routing: handle introduction node failure to convert error

This commit adds handling for errors that originate after the
introduction node when making payment to a blinded route. This
indicates that the introduction node is not obeying the spec, so
it is punished for the violation.

routing/result_interpretation.go

@@ -108,6 +108,16 @@ func (i *interpretedResult) processFail(
 		return
 	}
 
+	// If the payment was to a blinded route and we received an error from
+	// after the introduction point, handle this error separately - there
+	// has been a protocol violation from the introduction node. This
+	// penalty applies regardless of the error code that is returned.
+	introIdx, isBlinded := introductionPointIndex(rt)
+	if isBlinded && introIdx < *errSourceIdx {
+		i.processPaymentOutcomeBadIntro(rt, introIdx, *errSourceIdx)
+		return
+	}
+
 	switch *errSourceIdx {
 
 	// We are the source of the failure.
@@ -129,6 +139,33 @@ func (i *interpretedResult) processFail(
 	}
 }
 
+// processPaymentOutcomeBadIntro handles the case where we have made payment
+// to a blinded route, but received an error from a node after the introduction
+// node. This indicates that the introduction node is not obeying the route
+// blinding specification, as we expect all errors from the introduction node
+// to be source from it.
+func (i *interpretedResult) processPaymentOutcomeBadIntro(route *route.Route,
+	introIdx, errSourceIdx int) {
+
+	// We fail the introduction node for not obeying the specification.
+	i.failNode(route, introIdx)
+
+	// Other preceding channels in the route forwarded correctly. Note
+	// that we do not assign success to the incoming link to the
+	// introduction node because it has not handled the error correctly.
+	if introIdx > 1 {
+		i.successPairRange(route, 0, introIdx-2)
+	}
+
+	// If the source of the failure was from the final node, we also set
+	// a final failure reason because the recipient can't process the
+	// payment (independent of the introduction failing to convert the
+	// error, we can't complete the payment if the last hop fails).
+	if errSourceIdx == len(route.Hops) {
+		i.finalFailureReason = &reasonError
+	}
+}
+
 // processPaymentOutcomeSelf handles failures sent by ourselves.
 func (i *interpretedResult) processPaymentOutcomeSelf(
 	rt *route.Route, failure lnwire.FailureMessage) {
@@ -401,6 +438,20 @@ func (i *interpretedResult) processPaymentOutcomeIntermediate(
 	}
 }
 
+// introductionPointIndex returns the index of an introduction point in a
+// route, using the same indexing in the route that we use for errorSourceIdx
+// (i.e., that we consider our own node to be at index zero). A boolean is
+// returned to indicate whether the route contains a blinded portion at all.
+func introductionPointIndex(route *route.Route) (int, bool) {
+	for i, hop := range route.Hops {
+		if hop.BlindingPoint != nil {
+			return i + 1, true
+		}
+	}
+
+	return 0, false
+}
+
 // processPaymentOutcomeUnknown processes a payment outcome for which no failure
 // message or source is available.
 func (i *interpretedResult) processPaymentOutcomeUnknown(route *route.Route) {

routing/result_interpretation_test.go

@@ -4,6 +4,7 @@ import (
 	"reflect"
 	"testing"
 
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/routing/route"
@@ -14,6 +15,10 @@ var (
 		{1, 0}, {1, 1}, {1, 2}, {1, 3}, {1, 4},
 	}
 
+	// blindingPoint provides a non-nil blinding point (value is never
+	// used).
+	blindingPoint = &btcec.PublicKey{}
+
 	routeOneHop = route.Route{
 		SourcePubKey: hops[0],
 		TotalAmount:  100,
@@ -51,6 +56,40 @@ var (
 			{PubKeyBytes: hops[4], AmtToForward: 90},
 		},
 	}
+
+	// blindedMultiHop is a blinded path where there are cleartext hops
+	// before the introduction node, and an intermediate blinded hop before
+	// the recipient after it.
+	blindedMultiHop = route.Route{
+		SourcePubKey: hops[0],
+		TotalAmount:  100,
+		Hops: []*route.Hop{
+			{PubKeyBytes: hops[1], AmtToForward: 99},
+			{
+				PubKeyBytes:   hops[2],
+				AmtToForward:  95,
+				BlindingPoint: blindingPoint,
+			},
+			{PubKeyBytes: hops[3], AmtToForward: 88},
+			{PubKeyBytes: hops[4], AmtToForward: 77},
+		},
+	}
+
+	// blindedMultiToIntroduction is a blinded path which goes directly
+	// to the introduction node, with multiple blinded hops after it.
+	blindedMultiToIntroduction = route.Route{
+		SourcePubKey: hops[0],
+		TotalAmount:  100,
+		Hops: []*route.Hop{
+			{
+				PubKeyBytes:   hops[1],
+				AmtToForward:  90,
+				BlindingPoint: blindingPoint,
+			},
+			{PubKeyBytes: hops[2], AmtToForward: 75},
+			{PubKeyBytes: hops[3], AmtToForward: 58},
+		},
+	}
 )
 
 func getTestPair(from, to int) DirectedNodePair {
@@ -366,6 +405,52 @@ var resultTestCases = []resultTestCase{
 			policyFailure: getPolicyFailure(1, 2),
 		},
 	},
+	// Test the case where a node after the introduction node returns a
+	// error. In this case the introduction node is penalized because it
+	// has not followed the specification properly.
+	{
+		name:          "error after introduction",
+		route:         &blindedMultiToIntroduction,
+		failureSrcIdx: 2,
+		// Note that the failure code doesn't matter in this case -
+		// all we're worried about is errors originating after the
+		// introduction node.
+		failure: &lnwire.FailExpiryTooSoon{},
+
+		expectedResult: &interpretedResult{
+			pairResults: map[DirectedNodePair]pairResult{
+				getTestPair(0, 1): failPairResult(0),
+				getTestPair(1, 0): failPairResult(0),
+				getTestPair(1, 2): failPairResult(0),
+				getTestPair(2, 1): failPairResult(0),
+			},
+			// Note: introduction node is failed even though the
+			// error source is after it.
+			nodeFailure: &hops[1],
+		},
+	},
+	// Test the case where we get a blinding failure from a blinded final
+	// hop when we expected the introduction node to convert.
+	{
+		name:          "final failure expected intro",
+		route:         &blindedMultiHop,
+		failureSrcIdx: 4,
+		failure:       &lnwire.FailInvalidBlinding{},
+
+		expectedResult: &interpretedResult{
+			pairResults: map[DirectedNodePair]pairResult{
+				getTestPair(0, 1): successPairResult(100),
+				getTestPair(1, 2): failPairResult(0),
+				getTestPair(2, 1): failPairResult(0),
+				getTestPair(2, 3): failPairResult(0),
+				getTestPair(3, 2): failPairResult(0),
+			},
+			// Note that the introduction node is penalized, not
+			// the final hop.
+			nodeFailure:        &hops[2],
+			finalFailureReason: &reasonError,
+		},
+	},
 }
 
 // TestResultInterpretation executes a list of test cases that test the result