Stateless verification of Invoice for Refund

Stateless verification of Invoice for Offer

Verify that an Invoice was produced from a Refund constructed by the
payer using the payer metadata reflected in the Invoice. The payer
metadata consists of a 128-bit encrypted nonce and possibly a 256-bit
HMAC over the nonce and Refund TLV records (excluding the payer id)
using an ExpandedKey.

Thus, the HMAC can be reproduced from the refund bytes using the nonce
and the original ExpandedKey, and then checked against the metadata. If
metadata does not contain an HMAC, then the reproduced HMAC was used to
form the signing keys, and thus can be checked against the payer id.

Author: jkczyz

lightning/src/offers/invoice.rs

@@ -535,7 +535,9 @@ impl InvoiceContents {
 			InvoiceContents::ForOffer { invoice_request, .. } => {
 				invoice_request.verify(tlv_stream, key, secp_ctx)
 			},
-			_ => todo!(),
+			InvoiceContents::ForRefund { refund, .. } => {
+				refund.verify(tlv_stream, key, secp_ctx)
+			},
 		}
 	}
 

lightning/src/offers/invoice_request.rs

@@ -602,12 +602,12 @@ impl Writeable for InvoiceRequestContents {
 }
 
 /// Valid type range for invoice_request TLV records.
-const INVOICE_REQUEST_TYPES: core::ops::Range<u64> = 80..160;
+pub(super) const INVOICE_REQUEST_TYPES: core::ops::Range<u64> = 80..160;
 
 /// TLV record type for [`InvoiceRequest::payer_id`] and [`Refund::payer_id`].
 ///
 /// [`Refund::payer_id`]: crate::offers::refund::Refund::payer_id
-const INVOICE_REQUEST_PAYER_ID_TYPE: u64 = 88;
+pub(super) const INVOICE_REQUEST_PAYER_ID_TYPE: u64 = 88;
 
 tlv_stream!(InvoiceRequestTlvStream, InvoiceRequestTlvStreamRef, INVOICE_REQUEST_TYPES, {
 	(80, chain: ChainHash),

lightning/src/offers/refund.rs

@@ -85,11 +85,12 @@ use crate::ln::features::InvoiceRequestFeatures;
 use crate::ln::inbound_payment::{ExpandedKey, IV_LEN, Nonce};
 use crate::ln::msgs::{DecodeError, MAX_VALUE_MSAT};
 use crate::offers::invoice::{BlindedPayInfo, InvoiceBuilder};
-use crate::offers::invoice_request::{InvoiceRequestTlvStream, InvoiceRequestTlvStreamRef};
-use crate::offers::offer::{OfferTlvStream, OfferTlvStreamRef};
+use crate::offers::invoice_request::{INVOICE_REQUEST_PAYER_ID_TYPE, INVOICE_REQUEST_TYPES, InvoiceRequestTlvStream, InvoiceRequestTlvStreamRef};
+use crate::offers::merkle::TlvStream;
+use crate::offers::offer::{OFFER_TYPES, OfferTlvStream, OfferTlvStreamRef};
 use crate::offers::parse::{Bech32Encode, ParseError, ParsedMessage, SemanticError};
-use crate::offers::payer::{PayerContents, PayerTlvStream, PayerTlvStreamRef};
-use crate::offers::signer::{Metadata, MetadataMaterial};
+use crate::offers::payer::{PAYER_METADATA_TYPE, PayerContents, PayerTlvStream, PayerTlvStreamRef};
+use crate::offers::signer::{Metadata, MetadataMaterial, self};
 use crate::onion_message::BlindedPath;
 use crate::util::ser::{SeekReadable, WithoutLength, Writeable, Writer};
 use crate::util::string::PrintableString;
@@ -343,7 +344,7 @@ impl Refund {
 	///
 	/// [`payer_id`]: Self::payer_id
 	pub fn metadata(&self) -> &[u8] {
-		self.contents.payer.0.as_bytes().map(|bytes| bytes.as_slice()).unwrap_or(&[])
+		self.contents.metadata()
 	}
 
 	/// A chain that the refund is valid for.
@@ -455,6 +456,10 @@ impl RefundContents {
 		}
 	}
 
+	fn metadata(&self) -> &[u8] {
+		self.payer.0.as_bytes().map(|bytes| bytes.as_slice()).unwrap_or(&[])
+	}
+
 	pub(super) fn chain(&self) -> ChainHash {
 		self.chain.unwrap_or_else(|| self.implied_chain())
 	}
@@ -463,6 +468,22 @@ impl RefundContents {
 		ChainHash::using_genesis_block(Network::Bitcoin)
 	}
 
+	/// Verifies that the payer metadata was produced from the refund in the TLV stream.
+	pub(super) fn verify<T: secp256k1::Signing>(
+		&self, tlv_stream: TlvStream<'_>, key: &ExpandedKey, secp_ctx: &Secp256k1<T>
+	) -> bool {
+		let offer_records = tlv_stream.clone().range(OFFER_TYPES);
+		let invreq_records = tlv_stream.range(INVOICE_REQUEST_TYPES).filter(|record| {
+			match record.r#type {
+				PAYER_METADATA_TYPE => false, // Should be outside range
+				INVOICE_REQUEST_PAYER_ID_TYPE => !self.payer.0.derives_keys(),
+				_ => true,
+			}
+		});
+		let tlv_stream = offer_records.chain(invreq_records);
+		signer::verify_metadata(self.metadata(), key, IV_BYTES, self.payer_id, tlv_stream, secp_ctx)
+	}
+
 	pub(super) fn as_tlv_stream(&self) -> RefundTlvStreamRef {
 		let payer = PayerTlvStreamRef {
 			metadata: self.payer.0.as_bytes(),
@@ -640,7 +661,9 @@ mod tests {
 	use bitcoin::secp256k1::{KeyPair, Secp256k1, SecretKey};
 	use core::convert::TryFrom;
 	use core::time::Duration;
+	use crate::chain::keysinterface::KeyMaterial;
 	use crate::ln::features::{InvoiceRequestFeatures, OfferFeatures};
+	use crate::ln::inbound_payment::ExpandedKey;
 	use crate::ln::msgs::{DecodeError, MAX_VALUE_MSAT};
 	use crate::offers::invoice_request::InvoiceRequestTlvStreamRef;
 	use crate::offers::offer::OfferTlvStreamRef;
@@ -726,6 +749,118 @@ mod tests {
 		}
 	}
 
+	#[test]
+	fn builds_refund_with_metadata_derived() {
+		let desc = "foo".to_string();
+		let node_id = payer_pubkey();
+		let expanded_key = ExpandedKey::new(&KeyMaterial([42; 32]));
+		let entropy = FixedEntropy {};
+		let secp_ctx = Secp256k1::new();
+
+		let refund = RefundBuilder
+			::deriving_payer_id(desc, node_id, &expanded_key, &entropy, &secp_ctx, 1000)
+			.unwrap()
+			.build().unwrap();
+		assert_eq!(refund.payer_id(), node_id);
+
+		// Fails verification with altered fields
+		let invoice = refund
+			.respond_with_no_std(payment_paths(), payment_hash(), recipient_pubkey(), now())
+			.unwrap()
+			.build().unwrap()
+			.sign(recipient_sign).unwrap();
+		assert!(invoice.verify(&expanded_key, &secp_ctx));
+
+		let mut tlv_stream = refund.as_tlv_stream();
+		tlv_stream.2.amount = Some(2000);
+
+		let mut encoded_refund = Vec::new();
+		tlv_stream.write(&mut encoded_refund).unwrap();
+
+		let invoice = Refund::try_from(encoded_refund).unwrap()
+			.respond_with_no_std(payment_paths(), payment_hash(), recipient_pubkey(), now())
+			.unwrap()
+			.build().unwrap()
+			.sign(recipient_sign).unwrap();
+		assert!(!invoice.verify(&expanded_key, &secp_ctx));
+
+		// Fails verification with altered metadata
+		let mut tlv_stream = refund.as_tlv_stream();
+		let metadata = tlv_stream.0.metadata.unwrap().iter().copied().rev().collect();
+		tlv_stream.0.metadata = Some(&metadata);
+
+		let mut encoded_refund = Vec::new();
+		tlv_stream.write(&mut encoded_refund).unwrap();
+
+		let invoice = Refund::try_from(encoded_refund).unwrap()
+			.respond_with_no_std(payment_paths(), payment_hash(), recipient_pubkey(), now())
+			.unwrap()
+			.build().unwrap()
+			.sign(recipient_sign).unwrap();
+		assert!(!invoice.verify(&expanded_key, &secp_ctx));
+	}
+
+	#[test]
+	fn builds_refund_with_derived_payer_id() {
+		let desc = "foo".to_string();
+		let node_id = payer_pubkey();
+		let expanded_key = ExpandedKey::new(&KeyMaterial([42; 32]));
+		let entropy = FixedEntropy {};
+		let secp_ctx = Secp256k1::new();
+
+		let blinded_path = BlindedPath {
+			introduction_node_id: pubkey(40),
+			blinding_point: pubkey(41),
+			blinded_hops: vec![
+				BlindedHop { blinded_node_id: pubkey(43), encrypted_payload: vec![0; 43] },
+				BlindedHop { blinded_node_id: node_id, encrypted_payload: vec![0; 44] },
+			],
+		};
+
+		let refund = RefundBuilder
+			::deriving_payer_id(desc, node_id, &expanded_key, &entropy, &secp_ctx, 1000)
+			.unwrap()
+			.path(blinded_path)
+			.build().unwrap();
+		assert_ne!(refund.payer_id(), node_id);
+
+		let invoice = refund
+			.respond_with_no_std(payment_paths(), payment_hash(), recipient_pubkey(), now())
+			.unwrap()
+			.build().unwrap()
+			.sign(recipient_sign).unwrap();
+		assert!(invoice.verify(&expanded_key, &secp_ctx));
+
+		// Fails verification with altered fields
+		let mut tlv_stream = refund.as_tlv_stream();
+		tlv_stream.2.amount = Some(2000);
+
+		let mut encoded_refund = Vec::new();
+		tlv_stream.write(&mut encoded_refund).unwrap();
+
+		let invoice = Refund::try_from(encoded_refund).unwrap()
+			.respond_with_no_std(payment_paths(), payment_hash(), recipient_pubkey(), now())
+			.unwrap()
+			.build().unwrap()
+			.sign(recipient_sign).unwrap();
+		assert!(!invoice.verify(&expanded_key, &secp_ctx));
+
+		// Fails verification with altered payer_id
+		let mut tlv_stream = refund.as_tlv_stream();
+		let payer_id = pubkey(1);
+		tlv_stream.2.payer_id = Some(&payer_id);
+
+		let mut encoded_refund = Vec::new();
+		tlv_stream.write(&mut encoded_refund).unwrap();
+
+		let invoice = Refund::try_from(encoded_refund).unwrap()
+			.respond_with_no_std(payment_paths(), payment_hash(), recipient_pubkey(), now())
+			.unwrap()
+			.build().unwrap()
+			.sign(recipient_sign).unwrap();
+		assert!(!invoice.verify(&expanded_key, &secp_ctx));
+	}
+
 	#[test]
 	fn builds_refund_with_absolute_expiry() {
 		let future_expiry = Duration::from_secs(u64::max_value());