lnwallet: extract SignDescriptor serialization and witness generator

lnwallet/interface.go

@@ -7,7 +7,6 @@ import (
 
 	"github.com/roasbeef/btcd/btcec"
 	"github.com/roasbeef/btcd/chaincfg/chainhash"
-	"github.com/roasbeef/btcd/txscript"
 	"github.com/roasbeef/btcd/wire"
 	"github.com/roasbeef/btcutil"
 )

lnwallet/signdescriptor.go

@@ -0,0 +1,134 @@
+package lnwallet
+
+import (
+	"encoding/binary"
+	"io"
+
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/roasbeef/btcd/btcec"
+	"github.com/roasbeef/btcd/txscript"
+	"github.com/roasbeef/btcd/wire"
+)
+
+// SignDescriptor houses the necessary information required to successfully sign
+// a given output. This struct is used by the Signer interface in order to gain
+// access to critical data needed to generate a valid signature.
+type SignDescriptor struct {
+	// Pubkey is the public key to which the signature should be generated
+	// over. The Signer should then generate a signature with the private
+	// key corresponding to this public key.
+	PubKey *btcec.PublicKey
+
+	// PrivateTweak is a scalar value that should be added to the private
+	// key corresponding to the above public key to obtain the private key
+	// to be used to sign this input. This value is typically a leaf node
+	// from the revocation tree.
+	//
+	// NOTE: If this value is nil, then the input can be signed using only
+	// the above public key.
+	PrivateTweak []byte
+
+	// WitnessScript is the full script required to properly redeem the
+	// output. This field will only be populated if a p2wsh or a p2sh
+	// output is being signed.
+	WitnessScript []byte
+
+	// Output is the target output which should be signed. The PkScript and
+	// Value fields within the output should be properly populated,
+	// otherwise an invalid signature may be generated.
+	Output *wire.TxOut
+
+	// HashType is the target sighash type that should be used when
+	// generating the final sighash, and signature.
+	HashType txscript.SigHashType
+
+	// SigHashes is the pre-computed sighash midstate to be used when
+	// generating the final sighash for signing.
+	SigHashes *txscript.TxSigHashes
+
+	// InputIndex is the target input within the transaction that should be
+	// signed.
+	InputIndex int
+}
+
+// WriteSignDescriptor serializes a SignDescriptor struct into the passed
+// io.Writer stream.
+// NOTE: We assume the SigHashes and InputIndex fields haven't been assigned
+// yet, since that is usually done just before broadcast by the witness
+// generator.
+func WriteSignDescriptor(w io.Writer, sd *SignDescriptor) error {
+	serializedPubKey := sd.PubKey.SerializeCompressed()
+	if err := wire.WriteVarBytes(w, 0, serializedPubKey); err != nil {
+		return err
+	}
+
+	if err := wire.WriteVarBytes(w, 0, sd.PrivateTweak); err != nil {
+		return err
+	}
+
+	if err := wire.WriteVarBytes(w, 0, sd.WitnessScript); err != nil {
+		return err
+	}
+
+	if err := lnwire.WriteTxOut(w, sd.Output); err != nil {
+		return err
+	}
+
+	var scratch [4]byte
+	binary.BigEndian.PutUint32(scratch[:], uint32(sd.HashType))
+	if _, err := w.Write(scratch[:]); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// ReadSignDescriptor deserializes a SignDescriptor struct from the passed
+// io.Reader stream.
+func ReadSignDescriptor(r io.Reader, sd *SignDescriptor) error {
+
+	pubKeyBytes, err := wire.ReadVarBytes(r, 0, 34, "pubkey")
+	if err != nil {
+		return err
+	}
+	sd.PubKey, err = btcec.ParsePubKey(pubKeyBytes, btcec.S256())
+	if err != nil {
+		return err
+	}
+
+	privateTweak, err := wire.ReadVarBytes(r, 0, 32, "privateTweak")
+	if err != nil {
+		return err
+	}
+
+	// Serializing a SignDescriptor with a nil-valued PrivateTweak results in
+	// deserializing a zero-length slice. Since a nil-valued PrivateTweak has
+	// special meaning and a zero-length slice for a PrivateTweak is invalid,
+	// we can use the zero-length slice as the flag for a nil-valued
+	// PrivateTweak.
+	if len(privateTweak) == 0 {
+		sd.PrivateTweak = nil
+	} else {
+		sd.PrivateTweak = privateTweak
+	}
+
+	witnessScript, err := wire.ReadVarBytes(r, 0, 100, "witnessScript")
+	if err != nil {
+		return err
+	}
+	sd.WitnessScript = witnessScript
+
+	txOut := &wire.TxOut{}
+	if err := lnwire.ReadTxOut(r, txOut); err != nil {
+		return err
+	}
+	sd.Output = txOut
+
+	var hashType [4]byte
+	if _, err := io.ReadFull(r, hashType[:]); err != nil {
+		return err
+	}
+	sd.HashType = txscript.SigHashType(binary.BigEndian.Uint32(hashType[:]))
+
+	return nil
+}

lnwallet/signdescriptor_test.go

@@ -0,0 +1,122 @@
+package lnwallet
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+
+	"github.com/roasbeef/btcd/btcec"
+	"github.com/roasbeef/btcd/txscript"
+	"github.com/roasbeef/btcd/wire"
+)
+
+func TestSignDescriptorSerialization(t *testing.T) {
+	keys := [][]byte{
+		{0x04, 0x11, 0xdb, 0x93, 0xe1, 0xdc, 0xdb, 0x8a,
+			0x01, 0x6b, 0x49, 0x84, 0x0f, 0x8c, 0x53, 0xbc, 0x1e,
+			0xb6, 0x8a, 0x38, 0x2e, 0x97, 0xb1, 0x48, 0x2e, 0xca,
+			0xd7, 0xb1, 0x48, 0xa6, 0x90, 0x9a, 0x5c, 0xb2, 0xe0,
+			0xea, 0xdd, 0xfb, 0x84, 0xcc, 0xf9, 0x74, 0x44, 0x64,
+			0xf8, 0x2e, 0x16, 0x0b, 0xfa, 0x9b, 0x8b, 0x64, 0xf9,
+			0xd4, 0xc0, 0x3f, 0x99, 0x9b, 0x86, 0x43, 0xf6, 0x56,
+			0xb4, 0x12, 0xa3,
+		},
+		{0x04, 0x11, 0xdb, 0x93, 0xe1, 0xdc, 0xdb, 0x8a,
+			0x01, 0x6b, 0x49, 0x84, 0x0f, 0x8c, 0x53, 0xbc, 0x1e,
+			0xb6, 0x8a, 0x38, 0x2e, 0x97, 0xb1, 0x48, 0x2e, 0xca,
+			0xd7, 0xb1, 0x48, 0xa6, 0x90, 0x9a, 0x5c, 0xb2, 0xe0,
+			0xea, 0xdd, 0xfb, 0x84, 0xcc, 0xf9, 0x74, 0x44, 0x64,
+			0xf8, 0x2e, 0x16, 0x0b, 0xfa, 0x9b, 0x8b, 0x64, 0xf9,
+			0xd4, 0xc0, 0x3f, 0x99, 0x9b, 0x86, 0x43, 0xf6, 0x56,
+			0xb4, 0x12, 0xa3,
+		},
+	}
+
+	signDescriptors := []SignDescriptor{
+		{
+			PrivateTweak: []byte{
+				0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02,
+				0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02,
+				0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02,
+				0x02, 0x02, 0x02, 0x02, 0x02,
+			},
+			WitnessScript: []byte{
+				0x00, 0x14, 0xee, 0x91, 0x41, 0x7e, 0x85, 0x6c, 0xde,
+				0x10, 0xa2, 0x91, 0x1e, 0xdc, 0xbd, 0xbd, 0x69, 0xe2,
+				0xef, 0xb5, 0x71, 0x48,
+			},
+			Output: &wire.TxOut{
+				Value: 5000000000,
+				PkScript: []byte{
+					0x41, // OP_DATA_65
+					0x04, 0xd6, 0x4b, 0xdf, 0xd0, 0x9e, 0xb1, 0xc5,
+					0xfe, 0x29, 0x5a, 0xbd, 0xeb, 0x1d, 0xca, 0x42,
+					0x81, 0xbe, 0x98, 0x8e, 0x2d, 0xa0, 0xb6, 0xc1,
+					0xc6, 0xa5, 0x9d, 0xc2, 0x26, 0xc2, 0x86, 0x24,
+					0xe1, 0x81, 0x75, 0xe8, 0x51, 0xc9, 0x6b, 0x97,
+					0x3d, 0x81, 0xb0, 0x1c, 0xc3, 0x1f, 0x04, 0x78,
+					0x34, 0xbc, 0x06, 0xd6, 0xd6, 0xed, 0xf6, 0x20,
+					0xd1, 0x84, 0x24, 0x1a, 0x6a, 0xed, 0x8b, 0x63,
+					0xa6, // 65-byte signature
+					0xac, // OP_CHECKSIG
+				},
+			},
+			HashType: txscript.SigHashAll,
+		},
+
+		// Test serializing a SignDescriptor with a nil-valued PrivateTweak
+		{
+			PrivateTweak: nil,
+			WitnessScript: []byte{
+				0x00, 0x14, 0xee, 0x91, 0x41, 0x7e, 0x85, 0x6c, 0xde,
+				0x10, 0xa2, 0x91, 0x1e, 0xdc, 0xbd, 0xbd, 0x69, 0xe2,
+				0xef, 0xb5, 0x71, 0x48,
+			},
+			Output: &wire.TxOut{
+				Value: 5000000000,
+				PkScript: []byte{
+					0x41, // OP_DATA_65
+					0x04, 0xd6, 0x4b, 0xdf, 0xd0, 0x9e, 0xb1, 0xc5,
+					0xfe, 0x29, 0x5a, 0xbd, 0xeb, 0x1d, 0xca, 0x42,
+					0x81, 0xbe, 0x98, 0x8e, 0x2d, 0xa0, 0xb6, 0xc1,
+					0xc6, 0xa5, 0x9d, 0xc2, 0x26, 0xc2, 0x86, 0x24,
+					0xe1, 0x81, 0x75, 0xe8, 0x51, 0xc9, 0x6b, 0x97,
+					0x3d, 0x81, 0xb0, 0x1c, 0xc3, 0x1f, 0x04, 0x78,
+					0x34, 0xbc, 0x06, 0xd6, 0xd6, 0xed, 0xf6, 0x20,
+					0xd1, 0x84, 0x24, 0x1a, 0x6a, 0xed, 0x8b, 0x63,
+					0xa6, // 65-byte signature
+					0xac, // OP_CHECKSIG
+				},
+			},
+			HashType: txscript.SigHashAll,
+		},
+	}
+
+	for i := 0; i < len(signDescriptors); i++ {
+		// Parse pubkeys for each sign descriptor.
+		sd := &signDescriptors[i]
+		pubkey, err := btcec.ParsePubKey(keys[i], btcec.S256())
+		if err != nil {
+			t.Fatalf("unable to parse pubkey: %v", err)
+		}
+		sd.PubKey = pubkey
+
+		// Test that serialize -> deserialize yields same result as original.
+		var buf bytes.Buffer
+		if err := WriteSignDescriptor(&buf, sd); err != nil {
+			t.Fatalf("unable to serialize sign descriptor[%v]: %v", i, sd)
+		}
+
+		desSd := &SignDescriptor{}
+		if err := ReadSignDescriptor(&buf, desSd); err != nil {
+			t.Fatalf("unable to deserialize sign descriptor[%v]: %v", i, sd)
+		}
+
+		if !reflect.DeepEqual(sd, desSd) {
+			t.Fatalf("original and deserialized sign descriptors not equal:\n"+
+				"original     : %+v\n"+
+				"deserialized : %+v\n",
+				sd, desSd)
+		}
+	}
+}

lnwallet/witnessgen.go

@@ -0,0 +1,59 @@
+package lnwallet
+
+import (
+	"fmt"
+
+	"github.com/roasbeef/btcd/txscript"
+	"github.com/roasbeef/btcd/wire"
+)
+
+// WitnessType determines how an output's witness will be generated. The
+// default commitmentTimeLock type will generate a witness that will allow
+// spending of a time-locked transaction enforced by CheckSequenceVerify.
+type WitnessType uint16
+
+const (
+	// Witness that allows us to spend the output of a commitment transaction
+	// after a relative lock-time lockout.
+	CommitmentTimeLock WitnessType = 0
+
+	// Witness that allows us to spend a settled no-delay output immediately on
+	// a counterparty's commitment transaction.
+	CommitmentNoDelay WitnessType = 1
+
+	// Witness that allows us to sweep the settled output of a malicious
+	// counterparty's who broadcasts a revoked commitment transaction.
+	CommitmentRevoke WitnessType = 2
+)
+
+// WitnessGenerator represents a function which is able to generate the final
+// witness for a particular public key script. This function acts as an
+// abstraction layer, hiding the details of the underlying script.
+type WitnessGenerator func(tx *wire.MsgTx, hc *txscript.TxSigHashes,
+	inputIndex int) ([][]byte, error)
+
+// GenWitnessFunc will return a WitnessGenerator function that an output
+// uses to generate the witness for a sweep transaction.
+func (wt WitnessType) GenWitnessFunc(signer *Signer,
+	descriptor *SignDescriptor) WitnessGenerator {
+
+	return func(tx *wire.MsgTx, hc *txscript.TxSigHashes,
+		inputIndex int) ([][]byte, error) {
+
+		desc := descriptor
+		desc.SigHashes = hc
+		desc.InputIndex = inputIndex
+
+		switch wt {
+		case CommitmentTimeLock:
+			return CommitSpendTimeout(*signer, desc, tx)
+		case CommitmentNoDelay:
+			return CommitSpendNoDelay(*signer, desc, tx)
+		case CommitmentRevoke:
+			return CommitSpendRevoke(*signer, desc, tx)
+		default:
+			return nil, fmt.Errorf("unknown witness type: %v", wt)
+		}
+	}
+
+}