watchtower: introduce CommitmentType

In this commit a new enum, CommitmentType, is introduced and initially
there are 3 CommitmentTypes: Legacy, LegacyTweakless and Anchor.

Then, various methods are added to `CommitmentType`. This allows us to
remove a bunch of "if-else" chains from the `wtclient` and `lookout`
code. This will also make things easier to extend when a new commitment
type (like Taproot) is added.

watchtower/blob/commitments.go

@@ -0,0 +1,174 @@
+package blob
+
+import (
+	"fmt"
+
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/input"
+	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+// CommitmentType characterises the various properties of the breach commitment
+// transaction.
+type CommitmentType uint8
+
+const (
+	// LegacyCommitment represents a legacy commitment transaction where
+	// anchor outputs are not yet used and so the to_remote output is just
+	// a regular but tweaked P2WKH.
+	LegacyCommitment CommitmentType = iota
+
+	// LegacyTweaklessCommitment is similar to the LegacyCommitment with the
+	// added detail of the to_remote output not being tweaked.
+	LegacyTweaklessCommitment
+
+	// AnchorCommitment represents the commitment transaction of an
+	// anchor channel. The key differences are that the to_remote is
+	// encumbered by a 1 block CSV and so is thus a P2WSH output.
+	AnchorCommitment
+)
+
+// ToLocalInput constructs the input that will be used to spend the to_local
+// output.
+func (c CommitmentType) ToLocalInput(info *lnwallet.BreachRetribution) (
+	input.Input, error) {
+
+	witnessType, err := c.ToLocalWitnessType()
+	if err != nil {
+		return nil, err
+	}
+
+	return input.NewBaseInput(
+		&info.RemoteOutpoint, witnessType, info.RemoteOutputSignDesc, 0,
+	), nil
+}
+
+// ToRemoteInput constructs the input that will be used to spend the to_remote
+// output.
+func (c CommitmentType) ToRemoteInput(info *lnwallet.BreachRetribution) (
+	input.Input, error) {
+
+	witnessType, err := c.ToRemoteWitnessType()
+	if err != nil {
+		return nil, err
+	}
+
+	switch c {
+	case LegacyCommitment, LegacyTweaklessCommitment:
+		return input.NewBaseInput(
+			&info.LocalOutpoint, witnessType,
+			info.LocalOutputSignDesc, 0,
+		), nil
+
+	case AnchorCommitment:
+		// Anchor channels have a CSV-encumbered to-remote output. We'll
+		// construct a CSV input and assign the proper CSV delay of 1.
+		return input.NewCsvInput(
+			&info.LocalOutpoint, witnessType,
+			info.LocalOutputSignDesc, 0, 1,
+		), nil
+
+	default:
+		return nil, fmt.Errorf("unknown commitment type: %v", c)
+	}
+}
+
+// ToLocalWitnessType is the input type of the to_local output.
+func (c CommitmentType) ToLocalWitnessType() (input.WitnessType, error) {
+	switch c {
+	case LegacyTweaklessCommitment, LegacyCommitment, AnchorCommitment:
+		return input.CommitmentRevoke, nil
+
+	default:
+		return nil, fmt.Errorf("unknown commitment type: %v", c)
+	}
+}
+
+// ToRemoteWitnessType is the input type of the to_remote output.
+func (c CommitmentType) ToRemoteWitnessType() (input.WitnessType, error) {
+	switch c {
+	case LegacyTweaklessCommitment:
+		return input.CommitSpendNoDelayTweakless, nil
+
+	case LegacyCommitment:
+		return input.CommitmentNoDelay, nil
+
+	case AnchorCommitment:
+		return input.CommitmentToRemoteConfirmed, nil
+
+	default:
+		return nil, fmt.Errorf("unknown commitment type: %v", c)
+	}
+}
+
+// ToRemoteWitnessSize is the size of the witness that will be required to spend
+// the to_remote output.
+func (c CommitmentType) ToRemoteWitnessSize() (int, error) {
+	switch c {
+	// Legacy channels (both tweaked and non-tweaked) spend from P2WKH
+	// output.
+	case LegacyTweaklessCommitment, LegacyCommitment:
+		return input.P2WKHWitnessSize, nil
+
+	// Anchor channels spend a to-remote confirmed P2WSH output.
+	case AnchorCommitment:
+		return input.ToRemoteConfirmedWitnessSize, nil
+
+	default:
+		return 0, fmt.Errorf("unknown commitment type: %v", c)
+	}
+}
+
+// ToLocalWitnessSize is the size of the witness that will be required to spend
+// the to_local output.
+func (c CommitmentType) ToLocalWitnessSize() (int, error) {
+	switch c {
+	// An older ToLocalPenaltyWitnessSize constant used to underestimate the
+	// size by one byte. The difference in weight can cause different output
+	// values on the sweep transaction, so we mimic the original bug and
+	// create signatures using the original weight estimate.
+	case LegacyTweaklessCommitment, LegacyCommitment:
+		return input.ToLocalPenaltyWitnessSize - 1, nil
+
+	case AnchorCommitment:
+		return input.ToLocalPenaltyWitnessSize, nil
+
+	default:
+		return 0, fmt.Errorf("unknown commitment type: %v", c)
+	}
+}
+
+// ParseRawSig parses a wire.TxWitness and creates an lnwire.Sig.
+func (c CommitmentType) ParseRawSig(witness wire.TxWitness) (lnwire.Sig,
+	error) {
+
+	switch c {
+	case LegacyCommitment, LegacyTweaklessCommitment, AnchorCommitment:
+		// Check that the witness has at least one item.
+		if len(witness) < 1 {
+			return lnwire.Sig{}, fmt.Errorf("the witness should " +
+				"have at least one element")
+		}
+
+		// Check that the first witness element is non-nil. This is to
+		// ensure that the witness length check below does not panic.
+		if witness[0] == nil {
+			return lnwire.Sig{}, fmt.Errorf("the first witness " +
+				"element should not be nil")
+		}
+
+		// Parse the DER-encoded signature from the first position of
+		// the resulting witness. We trim an extra byte to remove the
+		// sighash flag.
+		rawSignature := witness[0][:len(witness[0])-1]
+
+		// Re-encode the DER signature into a fixed-size 64 byte
+		// signature.
+		return lnwire.NewSigFromECDSARawSignature(rawSignature)
+
+	default:
+		return lnwire.Sig{}, fmt.Errorf("unknown commitment type: %v",
+			c)
+	}
+}

watchtower/blob/type.go

@@ -3,6 +3,8 @@ package blob
 import (
 	"fmt"
 	"strings"
+
+	"github.com/lightningnetwork/lnd/channeldb"
 )
 
 // Flag represents a specify option that can be present in a Type.
@@ -81,6 +83,27 @@ func (t Type) Identifier() (string, error) {
 	}
 }
 
+// CommitmentType returns the appropriate CommitmentType for the given blob Type
+// and channel type.
+func (t Type) CommitmentType(chanType *channeldb.ChannelType) (CommitmentType,
+	error) {
+
+	switch {
+	case t.Has(FlagAnchorChannel):
+		return AnchorCommitment, nil
+
+	case t.Has(FlagCommitOutputs):
+		if chanType != nil && chanType.IsTweakless() {
+			return LegacyTweaklessCommitment, nil
+		}
+
+		return LegacyCommitment, nil
+
+	default:
+		return 0, ErrUnknownBlobType
+	}
+}
+
 // Has returns true if the Type has the passed flag enabled.
 func (t Type) Has(flag Flag) bool {
 	return Flag(t)&flag == flag