Account for fully private paths.

Author: tnull

lightning/src/routing/router.rs

@@ -1521,49 +1521,58 @@ where L::Target: Logger {
 		let mut shadow_ctlv_expiry_delta_offset: u32 = 0;
 
 		// Choose the last publicly known node as the starting point for the random walk
-		let mut cur_node_iter = path.iter().rev();
-		let mut cur_node_id = NodeId::from_pubkey(&cur_node_iter.next().unwrap().as_ref().unwrap().pubkey);
-		assert_eq!(cur_node_id, payee_node_id);
-		while !network_nodes.contains_key(&cur_node_id) {
-			cur_node_id = NodeId::from_pubkey(&cur_node_iter.next().unwrap().as_ref().unwrap().pubkey);
-		}
-
-		// Init PRNG
-		let mut path_nonce = [0u8; 8];
-		path_nonce.copy_from_slice(&cur_node_id.as_slice()[..8]);
-		let mut prng = ChaCha20::new(random_seed_bytes, &path_nonce);
-		let mut random_path_bytes = [0u8; 8];
-
-		// Pick a random path length in [1 .. 3]
-		prng.process_in_place(&mut random_path_bytes);
-		let random_walk_length = usize::from_be_bytes(random_path_bytes).wrapping_rem(3).wrapping_add(1);
-
-		for _random_hop in 0..random_walk_length { 
-			if let Some(cur_node) = network_nodes.get(&cur_node_id) {
-				// Randomly choose the next hop
-				prng.process_in_place(&mut random_path_bytes);
-				if let Some(random_index) = usize::from_be_bytes(random_path_bytes).checked_rem(cur_node.channels.len()) {
-					if let Some(random_channel_id) = cur_node.channels.get(random_index) {
-						if let Some(random_channel) = network_channels.get(random_channel_id) {
-							if random_channel.node_one == cur_node_id {
-								cur_node_id = random_channel.node_two;
-								if let Some(dir_info) = &random_channel.one_to_two {
-									shadow_ctlv_expiry_delta_offset = shadow_ctlv_expiry_delta_offset
-										.checked_add(dir_info.cltv_expiry_delta.into()).unwrap_or(shadow_ctlv_expiry_delta_offset);
-								}
-							} else {
-								cur_node_id = random_channel.node_one;
-								if let Some(dir_info) = &random_channel.two_to_one {
-									shadow_ctlv_expiry_delta_offset = shadow_ctlv_expiry_delta_offset
-										.checked_add(dir_info.cltv_expiry_delta.into()).unwrap_or(shadow_ctlv_expiry_delta_offset);
-								}
-							};
+		if let Some(starting_hop) = path.iter().rev().find(|h| network_nodes.contains_key(&NodeId::from_pubkey(&h.as_ref().unwrap().pubkey))) {
+			let mut cur_node_id = NodeId::from_pubkey(&starting_hop.as_ref().unwrap().pubkey);
+
+			// Init PRNG with path nonce
+			let mut path_nonce = [0u8; 8];
+			path_nonce.copy_from_slice(&cur_node_id.as_slice()[..8]);
+			let mut prng = ChaCha20::new(random_seed_bytes, &path_nonce);
+			let mut random_path_bytes = [0u8; 8];
+
+			// Pick a random path length in [1 .. 3]
+			prng.process_in_place(&mut random_path_bytes);
+			let random_walk_length = usize::from_be_bytes(random_path_bytes).wrapping_rem(3).wrapping_add(1);
+
+			for _random_hop in 0..random_walk_length { 
+				if let Some(cur_node) = network_nodes.get(&cur_node_id) {
+					// Randomly choose the next hop
+					prng.process_in_place(&mut random_path_bytes);
+					if let Some(random_index) = usize::from_be_bytes(random_path_bytes).checked_rem(cur_node.channels.len()) {
+						if let Some(random_channel_id) = cur_node.channels.get(random_index) {
+							if let Some(random_channel) = network_channels.get(random_channel_id) {
+								if random_channel.node_one == cur_node_id {
+									cur_node_id = random_channel.node_two;
+									if let Some(dir_info) = &random_channel.one_to_two {
+										shadow_ctlv_expiry_delta_offset = shadow_ctlv_expiry_delta_offset
+											.checked_add(dir_info.cltv_expiry_delta.into()).unwrap_or(shadow_ctlv_expiry_delta_offset);
+									}
+								} else {
+									cur_node_id = random_channel.node_one;
+									if let Some(dir_info) = &random_channel.two_to_one {
+										shadow_ctlv_expiry_delta_offset = shadow_ctlv_expiry_delta_offset
+											.checked_add(dir_info.cltv_expiry_delta.into()).unwrap_or(shadow_ctlv_expiry_delta_offset);
+									}
+								};
+							}
 						}
 					}
 				}
 			}
+		} else {
+			// If the entire path is private, choose a random offset from multiples of 144, i.e., our
+			// default cltv_expiry_delta
+			let mut prng = ChaCha20::new(random_seed_bytes, &[0u8; 8]);
+			let mut random_bytes = [0u8; 4];
+			prng.process_in_place(&mut random_bytes);
+			let random_walk_length = u32::from_be_bytes(random_bytes).wrapping_rem(3).wrapping_add(1);
+			shadow_ctlv_expiry_delta_offset = random_walk_length.wrapping_mul(144);
 		}
 
+		// Limit the offset to reduce the payment failure probabilty
+		const MAX_SHADOW_CLTV_EXPIRY_DELTA_OFFSET: u32 = 432; // 3*144
+		shadow_ctlv_expiry_delta_offset = cmp::min(shadow_ctlv_expiry_delta_offset, MAX_SHADOW_CLTV_EXPIRY_DELTA_OFFSET);
+
 		// Add 'shadow' CLTV offset to all hops but the final one
 		for h in path {
 			if let Ok(hop) = h {