chantools/signer.go

package chantools

import (
	"fmt"

	"github.com/btcsuite/btcd/btcec"
	"github.com/btcsuite/btcd/txscript"
	"github.com/btcsuite/btcd/wire"
	"github.com/btcsuite/btcutil/hdkeychain"
	"github.com/lightningnetwork/lnd/input"
	"github.com/lightningnetwork/lnd/keychain"
)

type signer struct {
	extendedKey *hdkeychain.ExtendedKey
}

func (s *signer) SignOutputRaw(tx *wire.MsgTx,
	signDesc *input.SignDescriptor) ([]byte, error) {
	witnessScript := signDesc.WitnessScript

	// First attempt to fetch the private key which corresponds to the
	// specified public key.
	privKey, err := s.fetchPrivKey(&signDesc.KeyDesc)
	if err != nil {
		return nil, err
	}

	privKey = maybeTweakPrivKey(signDesc, privKey)
	amt := signDesc.Output.Value
	sig, err := txscript.RawTxInWitnessSignature(
		tx, signDesc.SigHashes, signDesc.InputIndex, amt,
		witnessScript, signDesc.HashType, privKey,
	)
	if err != nil {
		return nil, err
	}

	// Chop off the sighash flag at the end of the signature.
	return sig[:len(sig)-1], nil
}

func (s *signer) ComputeInputScript(tx *wire.MsgTx,
	signDesc *input.SignDescriptor) (*input.Script, error) {
	return nil, fmt.Errorf("unimplemented")
}

func (s *signer) fetchPrivKey(descriptor *keychain.KeyDescriptor) (
	*btcec.PrivateKey, error) {

	key, err := deriveChildren(s.extendedKey, []uint32{
		hardenedKeyStart + uint32(keychain.BIP0043Purpose),
		hardenedKeyStart + chainParams.HDCoinType,
		hardenedKeyStart + uint32(descriptor.Family),
		0,
		descriptor.Index,
	})
	if err != nil {
		return nil, err
	}
	return key.ECPrivKey()
}

// maybeTweakPrivKey examines the single tweak parameters on the passed sign
// descriptor and may perform a mapping on the passed private key in order to
// utilize the tweaks, if populated.
func maybeTweakPrivKey(signDesc *input.SignDescriptor,
	privKey *btcec.PrivateKey) *btcec.PrivateKey {

	if signDesc.SingleTweak != nil {
		return input.TweakPrivKey(privKey, signDesc.SingleTweak)
	}
	return privKey
}