Compute aggregated BlindedPayInfo in path construction

lightning/src/blinded_path/mod.rs

@@ -16,8 +16,9 @@ pub(crate) mod utils;
 use bitcoin::secp256k1::{self, PublicKey, Secp256k1, SecretKey};
 
 use crate::blinded_path::payment::BlindedPaymentTlvs;
-use crate::sign::EntropySource;
+use crate::offers::invoice::BlindedPayInfo;
 use crate::ln::msgs::DecodeError;
+use crate::sign::EntropySource;
 use crate::util::ser::{Readable, Writeable, Writer};
 
 use crate::io;
@@ -79,20 +80,22 @@ impl BlindedPath {
 	/// Create a blinded path for a payment, to be forwarded along `path`. The last node
 	/// in `path` will be the destination node.
 	///
-	/// Errors if `path` is empty or a node id in `path` is invalid.
+	/// Errors if `path` is empty, a node id in `path` is invalid, or [`BlindedPayInfo`] calculation
+	/// results in an integer overflow.
 	//  TODO: make all payloads the same size with padding + add dummy hops
 	pub fn new_for_payment<ES: EntropySource, T: secp256k1::Signing + secp256k1::Verification>(
 		path: &[(PublicKey, BlindedPaymentTlvs)], entropy_source: &ES, secp_ctx: &Secp256k1<T>
-	) -> Result<Self, ()> {
+	) -> Result<(BlindedPayInfo, Self), ()> {
 		if path.len() < 1 { return Err(()) }
 		let blinding_secret_bytes = entropy_source.get_secure_random_bytes();
 		let blinding_secret = SecretKey::from_slice(&blinding_secret_bytes[..]).expect("RNG is busted");
 
-		Ok(BlindedPath {
+		let blinded_payinfo = payment::compute_payinfo(path)?;
+		Ok((blinded_payinfo, BlindedPath {
 			introduction_node_id: path[0].0,
 			blinding_point: PublicKey::from_secret_key(secp_ctx, &blinding_secret),
 			blinded_hops: payment::blinded_hops(secp_ctx, path, &blinding_secret).map_err(|_| ())?,
-		})
+		}))
 	}
 }
 

lightning/src/blinded_path/payment.rs

@@ -3,13 +3,14 @@
 //! [`BlindedPath`]: crate::blinded_path::BlindedPath
 
 use bitcoin::secp256k1::{self, PublicKey, Secp256k1, SecretKey};
-
+use core::convert::TryFrom;
 use crate::blinded_path::BlindedHop;
 use crate::blinded_path::utils;
 use crate::io;
 use crate::ln::PaymentSecret;
 use crate::ln::features::BlindedHopFeatures;
 use crate::ln::msgs::DecodeError;
+use crate::offers::invoice::BlindedPayInfo;
 use crate::prelude::*;
 use crate::util::ser::{Readable, Writeable, Writer};
 
@@ -45,6 +46,36 @@ pub enum BlindedPaymentTlvs {
 	},
 }
 
+impl BlindedPaymentTlvs {
+	// The fee used to get from the current hop to the next hop in the path.
+	fn fee_base_msat(&self) -> u32 {
+		match self {
+			Self::Forward { payment_relay, .. } => payment_relay.fee_base_msat,
+			_ => 0,
+		}
+	}
+	// The fee used to get from the current hop to the next hop in the path.
+	fn fee_proportional_millionths(&self) -> u32 {
+		match self {
+			Self::Forward { payment_relay, .. } => payment_relay.fee_proportional_millionths,
+			_ => 0,
+		}
+	}
+	// The delta used to get from the current hop to the next hop in the path.
+	fn cltv_expiry_delta(&self) -> u16 {
+		match self {
+			Self::Forward { payment_relay, .. } => payment_relay.cltv_expiry_delta,
+			_ => 0,
+		}
+	}
+	fn htlc_minimum_msat(&self) -> u64 {
+		match self {
+			Self::Forward { payment_constraints, .. } | Self::Receive { payment_constraints, .. } =>
+				payment_constraints.htlc_minimum_msat,
+		}
+	}
+}
+
 /// Parameters for relaying over a given [`BlindedHop`].
 ///
 /// [`BlindedHop`]: crate::blinded_path::BlindedHop
@@ -148,6 +179,34 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 	Ok(blinded_hops)
 }
 
+pub(super) fn compute_payinfo(
+	path: &[(PublicKey, BlindedPaymentTlvs)]
+) -> Result<BlindedPayInfo, ()> {
+	let mut curr_base_fee: u128 = 0;
+	let mut curr_prop_mil: u128 = 0;
+	for (_, payment_tlvs) in path.iter().rev().skip(1) {
+		let next_base_fee = payment_tlvs.fee_base_msat() as u128;
+		let next_prop_mil = payment_tlvs.fee_proportional_millionths() as u128;
+		curr_base_fee =
+			((next_base_fee * 1_000_000 + (curr_base_fee * (1_000_000 + next_prop_mil))) + 1_000_000 - 1)
+			 / 1_000_000;
+		curr_prop_mil =
+			(((curr_prop_mil + next_prop_mil) * 1_000_000 + curr_prop_mil * next_prop_mil) + 1_000_000 - 1)
+			 / 1_000_000;
+	}
+	Ok(BlindedPayInfo {
+		fee_base_msat: u32::try_from(curr_base_fee).map_err(|_| ())?,
+		fee_proportional_millionths: u32::try_from(curr_prop_mil).map_err(|_| ())?,
+		cltv_expiry_delta: path.iter().map(|(_, tlvs)| tlvs.cltv_expiry_delta())
+			.try_fold(0u16, |acc, delta| acc.checked_add(delta)).ok_or(())?,
+		htlc_minimum_msat: path.iter().map(|(_, tlvs)| tlvs.htlc_minimum_msat()).max().unwrap_or(0),
+		// TODO: this field isn't present in route blinding encrypted data
+		htlc_maximum_msat: 21_000_000 * 100_000_000 * 1_000, // Total bitcoin supply
+		// TODO: when there are blinded hop features, take the subset of them here
+		features: BlindedHopFeatures::empty(),
+	})
+}
+
 impl_writeable_msg!(PaymentRelay, {
 	cltv_expiry_delta,
 	fee_proportional_millionths,
@@ -158,3 +217,72 @@ impl_writeable_msg!(PaymentConstraints, {
 	max_cltv_expiry,
 	htlc_minimum_msat
 }, {});
+
+#[cfg(test)]
+mod tests {
+	use bitcoin::secp256k1::PublicKey;
+	use crate::blinded_path::payment::{BlindedPaymentTlvs, PaymentConstraints, PaymentRelay};
+	use crate::ln::PaymentSecret;
+	use crate::ln::features::BlindedHopFeatures;
+
+	#[test]
+	fn compute_payinfo() {
+		// Taken from the spec example for aggregating blinded payment info.
+		let dummy_pk = PublicKey::from_slice(&[2; 33]).unwrap();
+		let path = vec![(dummy_pk, BlindedPaymentTlvs::Forward {
+			short_channel_id: 0,
+			payment_relay: PaymentRelay {
+				cltv_expiry_delta: 144,
+				fee_proportional_millionths: 500,
+				fee_base_msat: 100,
+			},
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 100,
+			},
+			features: BlindedHopFeatures::empty(),
+		}), (dummy_pk, BlindedPaymentTlvs::Forward {
+			short_channel_id: 0,
+			payment_relay: PaymentRelay {
+				cltv_expiry_delta: 144,
+				fee_proportional_millionths: 500,
+				fee_base_msat: 100,
+			},
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 1_000,
+			},
+			features: BlindedHopFeatures::empty(),
+		}), (dummy_pk, BlindedPaymentTlvs::Receive {
+			payment_secret: PaymentSecret([0; 32]),
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 1,
+			},
+			features: BlindedHopFeatures::empty(),
+		})];
+		let blinded_payinfo = super::compute_payinfo(&path[..]).unwrap();
+		assert_eq!(blinded_payinfo.fee_base_msat, 201);
+		assert_eq!(blinded_payinfo.fee_proportional_millionths, 1001);
+		assert_eq!(blinded_payinfo.cltv_expiry_delta, 288);
+		assert_eq!(blinded_payinfo.htlc_minimum_msat, 1_000);
+	}
+
+	#[test]
+	fn compute_payinfo_1_hop() {
+		let dummy_pk = PublicKey::from_slice(&[2; 33]).unwrap();
+		let path = vec![(dummy_pk, BlindedPaymentTlvs::Receive {
+			payment_secret: PaymentSecret([0; 32]),
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 1,
+			},
+			features: BlindedHopFeatures::empty(),
+		})];
+		let blinded_payinfo = super::compute_payinfo(&path[..]).unwrap();
+		assert_eq!(blinded_payinfo.fee_base_msat, 0);
+		assert_eq!(blinded_payinfo.fee_proportional_millionths, 0);
+		assert_eq!(blinded_payinfo.cltv_expiry_delta, 0);
+		assert_eq!(blinded_payinfo.htlc_minimum_msat, 1);
+	}
+}