Include a one-hop blinded path in Offer and Refund

While this doesn't add much privacy over not including any blinded
paths, it allows us to exercise code for receiving on blinded paths.

lightning/src/blinded_path/mod.rs

@@ -57,7 +57,7 @@ pub struct BlindedHop {
 
 impl BlindedPath {
 	/// Create a one-hop blinded path for a message.
-	pub fn one_hop_for_message<ES: EntropySource, T: secp256k1::Signing + secp256k1::Verification>(
+	pub fn one_hop_for_message<ES: EntropySource + ?Sized, T: secp256k1::Signing + secp256k1::Verification>(
 		recipient_node_id: PublicKey, entropy_source: &ES, secp_ctx: &Secp256k1<T>
 	) -> Result<Self, ()> {
 		Self::new_for_message(&[recipient_node_id], entropy_source, secp_ctx)
@@ -68,7 +68,7 @@ impl BlindedPath {
 	///
 	/// Errors if no hops are provided or if `node_pk`(s) are invalid.
 	//  TODO: make all payloads the same size with padding + add dummy hops
-	pub fn new_for_message<ES: EntropySource, T: secp256k1::Signing + secp256k1::Verification>(
+	pub fn new_for_message<ES: EntropySource + ?Sized, T: secp256k1::Signing + secp256k1::Verification>(
 		node_pks: &[PublicKey], entropy_source: &ES, secp_ctx: &Secp256k1<T>
 	) -> Result<Self, ()> {
 		if node_pks.is_empty() { return Err(()) }

lightning/src/ln/channelmanager.rs

@@ -30,6 +30,7 @@ use bitcoin::secp256k1::{SecretKey,PublicKey};
 use bitcoin::secp256k1::Secp256k1;
 use bitcoin::{LockTime, secp256k1, Sequence};
 
+use crate::blinded_path::BlindedPath;
 use crate::chain;
 use crate::chain::{Confirm, ChannelMonitorUpdateStatus, Watch, BestBlock};
 use crate::chain::chaininterface::{BroadcasterInterface, ConfirmationTarget, FeeEstimator, LowerBoundedFeeEstimator};
@@ -7093,6 +7094,11 @@ where
 	/// [`ChannelManager`] when handling [`InvoiceRequest`] messages for the offer. The offer will
 	/// not have an expiration unless otherwise set on the builder.
 	///
+	/// Uses a one-hop [`BlindedPath`] for the offer with [`ChannelManager::get_our_node_id`] as the
+	/// introduction node and a derived signing pubkey for recipient privacy. As such, currently,
+	/// the node must be announced. Otherwise, there is no way to find a path to the introduction
+	/// node in order to send the [`InvoiceRequest`].
+	///
 	/// [`Offer`]: crate::offers::offer::Offer
 	/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
 	pub fn create_offer_builder(
@@ -7102,10 +7108,11 @@ where
 		let expanded_key = &self.inbound_payment_key;
 		let entropy = &*self.entropy_source;
 		let secp_ctx = &self.secp_ctx;
+		let path = self.create_one_hop_blinded_path();
 
-		// TODO: Set blinded paths
 		OfferBuilder::deriving_signing_pubkey(description, node_id, expanded_key, entropy, secp_ctx)
 			.chain_hash(self.chain_hash)
+			.path(path)
 	}
 
 	/// Creates a [`RefundBuilder`] such that the [`Refund`] it builds is recognized by the
@@ -7115,6 +7122,11 @@ where
 	///
 	/// The provided `payment_id` is used to ensure that only one invoice is paid for the refund.
 	///
+	/// Uses a one-hop [`BlindedPath`] for the refund with [`ChannelManager::get_our_node_id`] as
+	/// the introduction node and a derived payer id for sender privacy. As such, currently, the
+	/// node must be announced. Otherwise, there is no way to find a path to the introduction node
+	/// in order to send the [`Bolt12Invoice`].
+	///
 	/// [`Refund`]: crate::offers::refund::Refund
 	/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
 	pub fn create_refund_builder(
@@ -7125,13 +7137,14 @@ where
 		let expanded_key = &self.inbound_payment_key;
 		let entropy = &*self.entropy_source;
 		let secp_ctx = &self.secp_ctx;
+		let path = self.create_one_hop_blinded_path();
 
-		// TODO: Set blinded paths
 		let builder = RefundBuilder::deriving_payer_id(
 			description, node_id, expanded_key, entropy, secp_ctx, amount_msats, payment_id
 		)?
 			.chain_hash(self.chain_hash)
-			.absolute_expiry(absolute_expiry);
+			.absolute_expiry(absolute_expiry)
+			.path(path);
 
 		self.pending_outbound_payments
 			.add_new_awaiting_invoice(
@@ -7242,6 +7255,14 @@ where
 		inbound_payment::get_payment_preimage(payment_hash, payment_secret, &self.inbound_payment_key)
 	}
 
+	/// Creates a one-hop blinded path with [`ChannelManager::get_our_node_id`] as the introduction
+	/// node.
+	fn create_one_hop_blinded_path(&self) -> BlindedPath {
+		let entropy_source = self.entropy_source.deref();
+		let secp_ctx = &self.secp_ctx;
+		BlindedPath::one_hop_for_message(self.get_our_node_id(), entropy_source, secp_ctx).unwrap()
+	}
+
 	/// Gets a fake short channel id for use in receiving [phantom node payments]. These fake scids
 	/// are used when constructing the phantom invoice's route hints.
 	///

lightning/src/routing/router.rs

@@ -90,6 +90,7 @@ pub trait Router {
 		&self, payer: &PublicKey, route_params: &RouteParameters,
 		first_hops: Option<&[&ChannelDetails]>, inflight_htlcs: InFlightHtlcs
 	) -> Result<Route, LightningError>;
+
 	/// Finds a [`Route`] for a payment between the given `payer` and a payee.
 	///
 	/// The `payee` and the payment's value are given in [`RouteParameters::payment_params`]