Elide metadata from Offer with derived keys

When an Offer uses blinded paths, its metadata consists of a nonce used
to derive its signing keys. Now that the blinded paths contain this
nonce, elide the metadata as it is now redundant. This saves space and
also makes it impossible to derive the signing keys if an invoice
request is received with the incorrect nonce. The nonce shouldn't be
revealed in this case either to prevent de-anonymization attacks.

Author: jkczyz

lightning/src/offers/invoice.rs

@@ -1776,7 +1776,7 @@ mod tests {
 			.sign(payer_sign).unwrap();
 
 		if let Err(e) = invoice_request.clone()
-			.verify(&expanded_key, &secp_ctx).unwrap()
+			.verify_using_nonce(nonce, &expanded_key, &secp_ctx).unwrap()
 			.respond_using_derived_keys_no_std(payment_paths(), payment_hash(), now()).unwrap()
 			.build_and_sign(&secp_ctx)
 		{

lightning/src/offers/offer.rs

@@ -399,13 +399,13 @@ macro_rules! offer_builder_methods { (
 				}
 
 				let (derived_metadata, keys) = metadata.derive_from(tlv_stream, $self.secp_ctx);
-				metadata = derived_metadata;
-				if let Some(keys) = keys {
-					$self.offer.signing_pubkey = Some(keys.public_key());
+				match keys {
+					Some(keys) => $self.offer.signing_pubkey = Some(keys.public_key()),
+					None => $self.offer.metadata = Some(derived_metadata),
 				}
+			} else {
+				$self.offer.metadata = Some(metadata);
 			}
-
-			$self.offer.metadata = Some(metadata);
 		}
 
 		let mut bytes = Vec::new();
@@ -667,9 +667,9 @@ impl Offer {
 
 	#[cfg(async_payments)]
 	pub(super) fn verify<T: secp256k1::Signing>(
-		&self, key: &ExpandedKey, secp_ctx: &Secp256k1<T>
+		&self, nonce: Nonce, key: &ExpandedKey, secp_ctx: &Secp256k1<T>
 	) -> Result<(OfferId, Option<Keypair>), ()> {
-		self.contents.verify(&self.bytes, key, secp_ctx)
+		self.contents.verify_using_nonce(&self.bytes, nonce, key, secp_ctx)
 	}
 }
 
@@ -1296,6 +1296,7 @@ mod tests {
 		let offer = OfferBuilder::deriving_signing_pubkey(node_id, &expanded_key, nonce, &secp_ctx)
 			.amount_msats(1000)
 			.build().unwrap();
+		assert!(offer.metadata().is_some());
 		assert_eq!(offer.signing_pubkey(), Some(node_id));
 
 		let invoice_request = offer.request_invoice(vec![1; 32], payer_pubkey()).unwrap()
@@ -1362,16 +1363,9 @@ mod tests {
 			.amount_msats(1000)
 			.path(blinded_path)
 			.build().unwrap();
+		assert!(offer.metadata().is_none());
 		assert_ne!(offer.signing_pubkey(), Some(node_id));
 
-		let invoice_request = offer.request_invoice(vec![1; 32], payer_pubkey()).unwrap()
-			.build().unwrap()
-			.sign(payer_sign).unwrap();
-		match invoice_request.verify(&expanded_key, &secp_ctx) {
-			Ok(invoice_request) => assert_eq!(invoice_request.offer_id, offer.id()),
-			Err(_) => panic!("unexpected error"),
-		}
-
 		let invoice_request = offer.request_invoice(vec![1; 32], payer_pubkey()).unwrap()
 			.build().unwrap()
 			.sign(payer_sign).unwrap();

lightning/src/offers/static_invoice.rs

@@ -22,6 +22,7 @@ use crate::offers::invoice_macros::{invoice_accessors_common, invoice_builder_me
 use crate::offers::merkle::{
 	self, SignError, SignFn, SignatureTlvStream, SignatureTlvStreamRef, TaggedHash,
 };
+use crate::offers::nonce::Nonce;
 use crate::offers::offer::{
 	Amount, Offer, OfferContents, OfferTlvStream, OfferTlvStreamRef, Quantity,
 };
@@ -97,7 +98,7 @@ impl<'a> StaticInvoiceBuilder<'a> {
 	pub fn for_offer_using_derived_keys<T: secp256k1::Signing>(
 		offer: &'a Offer, payment_paths: Vec<(BlindedPayInfo, BlindedPath)>,
 		message_paths: Vec<BlindedPath>, created_at: Duration, expanded_key: &ExpandedKey,
-		secp_ctx: &Secp256k1<T>,
+		nonce: Nonce, secp_ctx: &Secp256k1<T>,
 	) -> Result<Self, Bolt12SemanticError> {
 		if offer.chains().len() > 1 {
 			return Err(Bolt12SemanticError::UnexpectedChain);
@@ -111,7 +112,7 @@ impl<'a> StaticInvoiceBuilder<'a> {
 			offer.signing_pubkey().ok_or(Bolt12SemanticError::MissingSigningPubkey)?;
 
 		let keys = offer
-			.verify(&expanded_key, &secp_ctx)
+			.verify(nonce, &expanded_key, &secp_ctx)
 			.map_err(|()| Bolt12SemanticError::InvalidMetadata)?
 			.1
 			.ok_or(Bolt12SemanticError::MissingSigningPubkey)?;
@@ -623,6 +624,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		)
 		.unwrap()
@@ -662,6 +664,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		)
 		.unwrap()
@@ -672,7 +675,7 @@ mod tests {
 		invoice.write(&mut buffer).unwrap();
 
 		assert_eq!(invoice.bytes, buffer.as_slice());
-		assert!(invoice.metadata().is_some());
+		assert_eq!(invoice.metadata(), None);
 		assert_eq!(invoice.amount(), None);
 		assert_eq!(invoice.description(), None);
 		assert_eq!(invoice.offer_features(), &OfferFeatures::empty());
@@ -698,13 +701,12 @@ mod tests {
 		);
 
 		let paths = vec![blinded_path()];
-		let metadata = vec![42; 16];
 		assert_eq!(
 			invoice.as_tlv_stream(),
 			(
 				OfferTlvStreamRef {
 					chains: None,
-					metadata: Some(&metadata),
+					metadata: None,
 					currency: None,
 					amount: None,
 					description: None,
@@ -762,6 +764,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		)
 		.unwrap()
@@ -782,6 +785,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		)
 		.unwrap()
@@ -815,6 +819,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		) {
 			assert_eq!(e, Bolt12SemanticError::MissingPaths);
@@ -829,6 +834,7 @@ mod tests {
 			Vec::new(),
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		) {
 			assert_eq!(e, Bolt12SemanticError::MissingPaths);
@@ -849,6 +855,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		) {
 			assert_eq!(e, Bolt12SemanticError::MissingPaths);
@@ -886,6 +893,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		) {
 			assert_eq!(e, Bolt12SemanticError::MissingSigningPubkey);
@@ -906,6 +914,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		) {
 			assert_eq!(e, Bolt12SemanticError::InvalidMetadata);
@@ -937,6 +946,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		) {
 			assert_eq!(e, Bolt12SemanticError::UnexpectedChain);
@@ -967,6 +977,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		)
 		.unwrap()
@@ -1007,6 +1018,7 @@ mod tests {
 			vec![blinded_path()],
 			now,
 			&expanded_key,
+			nonce,
 			&secp_ctx,
 		)
 		.unwrap()