From 65aef6a69cae2fb30d16b200f7256e570307f1ae Mon Sep 17 00:00:00 2001
From: Elle Mouton <elle.mouton@gmail.com>
Date: Wed, 10 Jul 2024 11:39:11 +0200
Subject: [PATCH] htlcswitch: handle blinded path dummy hops

If a blinded path payload contains a signal that the following hop on
the path is a dummy hop, then we iteratively peel the dummy hops until
the final payload is reached.
---
 htlcswitch/hop/iterator.go      | 72 +++++++++++++++++++++++++++++----
 htlcswitch/hop/iterator_test.go |  7 ++++
 2 files changed, 72 insertions(+), 7 deletions(-)

diff --git a/htlcswitch/hop/iterator.go b/htlcswitch/hop/iterator.go
index bb3fb12d7..83b5e3f52 100644
--- a/htlcswitch/hop/iterator.go
+++ b/htlcswitch/hop/iterator.go
@@ -284,13 +284,6 @@ func deriveBlindedRouteForwardingInfo(r *sphinxHopIterator,
 		return nil, routeRole, err
 	}
 
-	nextSCID, err := routeData.ShortChannelID.UnwrapOrErr(
-		fmt.Errorf("next SCID not set for non-final blinded hop"),
-	)
-	if err != nil {
-		return nil, routeRole, err
-	}
-
 	fwdAmt, err := calculateForwardingAmount(
 		r.blindingKit.IncomingAmount, relayInfo.Val.BaseFee,
 		relayInfo.Val.FeeRate,
@@ -315,6 +308,22 @@ func deriveBlindedRouteForwardingInfo(r *sphinxHopIterator,
 		return nil, routeRole, err
 	}
 
+	// If the payload signals that the following hop is a dummy hop, then
+	// we will iteratively peel the dummy hop until we reach the final
+	// payload.
+	if checkForDummyHop(routeData, r.router.OnionPublicKey()) {
+		return peelBlindedPathDummyHop(
+			r, uint32(relayInfo.Val.CltvExpiryDelta), fwdAmt,
+			routeRole, nextEph,
+		)
+	}
+
+	nextSCID, err := routeData.ShortChannelID.UnwrapOrErr(
+		fmt.Errorf("next SCID not set for non-final blinded hop"),
+	)
+	if err != nil {
+		return nil, routeRole, err
+	}
 	payload.FwdInfo = ForwardingInfo{
 		NextHop:         nextSCID.Val,
 		AmountToForward: fwdAmt,
@@ -332,6 +341,55 @@ func deriveBlindedRouteForwardingInfo(r *sphinxHopIterator,
 	return payload, routeRole, nil
 }
 
+// checkForDummyHop returns whether the given BlindedRouteData packet indicates
+// the presence of a dummy hop.
+func checkForDummyHop(routeData *record.BlindedRouteData,
+	routerPubKey *btcec.PublicKey) bool {
+
+	var isDummy bool
+	routeData.NextNodeID.WhenSome(
+		func(r tlv.RecordT[tlv.TlvType4, *btcec.PublicKey]) {
+			isDummy = r.Val.IsEqual(routerPubKey)
+		},
+	)
+
+	return isDummy
+}
+
+// peelBlindedPathDummyHop packages the next onion packet and then constructs
+// a new hop iterator using our router and then proceeds to process the next
+// packet. This can only be done for blinded route dummy hops since we expect
+// to be the final hop on the path.
+func peelBlindedPathDummyHop(r *sphinxHopIterator, cltvExpiryDelta uint32,
+	fwdAmt lnwire.MilliSatoshi, routeRole RouteRole,
+	nextEph tlv.RecordT[tlv.TlvType8, *btcec.PublicKey]) (*Payload,
+	RouteRole, error) {
+
+	onionPkt := r.processedPacket.NextPacket
+	sphinxPacket, err := r.router.ReconstructOnionPacket(
+		onionPkt, r.rHash, sphinx.WithBlindingPoint(nextEph.Val),
+	)
+	if err != nil {
+		return nil, routeRole, err
+	}
+
+	iterator := makeSphinxHopIterator(
+		r.router, onionPkt, sphinxPacket, BlindingKit{
+			Processor: r.router,
+			UpdateAddBlinding: tlv.SomeRecordT(
+				tlv.NewPrimitiveRecord[lnwire.BlindingPointTlvType]( //nolint:lll
+					nextEph.Val,
+				),
+			),
+			IncomingAmount: fwdAmt,
+			IncomingCltv: r.blindingKit.IncomingCltv -
+				cltvExpiryDelta,
+		}, r.rHash,
+	)
+
+	return extractTLVPayload(iterator)
+}
+
 // decryptAndValidateBlindedRouteData decrypts the encrypted payload from the
 // payment recipient using a blinding key. The incoming HTLC amount and CLTV
 // values are then verified against the policy values from the recipient.
diff --git a/htlcswitch/hop/iterator_test.go b/htlcswitch/hop/iterator_test.go
index 74be6b190..fff9ae17d 100644
--- a/htlcswitch/hop/iterator_test.go
+++ b/htlcswitch/hop/iterator_test.go
@@ -206,6 +206,9 @@ func TestParseAndValidateRecipientData(t *testing.T) {
 	// Mocked error.
 	errDecryptFailed := errors.New("could not decrypt")
 
+	nodeKey, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+
 	tests := []struct {
 		name              string
 		data              []byte
@@ -284,6 +287,10 @@ func TestParseAndValidateRecipientData(t *testing.T) {
 			}
 			iterator := &sphinxHopIterator{
 				blindingKit: kit,
+				router: sphinx.NewRouter(
+					&sphinx.PrivKeyECDH{PrivKey: nodeKey},
+					sphinx.NewMemoryReplayLog(),
+				),
 			}
 
 			_, _, err = parseAndValidateRecipientData(