routing: swap out final hop blinded route pub keys

If multiple blinded paths are provided, they will each have a different
pub key for the destination node. This makes using our existing
pathfinding logic tricky since it depends on having a single destination
node (characterised by a single pub key). We want to re-use this logic.
So what we do is swap out the pub keys of the destinaion hop with a
pseudo target pub key. This will then be used during pathfinding. Later
on once a path is found, we will swap the real destination keys back in
so that onion creation can be done.

routing/blinding.go

@@ -7,6 +7,7 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/routing/route"
 )
@@ -86,16 +87,35 @@ func NewBlindedPaymentPathSet(paths []*BlindedPayment) (*BlindedPaymentPathSet,
 		}
 	}
 
-	// NOTE: for now, we just take a single path. By the end of this PR
-	// series, all paths will be kept.
-	path := paths[0]
+	// Derive an ephemeral target priv key that will be injected into each
+	// blinded path final hop.
+	targetPriv, err := btcec.NewPrivateKey()
+	if err != nil {
+		return nil, err
+	}
+	targetPub := targetPriv.PubKey()
 
-	finalHop := path.BlindedPath.
-		BlindedHops[len(path.BlindedPath.BlindedHops)-1]
+	// If any provided blinded path only has a single hop (ie, the
+	// destination node is also the introduction node), then we discard all
+	// other paths since we know the real pub key of the destination node.
+	// For a single hop path, there is also no need for the pseudo target
+	// pub key replacement, so our target pub key in this case just remains
+	// the real introduction node ID.
+	var pathSet = paths
+	for _, path := range paths {
+		if len(path.BlindedPath.BlindedHops) != 1 {
+			continue
+		}
+
+		pathSet = []*BlindedPayment{path}
+		targetPub = path.BlindedPath.IntroductionPoint
+
+		break
+	}
 
 	return &BlindedPaymentPathSet{
-		paths:        paths,
-		targetPubKey: finalHop.BlindedNodePub,
+		paths:        pathSet,
+		targetPubKey: targetPub,
 		features:     features,
 	}, nil
 }
@@ -144,7 +164,7 @@ func (s *BlindedPaymentPathSet) ToRouteHints() (RouteHints, error) {
 	hints := make(RouteHints)
 
 	for _, path := range s.paths {
-		pathHints, err := path.toRouteHints()
+		pathHints, err := path.toRouteHints(fn.Some(s.targetPubKey))
 		if err != nil {
 			return nil, err
 		}
@@ -223,8 +243,11 @@ func (b *BlindedPayment) Validate() error {
 // effectively the final_cltv_delta for the receiving introduction node). In
 // the case of multiple blinded hops, CLTV delta is fully accounted for in the
 // hints (both for intermediate hops and the final_cltv_delta for the receiving
-// node).
-func (b *BlindedPayment) toRouteHints() (RouteHints, error) {
+// node). The pseudoTarget, if provided,  will be used to override the pub key
+// of the destination node in the path.
+func (b *BlindedPayment) toRouteHints(
+	pseudoTarget fn.Option[*btcec.PublicKey]) (RouteHints, error) {
+
 	// If we just have a single hop in our blinded route, it just contains
 	// an introduction node (this is a valid path according to the spec).
 	// Since we have the un-blinded node ID for the introduction node, we
@@ -272,12 +295,12 @@ func (b *BlindedPayment) toRouteHints() (RouteHints, error) {
 		ToNodeFeatures: features,
 	}
 
-	edge, err := NewBlindedEdge(edgePolicy, b, 0)
+	lastEdge, err := NewBlindedEdge(edgePolicy, b, 0)
 	if err != nil {
 		return nil, err
 	}
 
-	hints[fromNode] = []AdditionalEdge{edge}
+	hints[fromNode] = []AdditionalEdge{lastEdge}
 
 	// Start at an offset of 1 because the first node in our blinded hops
 	// is the introduction node and terminate at the second-last node
@@ -304,13 +327,24 @@ func (b *BlindedPayment) toRouteHints() (RouteHints, error) {
 			ToNodeFeatures: features,
 		}
 
-		edge, err := NewBlindedEdge(edgePolicy, b, i)
+		lastEdge, err = NewBlindedEdge(edgePolicy, b, i)
 		if err != nil {
 			return nil, err
 		}
 
-		hints[fromNode] = []AdditionalEdge{edge}
+		hints[fromNode] = []AdditionalEdge{lastEdge}
 	}
 
+	pseudoTarget.WhenSome(func(key *btcec.PublicKey) {
+		// For the very last hop on the path, switch out the ToNodePub
+		// for the pseudo target pub key.
+		lastEdge.policy.ToNodePubKey = func() route.Vertex {
+			return route.NewVertex(key)
+		}
+
+		// Then override the final hint with this updated edge.
+		hints[fromNode] = []AdditionalEdge{lastEdge}
+	})
+
 	return hints, nil
 }

routing/blinding_test.go

@@ -7,6 +7,7 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/stretchr/testify/require"
@@ -128,7 +129,7 @@ func TestBlindedPaymentToHints(t *testing.T) {
 		HtlcMaximum:         htlcMax,
 		Features:            features,
 	}
-	hints, err := blindedPayment.toRouteHints()
+	hints, err := blindedPayment.toRouteHints(fn.None[*btcec.PublicKey]())
 	require.NoError(t, err)
 	require.Nil(t, hints)
 
@@ -183,7 +184,7 @@ func TestBlindedPaymentToHints(t *testing.T) {
 		},
 	}
 
-	actual, err := blindedPayment.toRouteHints()
+	actual, err := blindedPayment.toRouteHints(fn.None[*btcec.PublicKey]())
 	require.NoError(t, err)
 
 	require.Equal(t, len(expected), len(actual))

routing/pathfind.go

@@ -153,19 +153,24 @@ func newRoute(sourceVertex route.Vertex,
 		// sender of the payment.
 		nextIncomingAmount lnwire.MilliSatoshi
 
-		blindedPath *sphinx.BlindedPath
+		blindedPayment *BlindedPayment
 	)
 
-	if blindedPathSet != nil {
-		blindedPath = blindedPathSet.GetPath().BlindedPath
-	}
-
 	pathLength := len(pathEdges)
 	for i := pathLength - 1; i >= 0; i-- {
 		// Now we'll start to calculate the items within the per-hop
 		// payload for the hop this edge is leading to.
 		edge := pathEdges[i].policy
 
+		// If this is an edge from a blinded path and the
+		// blindedPayment variable has not been set yet, then set it now
+		// by extracting the corresponding blinded payment from the
+		// edge.
+		isBlindedEdge := pathEdges[i].blindedPayment != nil
+		if isBlindedEdge && blindedPayment == nil {
+			blindedPayment = pathEdges[i].blindedPayment
+		}
+
 		// We'll calculate the amounts, timelocks, and fees for each hop
 		// in the route. The base case is the final hop which includes
 		// their amount and timelocks. These values will accumulate
@@ -212,8 +217,9 @@ func newRoute(sourceVertex route.Vertex,
 			// node's CLTV delta. The exception is for the case
 			// where the final hop is the blinded path introduction
 			// node.
-			if blindedPath == nil ||
-				len(blindedPath.BlindedHops) == 1 {
+			if blindedPathSet == nil ||
+				len(blindedPathSet.GetPath().BlindedPath.
+					BlindedHops) == 1 {
 
 				// As this is the last hop, we'll use the
 				// specified final CLTV delta value instead of
@@ -245,7 +251,7 @@ func newRoute(sourceVertex route.Vertex,
 
 			metadata = finalHop.metadata
 
-			if blindedPath != nil {
+			if blindedPathSet != nil {
 				totalAmtMsatBlinded = finalHop.totalAmt
 			}
 		} else {
@@ -305,11 +311,25 @@ func newRoute(sourceVertex route.Vertex,
 	// If we are creating a route to a blinded path, we need to add some
 	// additional data to the route that is required for blinded forwarding.
 	// We do another pass on our edges to append this data.
-	if blindedPath != nil {
+	if blindedPathSet != nil {
+		// If the passed in BlindedPaymentPathSet is non-nil but no
+		// edge had a BlindedPayment attached, it means that the path
+		// chosen was an introduction-node-only path. So in this case,
+		// we can assume the relevant payment is the only one in the
+		// payment set.
+		if blindedPayment == nil {
+			blindedPayment = blindedPathSet.GetPath()
+		}
+
 		var (
 			inBlindedRoute bool
 			dataIndex      = 0
 
+			blindedPath = blindedPayment.BlindedPath
+			numHops     = len(blindedPath.BlindedHops)
+			realFinal   = blindedPath.BlindedHops[numHops-1].
+					BlindedNodePub
+
 			introVertex = route.NewVertex(
 				blindedPath.IntroductionPoint,
 			)
@@ -337,6 +357,11 @@ func newRoute(sourceVertex route.Vertex,
 			if i != len(hops)-1 {
 				hop.AmtToForward = 0
 				hop.OutgoingTimeLock = 0
+			} else {
+				// For the final hop, we swap out the pub key
+				// bytes to the original destination node pub
+				// key for that payment path.
+				hop.PubKeyBytes = route.NewVertex(realFinal)
 			}
 
 			dataIndex++

routing/pathfind_test.go

@@ -23,6 +23,7 @@ import (
 	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	switchhop "github.com/lightningnetwork/lnd/htlcswitch/hop"
 	"github.com/lightningnetwork/lnd/kvdb"
@@ -3286,7 +3287,9 @@ func TestBlindedRouteConstruction(t *testing.T) {
 	// that make up the graph we'll give to route construction. The hints
 	// map is keyed by source node, so we can retrieve our blinded edges
 	// accordingly.
-	blindedEdges, err := blindedPayment.toRouteHints()
+	blindedEdges, err := blindedPayment.toRouteHints(
+		fn.None[*btcec.PublicKey](),
+	)
 	require.NoError(t, err)
 
 	carolDaveEdge := blindedEdges[carolVertex][0]

routing/router.go

@@ -573,20 +573,7 @@ func getTargetNode(target *route.Vertex,
 		return route.Vertex{}, ErrTargetAndBlinded
 
 	case blinded:
-		blindedPayment := blindedPathSet.GetPath()
-
-		// If we're dealing with an edge-case blinded path that just
-		// has an introduction node (first hop expected to be the intro
-		// hop), then we return the unblinded introduction node as our
-		// target.
-		hops := blindedPayment.BlindedPath.BlindedHops
-		if len(hops) == 1 {
-			return route.NewVertex(
-				blindedPayment.BlindedPath.IntroductionPoint,
-			), nil
-		}
-
-		return route.NewVertex(hops[len(hops)-1].BlindedNodePub), nil
+		return route.NewVertex(blindedPathSet.TargetPubKey()), nil
 
 	case targetSet:
 		return *target, nil

routing/router_test.go

@@ -2231,7 +2231,10 @@ func TestNewRouteRequest(t *testing.T) {
 		t.Run(testCase.name, func(t *testing.T) {
 			t.Parallel()
 
-			var blindedPathInfo *BlindedPaymentPathSet
+			var (
+				blindedPathInfo *BlindedPaymentPathSet
+				expectedTarget  = testCase.expectedTarget
+			)
 			if testCase.blindedPayment != nil {
 				blindedPathInfo, err = NewBlindedPaymentPathSet(
 					[]*BlindedPayment{
@@ -2239,6 +2242,10 @@ func TestNewRouteRequest(t *testing.T) {
 					},
 				)
 				require.NoError(t, err)
+
+				expectedTarget = route.NewVertex(
+					blindedPathInfo.TargetPubKey(),
+				)
 			}
 
 			req, err := NewRouteRequest(
@@ -2253,7 +2260,7 @@ func TestNewRouteRequest(t *testing.T) {
 				return
 			}
 
-			require.Equal(t, req.Target, testCase.expectedTarget)
+			require.Equal(t, req.Target, expectedTarget)
 			require.Equal(
 				t, req.FinalExpiry, testCase.expectedCltv,
 			)