multi: send to a blinded path in an invoice

Update the SendPayment flow so that it is able to send to an invoice
containing a blinded path.

lnrpc/routerrpc/router_backend.go

@@ -999,6 +999,32 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 		payIntent.PaymentAddr = payAddr
 		payIntent.PaymentRequest = []byte(rpcPayReq.PaymentRequest)
 		payIntent.Metadata = payReq.Metadata
+
+		if len(payReq.BlindedPaymentPaths) > 0 {
+			// NOTE: Currently we only choose a single payment path.
+			// This will be updated in a future PR to handle
+			// multiple blinded payment paths.
+			path := payReq.BlindedPaymentPaths[0]
+			if len(path.Hops) == 0 {
+				return nil, fmt.Errorf("a blinded payment " +
+					"must have at least 1 hop")
+			}
+
+			finalHop := path.Hops[len(path.Hops)-1]
+
+			payIntent.BlindedPayment = MarshalBlindedPayment(path)
+
+			// Replace the target node with the blinded public key
+			// of the blinded path's final node.
+			copy(
+				payIntent.Target[:],
+				finalHop.BlindedNodePub.SerializeCompressed(),
+			)
+
+			if !path.Features.IsEmpty() {
+				payIntent.DestFeatures = path.Features.Clone()
+			}
+		}
 	} else {
 		// Otherwise, If the payment request field was not specified
 		// (and a custom route wasn't specified), construct the payment
@@ -1137,6 +1163,26 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 	return payIntent, nil
 }
 
+// MarshalBlindedPayment marshals a zpay32.BLindedPaymentPath into a
+// routing.BlindedPayment.
+func MarshalBlindedPayment(
+	path *zpay32.BlindedPaymentPath) *routing.BlindedPayment {
+
+	return &routing.BlindedPayment{
+		BlindedPath: &sphinx.BlindedPath{
+			IntroductionPoint: path.Hops[0].BlindedNodePub,
+			BlindingPoint:     path.FirstEphemeralBlindingPoint,
+			BlindedHops:       path.Hops,
+		},
+		BaseFee:             path.FeeBaseMsat,
+		ProportionalFeeRate: path.FeeRate,
+		CltvExpiryDelta:     path.CltvExpiryDelta,
+		HtlcMinimum:         path.HTLCMinMsat,
+		HtlcMaximum:         path.HTLCMaxMsat,
+		Features:            path.Features,
+	}
+}
+
 // unmarshallRouteHints unmarshalls a list of route hints.
 func unmarshallRouteHints(rpcRouteHints []*lnrpc.RouteHint) (
 	[][]zpay32.HopHint, error) {

routing/payment_session.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btclog"
+	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/build"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
@@ -205,6 +206,18 @@ func newPaymentSession(p *LightningPayment, selfNode route.Vertex,
 		return nil, err
 	}
 
+	if p.BlindedPayment != nil {
+		if len(edges) != 0 {
+			return nil, fmt.Errorf("cannot have both route hints " +
+				"and blinded path")
+		}
+
+		edges, err = p.BlindedPayment.toRouteHints()
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	logPrefix := fmt.Sprintf("PaymentSession(%x):", p.Identifier())
 
 	return &paymentSession{
@@ -389,6 +402,11 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 			return nil, err
 		}
 
+		var blindedPath *sphinx.BlindedPath
+		if p.payment.BlindedPayment != nil {
+			blindedPath = p.payment.BlindedPayment.BlindedPath
+		}
+
 		// With the next candidate path found, we'll attempt to turn
 		// this into a route by applying the time-lock and fee
 		// requirements.
@@ -401,7 +419,7 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 				records:     p.payment.DestCustomRecords,
 				paymentAddr: p.payment.PaymentAddr,
 				metadata:    p.payment.Metadata,
-			}, nil,
+			}, blindedPath,
 		)
 		if err != nil {
 			return nil, err

routing/router.go

@@ -922,9 +922,19 @@ type LightningPayment struct {
 	// NOTE: This is optional unless required by the payment. When providing
 	// multiple routes, ensure the hop hints within each route are chained
 	// together and sorted in forward order in order to reach the
-	// destination successfully.
+	// destination successfully. This is mutually exclusive to the
+	// BlindedPayment field.
 	RouteHints [][]zpay32.HopHint
 
+	// BlindedPayment holds the information about a blinded path to the
+	// payment recipient. This is mutually exclusive to the RouteHints
+	// field.
+	//
+	// NOTE: a recipient may provide multiple blinded payment paths in the
+	// same invoice. Currently, LND will only attempt to use the first one.
+	// A future PR will handle multiple blinded payment paths.
+	BlindedPayment *BlindedPayment
+
 	// OutgoingChannelIDs is the list of channels that are allowed for the
 	// first hop. If nil, any channel may be used.
 	OutgoingChannelIDs []uint64

rpcserver.go

@@ -5110,6 +5110,7 @@ type rpcPaymentIntent struct {
 	paymentAddr        *[32]byte
 	payReq             []byte
 	metadata           []byte
+	blindedPayment     *routing.BlindedPayment
 
 	destCustomRecords record.CustomSet
 
@@ -5245,6 +5246,32 @@ func (r *rpcServer) extractPaymentIntent(rpcPayReq *rpcPaymentRequest) (rpcPayme
 		payIntent.paymentAddr = payReq.PaymentAddr
 		payIntent.metadata = payReq.Metadata
 
+		if len(payReq.BlindedPaymentPaths) > 0 {
+			// NOTE: Currently we only choose a single payment path.
+			// This will be updated in a future PR to handle
+			// multiple blinded payment paths.
+			path := payReq.BlindedPaymentPaths[0]
+			if len(path.Hops) == 0 {
+				return payIntent, fmt.Errorf("a blinded " +
+					"payment must have at least 1 hop")
+			}
+
+			finalHop := path.Hops[len(path.Hops)-1]
+			payIntent.blindedPayment =
+				routerrpc.MarshalBlindedPayment(path)
+
+			// Replace the target node with the blinded public key
+			// of the blinded path's final node.
+			copy(
+				payIntent.dest[:],
+				finalHop.BlindedNodePub.SerializeCompressed(),
+			)
+
+			if !payReq.BlindedPaymentPaths[0].Features.IsEmpty() {
+				payIntent.destFeatures = path.Features.Clone()
+			}
+		}
+
 		if err := validateDest(payIntent.dest); err != nil {
 			return payIntent, err
 		}
@@ -5399,6 +5426,7 @@ func (r *rpcServer) dispatchPaymentIntent(
 			DestFeatures:       payIntent.destFeatures,
 			PaymentAddr:        payIntent.paymentAddr,
 			Metadata:           payIntent.metadata,
+			BlindedPayment:     payIntent.blindedPayment,
 
 			// Don't enable multi-part payments on the main rpc.
 			// Users need to use routerrpc for that.