record: add NextNodeID type to BlindedRouteData

Add the NextNodeID TLV (tlv type 4) to the BlindedRouteData TLV stream.
This will be used during the dummy hop payload construction.

record/blinded_data.go

@@ -22,6 +22,11 @@ type BlindedRouteData struct {
 	// ShortChannelID is the channel ID of the next hop.
 	ShortChannelID tlv.OptionalRecordT[tlv.TlvType2, lnwire.ShortChannelID]
 
+	// NextNodeID is the node ID of the next node on the path. In the
+	// context of blinded path payments, this is used to indicate the
+	// presence of dummy hops that need to be peeled from the onion.
+	NextNodeID tlv.OptionalRecordT[tlv.TlvType4, *btcec.PublicKey]
+
 	// PathID is a secret set of bytes that the blinded path creator will
 	// set so that they can check the value on decryption to ensure that the
 	// path they created was used for the intended purpose.
@@ -98,6 +103,26 @@ func NewFinalHopBlindedRouteData(constraints *PaymentConstraints,
 	return &data
 }
 
+// NewDummyHopRouteData creates the data that's provided for any hop preceding
+// a dummy hop. The presence of such a payload indicates to the reader that
+// they are the intended recipient and should peel the remainder of the onion.
+func NewDummyHopRouteData(ourPubKey *btcec.PublicKey,
+	relayInfo PaymentRelayInfo,
+	constraints PaymentConstraints) *BlindedRouteData {
+
+	return &BlindedRouteData{
+		NextNodeID: tlv.SomeRecordT(
+			tlv.NewPrimitiveRecord[tlv.TlvType4](ourPubKey),
+		),
+		RelayInfo: tlv.SomeRecordT(
+			tlv.NewRecordT[tlv.TlvType10](relayInfo),
+		),
+		Constraints: tlv.SomeRecordT(
+			tlv.NewRecordT[tlv.TlvType12](constraints),
+		),
+	}
+}
+
 // DecodeBlindedRouteData decodes the data provided within a blinded route.
 func DecodeBlindedRouteData(r io.Reader) (*BlindedRouteData, error) {
 	var (
@@ -105,6 +130,7 @@ func DecodeBlindedRouteData(r io.Reader) (*BlindedRouteData, error) {
 
 		padding          = d.Padding.Zero()
 		scid             = d.ShortChannelID.Zero()
+		nextNodeID       = d.NextNodeID.Zero()
 		pathID           = d.PathID.Zero()
 		blindingOverride = d.NextBlindingOverride.Zero()
 		relayInfo        = d.RelayInfo.Zero()
@@ -118,8 +144,8 @@ func DecodeBlindedRouteData(r io.Reader) (*BlindedRouteData, error) {
 	}
 
 	typeMap, err := tlvRecords.ExtractRecords(
-		&padding, &scid, &pathID, &blindingOverride, &relayInfo,
-		&constraints, &features,
+		&padding, &scid, &nextNodeID, &pathID, &blindingOverride,
+		&relayInfo, &constraints, &features,
 	)
 	if err != nil {
 		return nil, err
@@ -134,6 +160,10 @@ func DecodeBlindedRouteData(r io.Reader) (*BlindedRouteData, error) {
 		d.ShortChannelID = tlv.SomeRecordT(scid)
 	}
 
+	if val, ok := typeMap[d.NextNodeID.TlvType()]; ok && val == nil {
+		d.NextNodeID = tlv.SomeRecordT(nextNodeID)
+	}
+
 	if val, ok := typeMap[d.PathID.TlvType()]; ok && val == nil {
 		d.PathID = tlv.SomeRecordT(pathID)
 	}
@@ -175,6 +205,12 @@ func EncodeBlindedRouteData(data *BlindedRouteData) ([]byte, error) {
 		recordProducers = append(recordProducers, &scid)
 	})
 
+	data.NextNodeID.WhenSome(func(f tlv.RecordT[tlv.TlvType4,
+		*btcec.PublicKey]) {
+
+		recordProducers = append(recordProducers, &f)
+	})
+
 	data.PathID.WhenSome(func(pathID tlv.RecordT[tlv.TlvType6, []byte]) {
 		recordProducers = append(recordProducers, &pathID)
 	})

record/blinded_data_test.go

@@ -171,6 +171,38 @@ func TestBlindedDataFinalHopEncoding(t *testing.T) {
 	}
 }
 
+// TestDummyHopBlindedDataEncoding tests the encoding and decoding of a blinded
+// data blob intended for hops preceding a dummy hop in a blinded path. These
+// hops provide the reader with a signal that the next hop may be a dummy hop.
+func TestDummyHopBlindedDataEncoding(t *testing.T) {
+	t.Parallel()
+
+	priv, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+
+	info := PaymentRelayInfo{
+		FeeRate:         2,
+		CltvExpiryDelta: 3,
+		BaseFee:         30,
+	}
+
+	constraints := PaymentConstraints{
+		MaxCltvExpiry:   4,
+		HtlcMinimumMsat: 100,
+	}
+
+	routeData := NewDummyHopRouteData(priv.PubKey(), info, constraints)
+
+	encoded, err := EncodeBlindedRouteData(routeData)
+	require.NoError(t, err)
+
+	b := bytes.NewBuffer(encoded)
+	decodedData, err := DecodeBlindedRouteData(b)
+	require.NoError(t, err)
+
+	require.Equal(t, routeData, decodedData)
+}
+
 // TestBlindedRouteDataPadding tests the PadBy method of BlindedRouteData.
 func TestBlindedRouteDataPadding(t *testing.T) {
 	newBlindedRouteData := func() *BlindedRouteData {