scbforceclose: classify the outputs and their amounts

Author: ziggie1984

cmd/chantools/scbforceclose.go

@@ -8,11 +8,14 @@ import (
 	"os"
 	"strings"
 
+	"github.com/btcsuite/btcd/wire"
 	"github.com/lightninglabs/chantools/btc"
 	"github.com/lightninglabs/chantools/lnd"
 	"github.com/lightninglabs/chantools/scbforceclose"
 	"github.com/lightningnetwork/lnd/chanbackup"
+	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/input"
+	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/spf13/cobra"
 )
 
@@ -209,7 +212,18 @@ func (c *scbForceCloseCommand) Execute(_ *cobra.Command, _ []string) error {
 		txHex := hex.EncodeToString(buf.Bytes())
 		fmt.Println("Channel point:", s.FundingOutpoint)
 		fmt.Println("Raw transaction hex:", txHex)
-		fmt.Println()
+
+		// Classify outputs: identify to_remote using known templates,
+		// anchors (330 sat), and log the rest as to_local/htlc without
+		// deriving per-commitment. The remote key is not tweaked in
+		// the backup (except for very old channels which we don't
+		// support anyway).
+		class, err := classifyOutputs(s, signedTx)
+		if err == nil {
+			printOutputClassification(class, signedTx)
+		} else {
+			fmt.Printf("failed to classify outputs: %v\n", err)
+		}
 
 		// Publish TX.
 		if c.Publish {
@@ -224,3 +238,154 @@ func (c *scbForceCloseCommand) Execute(_ *cobra.Command, _ []string) error {
 
 	return nil
 }
+
+// classifyAndLogOutputs attempts to identify the to_remote output by comparing
+// against known script templates (p2wkh, delayed p2wsh, lease), marks 330-sat
+// anchors, and logs remaining outputs as to_local/htlc without deriving
+// per-commitment data.
+func printOutputClassification(class outputClassification, tx *wire.MsgTx) {
+
+	if class.ToRemoteIdx >= 0 {
+		log.Infof("to_remote: idx=%d amount=%d sat", class.ToRemoteIdx,
+			class.ToRemoteAmt)
+
+		if len(class.ToRemotePkScript) != 0 {
+			log.Infof("to_remote PkScript (hex): %x",
+				class.ToRemotePkScript)
+		}
+	} else {
+		log.Infof("to_remote: not identified")
+	}
+
+	for _, idx := range class.AnchorIdxs {
+		log.Infof("possible anchor: idx=%d amount=%d sat", idx,
+			tx.TxOut[idx].Value)
+	}
+
+	for _, idx := range class.OtherIdxs {
+		log.Infof("possible to_local/htlc: idx=%d amount=%d sat", idx,
+			tx.TxOut[idx].Value)
+	}
+
+}
+
+// outputClassification is the result of classifying the outputs of a channel
+// force close transaction.
+type outputClassification struct {
+	ToRemoteIdx      int
+	ToRemoteAmt      int64
+	ToRemotePkScript []byte
+	AnchorIdxs       []int
+	OtherIdxs        []int
+}
+
+// classifyOutputs attempts to identify the to_remote output and classify the
+// other outputs into anchors and to_local/htlc.
+func classifyOutputs(s chanbackup.Single, tx *wire.MsgTx) (outputClassification,
+	error) {
+
+	// Best-effort get the remote key used for to_remote.
+	remoteDesc := s.RemoteChanCfg.PaymentBasePoint
+	remoteKey := remoteDesc.PubKey
+
+	// Compute the expected to_remote pkScript.
+	var toRemotePkScript []byte
+	if remoteKey != nil {
+		chanType, err := chanTypeFromBackupVersion(s.Version)
+		if err != nil {
+			return outputClassification{}, fmt.Errorf("failed to "+
+				"get channel type: %w", err)
+		}
+		desc, _, err := lnwallet.CommitScriptToRemote(
+			chanType, s.IsInitiator, remoteKey,
+			s.LeaseExpiry,
+			input.NoneTapLeaf(),
+		)
+		if err != nil {
+			return outputClassification{}, fmt.Errorf("failed to "+
+				"get commit script to remote: %w", err)
+		}
+		toRemotePkScript = desc.PkScript()
+	}
+
+	// anchorSats is anchor output value in sats.
+	const anchorSats = 330
+
+	result := outputClassification{
+		ToRemoteIdx:      -1,
+		ToRemotePkScript: toRemotePkScript,
+	}
+
+	// First pass: find to_remote by script match.
+	for idx, out := range tx.TxOut {
+		if len(toRemotePkScript) != 0 &&
+			bytes.Equal(out.PkScript, toRemotePkScript) {
+
+			result.ToRemoteIdx = idx
+			result.ToRemoteAmt = out.Value
+			break
+		}
+	}
+
+	// Second pass: classify anchors and the rest.
+	for idx, out := range tx.TxOut {
+		if idx == result.ToRemoteIdx {
+			continue
+		}
+		if out.Value == anchorSats {
+			result.AnchorIdxs = append(result.AnchorIdxs, idx)
+		} else {
+			result.OtherIdxs = append(result.OtherIdxs, idx)
+		}
+	}
+
+	return result, nil
+}
+
+// chanTypeFromBackupVersion maps a backup SingleBackupVersion to an approximate
+// channeldb.ChannelType sufficient for deriving to_remote scripts.
+func chanTypeFromBackupVersion(v chanbackup.SingleBackupVersion) (
+	channeldb.ChannelType, error) {
+
+	var chanType channeldb.ChannelType
+	switch v {
+	case chanbackup.DefaultSingleVersion:
+		chanType = channeldb.SingleFunderBit
+
+	case chanbackup.TweaklessCommitVersion:
+		chanType = channeldb.SingleFunderTweaklessBit
+
+	case chanbackup.AnchorsCommitVersion:
+		chanType = channeldb.AnchorOutputsBit
+		chanType |= channeldb.SingleFunderTweaklessBit
+
+	case chanbackup.AnchorsZeroFeeHtlcTxCommitVersion:
+		chanType = channeldb.ZeroHtlcTxFeeBit
+		chanType |= channeldb.AnchorOutputsBit
+		chanType |= channeldb.SingleFunderTweaklessBit
+
+	case chanbackup.ScriptEnforcedLeaseVersion:
+		chanType = channeldb.LeaseExpirationBit
+		chanType |= channeldb.ZeroHtlcTxFeeBit
+		chanType |= channeldb.AnchorOutputsBit
+		chanType |= channeldb.SingleFunderTweaklessBit
+
+	case chanbackup.SimpleTaprootVersion:
+		chanType = channeldb.ZeroHtlcTxFeeBit
+		chanType |= channeldb.AnchorOutputsBit
+		chanType |= channeldb.SingleFunderTweaklessBit
+		chanType |= channeldb.SimpleTaprootFeatureBit
+
+	case chanbackup.TapscriptRootVersion:
+		chanType = channeldb.ZeroHtlcTxFeeBit
+		chanType |= channeldb.AnchorOutputsBit
+		chanType |= channeldb.SingleFunderTweaklessBit
+		chanType |= channeldb.SimpleTaprootFeatureBit
+		chanType |= channeldb.TapscriptRootBit
+
+	default:
+		return 0, fmt.Errorf("unknown Single version: %v", v)
+	}
+
+	return chanType, nil
+}

cmd/chantools/scbforceclose_test.go

@@ -0,0 +1,94 @@
+package main
+
+import (
+	"bytes"
+	_ "embed"
+	"encoding/hex"
+	"encoding/json"
+	"testing"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/chanbackup"
+	"github.com/lightningnetwork/lnd/keychain"
+	"github.com/stretchr/testify/require"
+)
+
+//go:embed testdata/scbforceclose_testdata.json
+var scbforcecloseTestData []byte
+
+// TestClassifyOutputs_RealData verifies we can identify the to_remote output
+// using lnwallet.CommitScriptToRemote with real world data provided.
+func TestClassifyOutputs_RealData(t *testing.T) {
+	// Load test data from embedded file.
+	var testData struct {
+		RemotePubkey   string `json:"remotePubkey"`
+		TransactionHex string `json:"transactionHex"`
+	}
+	err := json.Unmarshal(scbforcecloseTestData, &testData)
+	require.NoError(t, err)
+
+	// Remote payment basepoint (compressed) provided by user.
+	remoteBytes, err := hex.DecodeString(testData.RemotePubkey)
+	require.NoError(t, err)
+	remoteKey, err := btcec.ParsePubKey(remoteBytes)
+	require.NoError(t, err)
+
+	// Example transaction hex from a real world channel.
+	txBytes, err := hex.DecodeString(testData.TransactionHex)
+	require.NoError(t, err)
+	var tx wire.MsgTx
+	require.NoError(t, tx.Deserialize(bytes.NewReader(txBytes)))
+
+	// Build a minimal Single with the remote payment basepoint.
+	makeSingle := func(version chanbackup.SingleBackupVersion,
+		initiator bool) chanbackup.Single {
+
+		s := chanbackup.Single{
+			Version:     version,
+			IsInitiator: initiator,
+		}
+		s.RemoteChanCfg.PaymentBasePoint = keychain.KeyDescriptor{
+			PubKey: remoteKey,
+		}
+
+		return s
+	}
+
+	// Try a set of plausible SCB versions and initiator roles to find
+	// a match.
+	versions := []chanbackup.SingleBackupVersion{
+		chanbackup.AnchorsCommitVersion,
+		chanbackup.AnchorsZeroFeeHtlcTxCommitVersion,
+		chanbackup.ScriptEnforcedLeaseVersion,
+		chanbackup.TweaklessCommitVersion,
+		chanbackup.DefaultSingleVersion,
+	}
+
+	found := false
+	var lastClass outputClassification
+	for _, v := range versions {
+		for _, initiator := range []bool{true, false} {
+			s := makeSingle(v, initiator)
+			class, err := classifyOutputs(s, &tx)
+			require.NoError(t, err)
+			if class.ToRemoteIdx >= 0 {
+				found = true
+				lastClass = class
+				t.Logf("Matched with version=%v initiator=%v",
+					v, initiator)
+
+				break
+			}
+		}
+		if found {
+			break
+		}
+	}
+
+	require.True(t, found, "to_remote output not identified for "+
+		"provided data")
+
+	// Log the results.
+	printOutputClassification(lastClass, &tx)
+}

cmd/chantools/testdata/scbforceclose_testdata.json

@@ -0,0 +1,5 @@
+{
+  "remotePubkey": "029e5f4d86d9d6c845fbcf37b09ac7d59c25c19932ab34a2757e8ea88437a876c3",
+  "transactionHex": "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"
+}
+