination };
+	nodes[0].messenger.send_onion_message(test_msg, instructions).unwrap();
+	nodes[1].custom_message_handler.expect_message(TestCustomMessage::Pong);
+	pass_along_path(&nodes);
+}
+
 #[test]
 fn two_unblinded_two_blinded() {
 	let nodes = create_nodes(5);
@@ -658,9 +686,10 @@ fn test_blinded_path_padding_for_full_length_path() {
 	let context = MessageContext::Custom(vec![0u8; 42]);
 	let entropy = &*nodes[3].entropy_source;
 	let receive_key = nodes[3].messenger.node_signer.get_receive_auth_key();
-	let blinded_path = BlindedMessagePath::new(
+	let blinded_path = BlindedMessagePath::new_with_dummy_hops(
 		&intermediate_nodes,
 		nodes[3].node_id,
+		5,
 		receive_key,
 		context,
 		entropy,
@@ -694,9 +723,10 @@ fn test_blinded_path_no_padding_for_compact_path() {
 	let context = MessageContext::Custom(vec![0u8; 42]);
 	let entropy = &*nodes[3].entropy_source;
 	let receive_key = nodes[3].messenger.node_signer.get_receive_auth_key();
-	let blinded_path = BlindedMessagePath::new(
+	let blinded_path = BlindedMessagePath::new_with_dummy_hops(
 		&intermediate_nodes,
 		nodes[3].node_id,
+		5,
 		receive_key,
 		context,
 		entropy,