Include PaymentId in payer metadata

When receiving a BOLT 12 invoice originating from either an invoice
request or a refund, the invoice should only be paid once. To accomplish
this, require that the invoice includes an encrypted payment id in the
payer metadata. This allows ChannelManager to track a payment when
requesting but prior to receiving the invoice. Thus, it can determine if
the invoice has already been paid.

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -221,7 +221,12 @@ struct ClaimableHTLC {
 ///
 /// This is not exported to bindings users as we just use [u8; 32] directly
 #[derive(Hash, Copy, Clone, PartialEq, Eq, Debug)]
-pub struct PaymentId(pub [u8; 32]);
+pub struct PaymentId(pub [u8; Self::LENGTH]);
+
+impl PaymentId {
+	/// Number of bytes in the id.
+	pub const LENGTH: usize = 32;
+}
 
 impl Writeable for PaymentId {
 	fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {

lightning/src/ln/inbound_payment.rs

@@ -78,6 +78,17 @@ impl ExpandedKey {
 		hmac.input(&nonce.0);
 		hmac
 	}
+
+	/// Encrypts or decrypts the given `bytes`. Used for data included in an offer message's
+	/// metadata (e.g., payment id).
+	pub(crate) fn crypt_for_offer(&self, mut bytes: [u8; 32], iv_bytes: &[u8; IV_LEN]) -> [u8; 32] {
+		let chacha_block = ChaCha20::get_single_block(&self.offers_base_key, iv_bytes);
+		for i in 0..bytes.len() {
+			bytes[i] = chacha_block[i] ^ bytes[i];
+		}
+
+		bytes
+	}
 }
 
 /// A 128-bit number used only once.

lightning/src/offers/invoice.rs

@@ -107,6 +107,7 @@ use core::time::Duration;
 use crate::io;
 use crate::blinded_path::BlindedPath;
 use crate::ln::PaymentHash;
+use crate::ln::channelmanager::PaymentId;
 use crate::ln::features::{BlindedHopFeatures, Bolt12InvoiceFeatures};
 use crate::ln::inbound_payment::ExpandedKey;
 use crate::ln::msgs::DecodeError;
@@ -593,10 +594,11 @@ impl Bolt12Invoice {
 		merkle::message_digest(SIGNATURE_TAG, &self.bytes).as_ref().clone()
 	}
 
-	/// Verifies that the invoice was for a request or refund created using the given key.
+	/// Verifies that the invoice was for a request or refund created using the given key. Returns
+	/// the associated [`PaymentId`] to use when sending the payment.
 	pub fn verify<T: secp256k1::Signing>(
 		&self, key: &ExpandedKey, secp_ctx: &Secp256k1<T>
-	) -> bool {
+	) -> Result<PaymentId, ()> {
 		self.contents.verify(TlvStream::new(&self.bytes), key, secp_ctx)
 	}
 
@@ -655,7 +657,7 @@ impl InvoiceContents {
 
 	fn verify<T: secp256k1::Signing>(
 		&self, tlv_stream: TlvStream<'_>, key: &ExpandedKey, secp_ctx: &Secp256k1<T>
-	) -> bool {
+	) -> Result<PaymentId, ()> {
 		let offer_records = tlv_stream.clone().range(OFFER_TYPES);
 		let invreq_records = tlv_stream.range(INVOICE_REQUEST_TYPES).filter(|record| {
 			match record.r#type {
@@ -675,10 +677,7 @@ impl InvoiceContents {
 			},
 		};
 
-		match signer::verify_metadata(metadata, key, iv_bytes, payer_id, tlv_stream, secp_ctx) {
-			Ok(_) => true,
-			Err(()) => false,
-		}
+		signer::verify_payer_metadata(metadata, key, iv_bytes, payer_id, tlv_stream, secp_ctx)
 	}
 
 	fn derives_keys(&self) -> bool {

lightning/src/offers/invoice_request.rs

@@ -63,6 +63,7 @@ use crate::sign::EntropySource;
 use crate::io;
 use crate::blinded_path::BlindedPath;
 use crate::ln::PaymentHash;
+use crate::ln::channelmanager::PaymentId;
 use crate::ln::features::InvoiceRequestFeatures;
 use crate::ln::inbound_payment::{ExpandedKey, IV_LEN, Nonce};
 use crate::ln::msgs::DecodeError;
@@ -127,10 +128,12 @@ impl<'a, 'b, T: secp256k1::Signing> InvoiceRequestBuilder<'a, 'b, ExplicitPayerI
 	}
 
 	pub(super) fn deriving_metadata<ES: Deref>(
-		offer: &'a Offer, payer_id: PublicKey, expanded_key: &ExpandedKey, entropy_source: ES
+		offer: &'a Offer, payer_id: PublicKey, expanded_key: &ExpandedKey, entropy_source: ES,
+		payment_id: PaymentId,
 	) -> Self where ES::Target: EntropySource {
 		let nonce = Nonce::from_entropy_source(entropy_source);
-		let derivation_material = MetadataMaterial::new(nonce, expanded_key, IV_BYTES);
+		let payment_id = Some(payment_id);
+		let derivation_material = MetadataMaterial::new(nonce, expanded_key, IV_BYTES, payment_id);
 		let metadata = Metadata::Derived(derivation_material);
 		Self {
 			offer,
@@ -144,10 +147,12 @@ impl<'a, 'b, T: secp256k1::Signing> InvoiceRequestBuilder<'a, 'b, ExplicitPayerI
 
 impl<'a, 'b, T: secp256k1::Signing> InvoiceRequestBuilder<'a, 'b, DerivedPayerId, T> {
 	pub(super) fn deriving_payer_id<ES: Deref>(
-		offer: &'a Offer, expanded_key: &ExpandedKey, entropy_source: ES, secp_ctx: &'b Secp256k1<T>
+		offer: &'a Offer, expanded_key: &ExpandedKey, entropy_source: ES,
+		secp_ctx: &'b Secp256k1<T>, payment_id: PaymentId
 	) -> Self where ES::Target: EntropySource {
 		let nonce = Nonce::from_entropy_source(entropy_source);
-		let derivation_material = MetadataMaterial::new(nonce, expanded_key, IV_BYTES);
+		let payment_id = Some(payment_id);
+		let derivation_material = MetadataMaterial::new(nonce, expanded_key, IV_BYTES, payment_id);
 		let metadata = Metadata::DerivedSigningPubkey(derivation_material);
 		Self {
 			offer,
@@ -258,7 +263,7 @@ impl<'a, 'b, P: PayerIdStrategy, T: secp256k1::Signing> InvoiceRequestBuilder<'a
 			let mut tlv_stream = self.invoice_request.as_tlv_stream();
 			debug_assert!(tlv_stream.2.payer_id.is_none());
 			tlv_stream.0.metadata = None;
-			if !metadata.derives_keys() {
+			if !metadata.derives_payer_keys() {
 				tlv_stream.2.payer_id = self.payer_id.as_ref();
 			}
 
@@ -645,7 +650,7 @@ impl InvoiceRequestContents {
 	}
 
 	pub(super) fn derives_keys(&self) -> bool {
-		self.inner.payer.0.derives_keys()
+		self.inner.payer.0.derives_payer_keys()
 	}
 
 	pub(super) fn chain(&self) -> ChainHash {
@@ -836,6 +841,7 @@ mod tests {
 	#[cfg(feature = "std")]
 	use core::time::Duration;
 	use crate::sign::KeyMaterial;
+	use crate::ln::channelmanager::PaymentId;
 	use crate::ln::features::InvoiceRequestFeatures;
 	use crate::ln::inbound_payment::ExpandedKey;
 	use crate::ln::msgs::{DecodeError, MAX_VALUE_MSAT};
@@ -940,12 +946,13 @@ mod tests {
 		let expanded_key = ExpandedKey::new(&KeyMaterial([42; 32]));
 		let entropy = FixedEntropy {};
 		let secp_ctx = Secp256k1::new();
+		let payment_id = PaymentId([1; 32]);
 
 		let offer = OfferBuilder::new("foo".into(), recipient_pubkey())
 			.amount_msats(1000)
 			.build().unwrap();
 		let invoice_request = offer
-			.request_invoice_deriving_metadata(payer_id, &expanded_key, &entropy)
+			.request_invoice_deriving_metadata(payer_id, &expanded_key, &entropy, payment_id)
 			.unwrap()
 			.build().unwrap()
 			.sign(payer_sign).unwrap();
@@ -955,7 +962,10 @@ mod tests {
 			.unwrap()
 			.build().unwrap()
 			.sign(recipient_sign).unwrap();
-		assert!(invoice.verify(&expanded_key, &secp_ctx));
+		match invoice.verify(&expanded_key, &secp_ctx) {
+			Ok(payment_id) => assert_eq!(payment_id, PaymentId([1; 32])),
+			Err(()) => panic!("verification failed"),
+		}
 
 		// Fails verification with altered fields
 		let (
@@ -978,7 +988,7 @@ mod tests {
 		signature_tlv_stream.write(&mut encoded_invoice).unwrap();
 
 		let invoice = Bolt12Invoice::try_from(encoded_invoice).unwrap();
-		assert!(!invoice.verify(&expanded_key, &secp_ctx));
+		assert!(invoice.verify(&expanded_key, &secp_ctx).is_err());
 
 		// Fails verification with altered metadata
 		let (
@@ -1001,20 +1011,21 @@ mod tests {
 		signature_tlv_stream.write(&mut encoded_invoice).unwrap();
 
 		let invoice = Bolt12Invoice::try_from(encoded_invoice).unwrap();
-		assert!(!invoice.verify(&expanded_key, &secp_ctx));
+		assert!(invoice.verify(&expanded_key, &secp_ctx).is_err());
 	}
 
 	#[test]
 	fn builds_invoice_request_with_derived_payer_id() {
 		let expanded_key = ExpandedKey::new(&KeyMaterial([42; 32]));
 		let entropy = FixedEntropy {};
 		let secp_ctx = Secp256k1::new();
+		let payment_id = PaymentId([1; 32]);
 
 		let offer = OfferBuilder::new("foo".into(), recipient_pubkey())
 			.amount_msats(1000)
 			.build().unwrap();
 		let invoice_request = offer
-			.request_invoice_deriving_payer_id(&expanded_key, &entropy, &secp_ctx)
+			.request_invoice_deriving_payer_id(&expanded_key, &entropy, &secp_ctx, payment_id)
 			.unwrap()
 			.build_and_sign()
 			.unwrap();
@@ -1023,7 +1034,10 @@ mod tests {
 			.unwrap()
 			.build().unwrap()
 			.sign(recipient_sign).unwrap();
-		assert!(invoice.verify(&expanded_key, &secp_ctx));
+		match invoice.verify(&expanded_key, &secp_ctx) {
+			Ok(payment_id) => assert_eq!(payment_id, PaymentId([1; 32])),
+			Err(()) => panic!("verification failed"),
+		}
 
 		// Fails verification with altered fields
 		let (
@@ -1046,7 +1060,7 @@ mod tests {
 		signature_tlv_stream.write(&mut encoded_invoice).unwrap();
 
 		let invoice = Bolt12Invoice::try_from(encoded_invoice).unwrap();
-		assert!(!invoice.verify(&expanded_key, &secp_ctx));
+		assert!(invoice.verify(&expanded_key, &secp_ctx).is_err());
 
 		// Fails verification with altered payer id
 		let (
@@ -1069,7 +1083,7 @@ mod tests {
 		signature_tlv_stream.write(&mut encoded_invoice).unwrap();
 
 		let invoice = Bolt12Invoice::try_from(encoded_invoice).unwrap();
-		assert!(!invoice.verify(&expanded_key, &secp_ctx));
+		assert!(invoice.verify(&expanded_key, &secp_ctx).is_err());
 	}
 
 	#[test]

lightning/src/offers/offer.rs

@@ -77,6 +77,7 @@ use core::time::Duration;
 use crate::sign::EntropySource;
 use crate::io;
 use crate::blinded_path::BlindedPath;
+use crate::ln::channelmanager::PaymentId;
 use crate::ln::features::OfferFeatures;
 use crate::ln::inbound_payment::{ExpandedKey, IV_LEN, Nonce};
 use crate::ln::msgs::MAX_VALUE_MSAT;
@@ -169,7 +170,7 @@ impl<'a, T: secp256k1::Signing> OfferBuilder<'a, DerivedMetadata, T> {
 		secp_ctx: &'a Secp256k1<T>
 	) -> Self where ES::Target: EntropySource {
 		let nonce = Nonce::from_entropy_source(entropy_source);
-		let derivation_material = MetadataMaterial::new(nonce, expanded_key, IV_BYTES);
+		let derivation_material = MetadataMaterial::new(nonce, expanded_key, IV_BYTES, None);
 		let metadata = Metadata::DerivedSigningPubkey(derivation_material);
 		OfferBuilder {
 			offer: OfferContents {
@@ -283,7 +284,7 @@ impl<'a, M: MetadataStrategy, T: secp256k1::Signing> OfferBuilder<'a, M, T> {
 				let mut tlv_stream = self.offer.as_tlv_stream();
 				debug_assert_eq!(tlv_stream.metadata, None);
 				tlv_stream.metadata = None;
-				if metadata.derives_keys() {
+				if metadata.derives_recipient_keys() {
 					tlv_stream.node_id = None;
 				}
 
@@ -450,10 +451,12 @@ impl Offer {
 
 	/// Similar to [`Offer::request_invoice`] except it:
 	/// - derives the [`InvoiceRequest::payer_id`] such that a different key can be used for each
-	///   request, and
+	///   request,
 	/// - sets the [`InvoiceRequest::metadata`] when [`InvoiceRequestBuilder::build`] is called such
 	///   that it can be used by [`Bolt12Invoice::verify`] to determine if the invoice was requested
-	///   using a base [`ExpandedKey`] from which the payer id was derived.
+	///   using a base [`ExpandedKey`] from which the payer id was derived, and
+	/// - includes the [`PaymentId`] encrypted in [`InvoiceRequest::metadata`] so that it can be
+	///   used when sending the payment for the requested invoice.
 	///
 	/// Useful to protect the sender's privacy.
 	///
@@ -464,7 +467,8 @@ impl Offer {
 	/// [`Bolt12Invoice::verify`]: crate::offers::invoice::Bolt12Invoice::verify
 	/// [`ExpandedKey`]: crate::ln::inbound_payment::ExpandedKey
 	pub fn request_invoice_deriving_payer_id<'a, 'b, ES: Deref, T: secp256k1::Signing>(
-		&'a self, expanded_key: &ExpandedKey, entropy_source: ES, secp_ctx: &'b Secp256k1<T>
+		&'a self, expanded_key: &ExpandedKey, entropy_source: ES, secp_ctx: &'b Secp256k1<T>,
+		payment_id: PaymentId
 	) -> Result<InvoiceRequestBuilder<'a, 'b, DerivedPayerId, T>, Bolt12SemanticError>
 	where
 		ES::Target: EntropySource,
@@ -473,7 +477,9 @@ impl Offer {
 			return Err(Bolt12SemanticError::UnknownRequiredFeatures);
 		}
 
-		Ok(InvoiceRequestBuilder::deriving_payer_id(self, expanded_key, entropy_source, secp_ctx))
+		Ok(InvoiceRequestBuilder::deriving_payer_id(
+				self, expanded_key, entropy_source, secp_ctx, payment_id
+		))
 	}
 
 	/// Similar to [`Offer::request_invoice_deriving_payer_id`] except uses `payer_id` for the
@@ -485,7 +491,8 @@ impl Offer {
 	///
 	/// [`InvoiceRequest::payer_id`]: crate::offers::invoice_request::InvoiceRequest::payer_id
 	pub fn request_invoice_deriving_metadata<ES: Deref>(
-		&self, payer_id: PublicKey, expanded_key: &ExpandedKey, entropy_source: ES
+		&self, payer_id: PublicKey, expanded_key: &ExpandedKey, entropy_source: ES,
+		payment_id: PaymentId
 	) -> Result<InvoiceRequestBuilder<ExplicitPayerId, secp256k1::SignOnly>, Bolt12SemanticError>
 	where
 		ES::Target: EntropySource,
@@ -494,7 +501,9 @@ impl Offer {
 			return Err(Bolt12SemanticError::UnknownRequiredFeatures);
 		}
 
-		Ok(InvoiceRequestBuilder::deriving_metadata(self, payer_id, expanded_key, entropy_source))
+		Ok(InvoiceRequestBuilder::deriving_metadata(
+				self, payer_id, expanded_key, entropy_source, payment_id
+		))
 	}
 
 	/// Creates an [`InvoiceRequestBuilder`] for the offer with the given `metadata` and `payer_id`,
@@ -641,11 +650,13 @@ impl OfferContents {
 				let tlv_stream = TlvStream::new(bytes).range(OFFER_TYPES).filter(|record| {
 					match record.r#type {
 						OFFER_METADATA_TYPE => false,
-						OFFER_NODE_ID_TYPE => !self.metadata.as_ref().unwrap().derives_keys(),
+						OFFER_NODE_ID_TYPE => {
+							!self.metadata.as_ref().unwrap().derives_recipient_keys()
+						},
 						_ => true,
 					}
 				});
-				signer::verify_metadata(
+				signer::verify_recipient_metadata(
 					metadata, key, IV_BYTES, self.signing_pubkey(), tlv_stream, secp_ctx
 				)
 			},