Don't include HMAC in Refund paths

Refunds are typically communicated via QR code, where a smaller size is
desirable. Make the HMAC in OutboundPayment data optional such that it
is elided from blinded paths used in refunds. This prevents abandoning
refunds if the reader sends an invoice_error instead of an invoice
message. However, this use case isn't necessary as the corresponding
outbound payment will either timeout when the refund expires or can be
explicitly abandoned by the creator.

Author: jkczyz

lightning/src/blinded_path/message.rs

@@ -153,7 +153,7 @@ pub enum OffersContext {
 		/// used with an [`InvoiceError`].
 		///
 		/// [`InvoiceError`]: crate::offers::invoice_error::InvoiceError
-		hmac: Hmac<Sha256>,
+		hmac: Option<Hmac<Sha256>>,
 	},
 	/// Context used by a [`BlindedPath`] as a reply path for a [`Bolt12Invoice`].
 	///
@@ -181,7 +181,7 @@ impl_writeable_tlv_based_enum!(OffersContext,
 	(1, OutboundPayment) => {
 		(0, payment_id, required),
 		(1, nonce, required),
-		(2, hmac, required),
+		(2, hmac, option),
 	},
 	(2, InboundPayment) => {
 		(0, payment_hash, required),

lightning/src/ln/channelmanager.rs

@@ -8883,8 +8883,7 @@ macro_rules! create_refund_builder { ($self: ident, $builder: ty) => {
 		let secp_ctx = &$self.secp_ctx;
 
 		let nonce = Nonce::from_entropy_source(entropy);
-		let hmac = signer::hmac_for_payment_id(payment_id, nonce, expanded_key);
-		let context = OffersContext::OutboundPayment { payment_id, nonce, hmac };
+		let context = OffersContext::OutboundPayment { payment_id, nonce, hmac: None };
 		let path = $self.create_blinded_paths_using_absolute_expiry(context, Some(absolute_expiry))
 			.and_then(|paths| paths.into_iter().next().ok_or(()))
 			.map_err(|_| Bolt12SemanticError::MissingPaths)?;
@@ -9020,7 +9019,7 @@ where
 		let invoice_request = builder.build_and_sign()?;
 
 		let hmac = signer::hmac_for_payment_id(payment_id, nonce, expanded_key);
-		let context = OffersContext::OutboundPayment { payment_id, nonce, hmac };
+		let context = OffersContext::OutboundPayment { payment_id, nonce, hmac: Some(hmac) };
 		let reply_paths = self.create_blinded_paths(context)
 			.map_err(|_| Bolt12SemanticError::MissingPaths)?;
 
@@ -10908,7 +10907,7 @@ where
 				log_trace!(logger, "Received invoice_error: {}", invoice_error);
 
 				match context {
-					Some(OffersContext::OutboundPayment { payment_id, nonce, hmac }) => {
+					Some(OffersContext::OutboundPayment { payment_id, nonce, hmac: Some(hmac) }) => {
 						if signer::verify_payment_id(payment_id, hmac, nonce, expanded_key) {
 							self.abandon_payment_with_reason(
 								payment_id, PaymentFailureReason::RecipientRejected,