Ensure that isOurOffer works for the default offer

The default offer also uses an empty `path_id` in the blinded path
(because it doesn't need to store any randomness), so an empty `path_id`
doesn't necessarily means that we're using a compact offer.

Author: t-bast

modules/core/src/commonMain/kotlin/fr/acinq/lightning/payment/OfferManager.kt

@@ -235,11 +235,11 @@ class OfferManager(val nodeParams: NodeParams, val walletParams: WalletParams, v
 
     /** This function verifies that the offer provided was generated by us. */
     private fun isOurOffer(offer: OfferTypes.Offer, pathId: ByteVector?, blindedPrivateKey: PrivateKey): Boolean = when {
-        pathId == null -> { // Compact offer
-            nodeParams.compactOfferKeys.value.contains(blindedPrivateKey.publicKey())
-        }
-        pathId.size() != 32 -> false
-        else -> { // Deterministic offer
+        pathId != null && pathId.size() != 32 -> false
+        // Compact offers are randomly generated, but they don't store the nonce in the path_id to save space.
+        // It is instead the wallet's responsibility to store the corresponding blinded public key(s).
+        pathId == null && nodeParams.compactOfferKeys.value.contains(blindedPrivateKey.publicKey()) -> true
+        else -> {
             val expected = deterministicOffer(nodeParams.chainHash, nodeParams.nodePrivateKey, walletParams.trampolineNode.id, offer.amount, offer.description, pathId?.let { ByteVector32(it) })
             expected == OfferTypes.OfferAndKey(offer, blindedPrivateKey)
         }
@@ -290,7 +290,7 @@ class OfferManager(val nodeParams: NodeParams, val walletParams: WalletParams, v
             val tweak = "bolt 12 compact offer".encodeToByteArray().byteVector() +
                     Crypto.sha256("offer nonce".encodeToByteArray().byteVector() + nonce)
             val sessionKey = PrivateKey(Crypto.sha256(tweak + trampolineNodeId.value + nodePrivateKey.value).byteVector32())
-            return OfferTypes.Offer.createBlindedOffer(chainHash, nodePrivateKey, trampolineNodeId, null, null, Features.empty, sessionKey, null, true)
+            return OfferTypes.Offer.createBlindedOffer(chainHash, nodePrivateKey, trampolineNodeId, null, null, Features.empty, sessionKey, null, allowCompactFormat = true)
         }
     }
 }