offers: verify invoice upon return

src/lib.rs

@@ -11,11 +11,12 @@ use crate::lnd::{
 use crate::lndk_offers::OfferError;
 use crate::onion_messenger::MessengerUtilities;
 use bitcoin::network::constants::Network;
-use bitcoin::secp256k1::{Error as Secp256k1Error, PublicKey};
+use bitcoin::secp256k1::{Error as Secp256k1Error, PublicKey, Secp256k1};
 use home::home_dir;
 use lightning::blinded_path::BlindedPath;
 use lightning::ln::inbound_payment::ExpandedKey;
 use lightning::ln::peer_handler::IgnoringMessageHandler;
+use lightning::offers::invoice_error::InvoiceError;
 use lightning::offers::offer::Offer;
 use lightning::onion_message::{
     DefaultMessageRouter, Destination, OffersMessage, OffersMessageHandler, OnionMessenger,
@@ -238,7 +239,20 @@ impl OffersMessageHandler for OfferHandler {
                 log::error!("Invoice request received, payment not yet supported.");
                 None
             }
-            OffersMessage::Invoice(_invoice) => None,
+            OffersMessage::Invoice(invoice) => {
+                let secp_ctx = &Secp256k1::new();
+                // We verify that this invoice is a response to the invoice request we just sent.
+                match invoice.verify(&self.expanded_key, secp_ctx) {
+                    // TODO: Eventually when we allow for multiple payments in flight, we can use the
+                    // returned payment id below to check if we already processed an invoice for
+                    // this payment. Right now it's safe to let this be because we won't try to pay
+                    // a second invoice (if it comes through).
+                    Ok(_payment_id) => Some(OffersMessage::Invoice(invoice)),
+                    Err(()) => Some(OffersMessage::InvoiceError(InvoiceError::from_string(
+                        String::from("invoice verification failure"),
+                    ))),
+                }
+            }
             OffersMessage::InvoiceError(_error) => None,
         }
     }