HMAC construction/verification for ReceiveTlvs

When receiving a PaymentContext from a blinded payment, the context must
be authenticated. Otherwise, the context can be forged and would appear
within a PaymentPurpose. Add functions for constructing and verifying an
HMAC for the ReceiveTlvs, which contains the PaymentContext.

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -480,6 +480,20 @@ impl Verification for PaymentHash {
 	}
 }
 
+impl Verification for ReceiveTlvs {
+	fn hmac_for_offer_payment(
+		&self, nonce: Nonce, expanded_key: &inbound_payment::ExpandedKey,
+	) -> Hmac<Sha256> {
+		signer::hmac_for_payment_tlvs(self, nonce, expanded_key)
+	}
+
+	fn verify_for_offer_payment(
+		&self, hmac: Hmac<Sha256>, nonce: Nonce, expanded_key: &inbound_payment::ExpandedKey,
+	) -> Result<(), ()> {
+		signer::verify_payment_tlvs(self, hmac, nonce, expanded_key)
+	}
+}
+
 /// A user-provided identifier in [`ChannelManager::send_payment`] used to uniquely identify
 /// a payment and ensure idempotency in LDK.
 ///

lightning/src/offers/signer.rs

@@ -16,6 +16,7 @@ use bitcoin::hashes::sha256::Hash as Sha256;
 use bitcoin::secp256k1::{Keypair, PublicKey, Secp256k1, SecretKey, self};
 use types::payment::PaymentHash;
 use core::fmt;
+use crate::blinded_path::payment::ReceiveTlvs;
 use crate::ln::channelmanager::PaymentId;
 use crate::ln::inbound_payment::{ExpandedKey, IV_LEN};
 use crate::offers::merkle::TlvRecord;
@@ -46,6 +47,9 @@ const ASYNC_PAYMENT_ID_HMAC_INPUT: &[u8; 16] = &[6; 16];
 // HMAC input for a `PaymentHash`. The HMAC is used in `OffersContext::InboundPayment`.
 const PAYMENT_HASH_HMAC_INPUT: &[u8; 16] = &[7; 16];
 
+// HMAC input for `ReceiveTlvs`. The HMAC is used in `blinded_path::payment::PaymentContext`.
+const PAYMENT_TLVS_HMAC_INPUT: &[u8; 16] = &[8; 16];
+
 /// Message metadata which possibly is derived from [`MetadataMaterial`] such that it can be
 /// verified.
 #[derive(Clone)]
@@ -459,3 +463,22 @@ fn hmac_for_payment_id(
 
 	Hmac::from_engine(hmac)
 }
+
+pub(crate) fn hmac_for_payment_tlvs(
+	receive_tlvs: &ReceiveTlvs, nonce: Nonce, expanded_key: &ExpandedKey,
+) -> Hmac<Sha256> {
+	const IV_BYTES: &[u8; IV_LEN] = b"LDK Payment TLVs";
+	let mut hmac = expanded_key.hmac_for_offer();
+	hmac.input(IV_BYTES);
+	hmac.input(&nonce.0);
+	hmac.input(PAYMENT_TLVS_HMAC_INPUT);
+	receive_tlvs.write(&mut hmac).unwrap();
+
+	Hmac::from_engine(hmac)
+}
+
+pub(crate) fn verify_payment_tlvs(
+	receive_tlvs: &ReceiveTlvs, hmac: Hmac<Sha256>, nonce: Nonce, expanded_key: &ExpandedKey,
+) -> Result<(), ()> {
+	if hmac_for_payment_tlvs(receive_tlvs, nonce, expanded_key) == hmac { Ok(()) } else { Err(()) }
+}