Validate amount_msats against invoice and refund amounts

Add a check to ensure that the amount_msats in an invoice matches the
amount_msats specified in the invoice_request or refund. Reject the
invoice as invalid if there is a mismatch between these amounts.
Otherwise, an invoice may be paid with an amount greater than the
requested amount.

Co-authored-by: Ian Slane <[email protected]>
Co-authored-by: Jeffrey Czyz <[email protected]>

Author: slanesuke

lightning/src/offers/invoice.rs

@@ -1531,6 +1531,11 @@ impl TryFrom<PartialInvoiceTlvStream> for InvoiceContents {
 					experimental_offer_tlv_stream, experimental_invoice_request_tlv_stream,
 				)
 			)?;
+
+			if amount_msats != refund.amount_msats() {
+				return Err(Bolt12SemanticError::InvalidAmount);
+			}
+
 			Ok(InvoiceContents::ForRefund { refund, fields })
 		} else {
 			let invoice_request = InvoiceRequestContents::try_from(
@@ -1539,6 +1544,13 @@ impl TryFrom<PartialInvoiceTlvStream> for InvoiceContents {
 					experimental_offer_tlv_stream, experimental_invoice_request_tlv_stream,
 				)
 			)?;
+
+			if let Some(requested_amount_msats) = invoice_request.amount_msats() {
+				if amount_msats != requested_amount_msats {
+					return Err(Bolt12SemanticError::InvalidAmount);
+				}
+			}
+
 			Ok(InvoiceContents::ForOffer { invoice_request, fields })
 		}
 	}
@@ -2469,6 +2481,7 @@ mod tests {
 			.amount_msats(1000)
 			.build().unwrap()
 			.request_invoice(&expanded_key, nonce, &secp_ctx, payment_id).unwrap()
+			.amount_msats(1000).unwrap()
 			.build_and_sign().unwrap()
 			.respond_with_no_std(payment_paths(), payment_hash(), now()).unwrap()
 			.build().unwrap()
@@ -2488,6 +2501,33 @@ mod tests {
 			Ok(_) => panic!("expected error"),
 			Err(e) => assert_eq!(e, Bolt12ParseError::InvalidSemantics(Bolt12SemanticError::MissingAmount)),
 		}
+
+		let mut tlv_stream = invoice.as_tlv_stream();
+		tlv_stream.3.amount = Some(2000);
+
+		match Bolt12Invoice::try_from(tlv_stream.to_bytes()) {
+			Ok(_) => panic!("expected error"),
+			Err(e) => assert_eq!(e, Bolt12ParseError::InvalidSemantics(Bolt12SemanticError::InvalidAmount)),
+		}
+
+		let invoice = RefundBuilder::new(vec![1; 32], payer_pubkey(), 1000).unwrap()
+			.build().unwrap()
+			.respond_using_derived_keys_no_std(
+				payment_paths(), payment_hash(), now(), &expanded_key, &entropy
+			)
+			.unwrap()
+			.build_and_sign(&secp_ctx).unwrap();
+
+		let mut buffer = Vec::new();
+		invoice.write(&mut buffer).unwrap();
+
+		let mut tlv_stream = invoice.as_tlv_stream();
+		tlv_stream.3.amount = Some(2000);
+
+		match Bolt12Invoice::try_from(tlv_stream.to_bytes()) {
+			Ok(_) => panic!("expected error"),
+			Err(e) => assert_eq!(e, Bolt12ParseError::InvalidSemantics(Bolt12SemanticError::InvalidAmount)),
+		}
 	}
 
 	#[test]

lightning/src/offers/parse.rs

@@ -146,7 +146,7 @@ pub enum Bolt12SemanticError {
 	UnexpectedChain,
 	/// An amount was expected but was missing.
 	MissingAmount,
-	/// The amount exceeded the total bitcoin supply.
+	/// The amount exceeded the total bitcoin supply or didn't match an expected amount.
 	InvalidAmount,
 	/// An amount was provided but was not sufficient in value.
 	InsufficientAmount,