multi: add blinded path TLVs to onion payload / hops

This commit adds the encrypted_data, blinding_point and total_amt_msat
tlvs to the known set of even tlvs for the onion payload. These TLVs
are added in two places (the onion payload and hop struct) because
lnd uses the same set of TLV types for both structs (and they
inherently represent the same thing).

Note: in some places, unit tests intentionally mimic the style
of older tests, so as to be more consistently readable.

record/hop.go

@@ -1,6 +1,7 @@
 package record
 
 import (
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/tlv"
 )
 
@@ -17,9 +18,21 @@ const (
 	// of the next hop.
 	NextHopOnionType tlv.Type = 6
 
+	// EncryptedDataOnionType is the type used to include encrypted data
+	// provided by the receiver in the onion for use in blinded paths.
+	EncryptedDataOnionType tlv.Type = 10
+
+	// BlindingPointOnionType is the type used to include receiver provided
+	// ephemeral keys in the onion that are used in blinded paths.
+	BlindingPointOnionType tlv.Type = 12
+
 	// MetadataOnionType is the type used in the onion for the payment
 	// metadata.
 	MetadataOnionType tlv.Type = 16
+
+	// TotalAmtMsatBlindedType is the type used in the onion for the total
+	// amount field that is included in the final hop for blinded payments.
+	TotalAmtMsatBlindedType tlv.Type = 18
 )
 
 // NewAmtToFwdRecord creates a tlv.Record that encodes the amount_to_forward
@@ -50,6 +63,18 @@ func NewNextHopIDRecord(cid *uint64) tlv.Record {
 	return tlv.MakePrimitiveRecord(NextHopOnionType, cid)
 }
 
+// NewEncryptedDataRecord creates a tlv.Record that encodes the encrypted_data
+// (type 10) record for an onion payload.
+func NewEncryptedDataRecord(data *[]byte) tlv.Record {
+	return tlv.MakePrimitiveRecord(EncryptedDataOnionType, data)
+}
+
+// NewBlindingPointRecord creates a tlv.Record that encodes the blinding_point
+// (type 12) record for an onion payload.
+func NewBlindingPointRecord(point **btcec.PublicKey) tlv.Record {
+	return tlv.MakePrimitiveRecord(BlindingPointOnionType, point)
+}
+
 // NewMetadataRecord creates a tlv.Record that encodes the metadata (type 10)
 // for an onion payload.
 func NewMetadataRecord(metadata *[]byte) tlv.Record {
@@ -61,3 +86,14 @@ func NewMetadataRecord(metadata *[]byte) tlv.Record {
 		tlv.EVarBytes, tlv.DVarBytes,
 	)
 }
+
+// NewTotalAmtMsatBlinded creates a tlv.Record that encodes the
+// total_amount_msat for the final an onion payload within a blinded route.
+func NewTotalAmtMsatBlinded(amt *uint64) tlv.Record {
+	return tlv.MakeDynamicRecord(
+		TotalAmtMsatBlindedType, amt, func() uint64 {
+			return tlv.SizeTUint64(*amt)
+		},
+		tlv.ETUint64, tlv.DTUint64,
+	)
+}