Trampoline payload construction method

Author: arik-so

lightning/src/ln/onion_utils.rs

@@ -14,7 +14,7 @@ use crate::ln::channel::TOTAL_BITCOIN_SUPPLY_SATOSHIS;
 use crate::ln::channelmanager::{HTLCSource, RecipientOnionFields};
 use crate::ln::msgs;
 use crate::routing::gossip::NetworkUpdate;
-use crate::routing::router::{Path, RouteHop, RouteParameters};
+use crate::routing::router::{Path, RouteHop, RouteParameters, TrampolineHop};
 use crate::sign::NodeSigner;
 use crate::types::features::{ChannelFeatures, NodeFeatures};
 use crate::types::payment::{PaymentHash, PaymentPreimage};
@@ -175,6 +175,122 @@ pub(super) fn construct_onion_keys<T: secp256k1::Signing>(
 	Ok(res)
 }
 
+fn build_trampoline_onion_payloads<'a>(
+	path: &'a Path, total_msat: u64, recipient_onion: &'a RecipientOnionFields,
+	starting_htlc_offset: u32, keysend_preimage: &Option<PaymentPreimage>,
+) -> Result<(Vec<msgs::OutboundTrampolinePayload<'a>>, u64, u32), APIError> {
+	let mut res: Vec<msgs::OutboundTrampolinePayload> = Vec::with_capacity(
+		path.trampoline_hops.len() + path.blinded_tail.as_ref().map_or(0, |t| t.hops.len()),
+	);
+	let blinded_tail = path.blinded_tail.as_ref().ok_or(APIError::InvalidRoute {
+		err: "Routes using Trampoline must terminate blindly.".to_string(),
+	})?;
+	let blinded_tail_with_hop_iter = BlindedTailHopIter {
+		hops: blinded_tail.hops.iter(),
+		blinding_point: blinded_tail.blinding_point,
+		final_value_msat: blinded_tail.final_value_msat,
+		excess_final_cltv_expiry_delta: blinded_tail.excess_final_cltv_expiry_delta,
+	};
+
+	let (value_msat, cltv) = build_trampoline_onion_payloads_callback(
+		path.trampoline_hops.iter(),
+		blinded_tail_with_hop_iter,
+		total_msat,
+		recipient_onion,
+		starting_htlc_offset,
+		keysend_preimage,
+		|action, payload| match action {
+			PayloadCallbackAction::PushBack => res.push(payload),
+			PayloadCallbackAction::PushFront => res.insert(0, payload),
+		},
+	)?;
+	Ok((res, value_msat, cltv))
+}
+
+fn build_trampoline_onion_payloads_callback<'a, H, B, F>(
+	hops: H, mut blinded_tail: BlindedTailHopIter<'a, B>, total_msat: u64,
+	recipient_onion: &'a RecipientOnionFields, starting_htlc_offset: u32,
+	keysend_preimage: &Option<PaymentPreimage>, mut callback: F,
+) -> Result<(u64, u32), APIError>
+where
+	H: DoubleEndedIterator<Item = &'a TrampolineHop>,
+	B: ExactSizeIterator<Item = &'a BlindedHop>,
+	F: FnMut(PayloadCallbackAction, msgs::OutboundTrampolinePayload<'a>),
+{
+	let mut cur_value_msat = 0u64;
+	let mut cur_cltv = starting_htlc_offset;
+	let mut last_node_id = None;
+
+	// appeasing the borrow checker
+	let mut blinded_tail_option = Some(blinded_tail);
+
+	for (idx, hop) in hops.rev().enumerate() {
+		// First hop gets special values so that it can check, on receipt, that everything is
+		// exactly as it should be (and the next hop isn't trying to probe to find out if we're
+		// the intended recipient).
+		let value_msat = if cur_value_msat == 0 { hop.fee_msat } else { cur_value_msat };
+		let cltv = if cur_cltv == starting_htlc_offset {
+			hop.cltv_expiry_delta + starting_htlc_offset
+		} else {
+			cur_cltv
+		};
+		if idx == 0 {
+			if let Some(BlindedTailHopIter {
+				blinding_point,
+				hops,
+				final_value_msat,
+				excess_final_cltv_expiry_delta,
+			}) = blinded_tail_option.take()
+			{
+				let mut blinding_point = Some(blinding_point);
+				let hops_len = hops.len();
+				for (i, blinded_hop) in hops.enumerate() {
+					if i == hops_len - 1 {
+						cur_value_msat += final_value_msat;
+						callback(
+							PayloadCallbackAction::PushBack,
+							msgs::OutboundTrampolinePayload::BlindedReceive {
+								sender_intended_htlc_amt_msat: final_value_msat,
+								total_msat,
+								cltv_expiry_height: cur_cltv + excess_final_cltv_expiry_delta,
+								encrypted_tlvs: &blinded_hop.encrypted_payload,
+								intro_node_blinding_point: blinding_point.take(),
+								keysend_preimage: *keysend_preimage,
+								custom_tlvs: &recipient_onion.custom_tlvs,
+							},
+						);
+					} else {
+						callback(
+							PayloadCallbackAction::PushBack,
+							msgs::OutboundTrampolinePayload::BlindedForward {
+								encrypted_tlvs: &blinded_hop.encrypted_payload,
+								intro_node_blinding_point: blinding_point.take(),
+							},
+						);
+					}
+				}
+			}
+		} else {
+			let payload = msgs::OutboundTrampolinePayload::Forward {
+				outgoing_node_id: last_node_id.unwrap(),
+				amt_to_forward: value_msat,
+				outgoing_cltv_value: cltv,
+			};
+			callback(PayloadCallbackAction::PushFront, payload);
+		}
+		cur_value_msat += hop.fee_msat;
+		if cur_value_msat >= 21000000 * 100000000 * 1000 {
+			return Err(APIError::InvalidRoute { err: "Channel fees overflowed?".to_owned() });
+		}
+		cur_cltv += hop.cltv_expiry_delta as u32;
+		if cur_cltv >= 500000000 {
+			return Err(APIError::InvalidRoute { err: "Channel CLTV overflowed?".to_owned() });
+		}
+		last_node_id = Some(hop.pubkey);
+	}
+	Ok((cur_value_msat, cur_cltv))
+}
+
 /// returns the hop data, as well as the first-hop value_msat and CLTV value we should send.
 pub(super) fn build_onion_payloads<'a>(
 	path: &'a Path, total_msat: u64, recipient_onion: &'a RecipientOnionFields,