lnd/lnwallet/btcwallet/psbt.go

package btcwallet

import (
	"bytes"
	"fmt"

	"github.com/btcsuite/btcd/btcec"
	"github.com/btcsuite/btcd/txscript"
	"github.com/btcsuite/btcutil"
	"github.com/btcsuite/btcutil/psbt"
	"github.com/btcsuite/btcwallet/waddrmgr"
	"github.com/lightningnetwork/lnd/input"
	"github.com/lightningnetwork/lnd/lnwallet"
	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
)

var (
	// PsbtKeyTypeInputSignatureTweakSingle is a custom/proprietary PSBT key
	// for an input that specifies what single tweak should be applied to
	// the key before signing the input. The value 51 is leet speak for
	// "si", short for "single".
	PsbtKeyTypeInputSignatureTweakSingle = []byte{0x51}

	// PsbtKeyTypeInputSignatureTweakDouble is a custom/proprietary PSBT key
	// for an input that specifies what double tweak should be applied to
	// the key before signing the input. The value d0 is leet speak for
	// "do", short for "double".
	PsbtKeyTypeInputSignatureTweakDouble = []byte{0xd0}
)

// FundPsbt creates a fully populated PSBT packet that contains enough inputs to
// fund the outputs specified in the passed in packet with the specified fee
// rate. If there is change left, a change output from the internal wallet is
// added and the index of the change output is returned. Otherwise no additional
// output is created and the index -1 is returned.
//
// NOTE: If the packet doesn't contain any inputs, coin selection is performed
// automatically. The account parameter must be non-empty as it determines which
// set of coins are eligible for coin selection. If the packet does contain any
// inputs, it is assumed that full coin selection happened externally and no
// additional inputs are added. If the specified inputs aren't enough to fund
// the outputs with the given fee rate, an error is returned. No lock lease is
// acquired for any of the selected/validated inputs. It is in the caller's
// responsibility to lock the inputs before handing them out.
//
// This is a part of the WalletController interface.
func (b *BtcWallet) FundPsbt(packet *psbt.Packet, minConfs int32,
	feeRate chainfee.SatPerKWeight, accountName string) (int32, error) {

	// The fee rate is passed in using units of sat/kw, so we'll convert
	// this to sat/KB as the CreateSimpleTx method requires this unit.
	feeSatPerKB := btcutil.Amount(feeRate.FeePerKVByte())

	var (
		keyScope   *waddrmgr.KeyScope
		accountNum uint32
	)
	switch accountName {
	// If the default/imported account name was specified, we'll provide a
	// nil key scope to FundPsbt, allowing it to select inputs from both key
	// scopes (NP2WKH, P2WKH).
	case lnwallet.DefaultAccountName:
		accountNum = defaultAccount

	case waddrmgr.ImportedAddrAccountName:
		accountNum = importedAccount

	// Otherwise, map the account name to its key scope and internal account
	// number to only select inputs from said account.
	default:
		scope, account, err := b.wallet.LookupAccount(accountName)
		if err != nil {
			return 0, err
		}
		keyScope = &scope
		accountNum = account
	}

	// Let the wallet handle coin selection and/or fee estimation based on
	// the partial TX information in the packet.
	return b.wallet.FundPsbt(
		packet, keyScope, minConfs, accountNum, feeSatPerKB,
		b.cfg.CoinSelectionStrategy,
	)
}

// SignPsbt expects a partial transaction with all inputs and outputs fully
// declared and tries to sign all unsigned inputs that have all required fields
// (UTXO information, BIP32 derivation information, witness or sig scripts) set.
// If no error is returned, the PSBT is ready to be given to the next signer or
// to be finalized if lnd was the last signer.
//
// NOTE: This method only signs inputs (and only those it can sign), it does not
// perform any other tasks (such as coin selection, UTXO locking or
// input/output/fee value validation, PSBT finalization). Any input that is
// incomplete will be skipped.
func (b *BtcWallet) SignPsbt(packet *psbt.Packet) error {
	// Let's check that this is actually something we can and want to sign.
	// We need at least one input and one output.
	err := psbt.VerifyInputOutputLen(packet, true, true)
	if err != nil {
		return err
	}

	// Go through each input that doesn't have final witness data attached
	// to it already and try to sign it. If there is nothing more to sign or
	// there are inputs that we don't know how to sign, we won't return any
	// error. So it's possible we're not the final signer.
	tx := packet.UnsignedTx
	sigHashes := txscript.NewTxSigHashes(tx)
	for idx := range tx.TxIn {
		in := packet.Inputs[idx]

		// We can only sign if we have UTXO information available. Since
		// we don't finalize, we just skip over any input that we know
		// we can't do anything with. Since we only support signing
		// witness inputs, we only look at the witness UTXO being set.
		if in.WitnessUtxo == nil {
			continue
		}

		// Skip this input if it's got final witness data attached.
		if len(in.FinalScriptWitness) > 0 {
			continue
		}

		// Skip this input if there is no BIP32 derivation info
		// available.
		if len(in.Bip32Derivation) == 0 {
			continue
		}

		// TODO(guggero): For multisig, we'll need to find out what key
		// to use and there should be multiple derivation paths in the
		// BIP32 derivation field.

		// Let's try and derive the key now. This method will decide if
		// it's a BIP49/84 key for normal on-chain funds or a key of the
		// custom purpose 1017 key scope.
		derivationInfo := in.Bip32Derivation[0]
		privKey, err := b.deriveKeyByBIP32Path(derivationInfo.Bip32Path)
		if err != nil {
			log.Warnf("SignPsbt: Skipping input %d, error "+
				"deriving signing key: %v", idx, err)
			continue
		}

		// We need to make sure we actually derived the key that was
		// expected to be derived.
		pubKeysEqual := bytes.Equal(
			derivationInfo.PubKey,
			privKey.PubKey().SerializeCompressed(),
		)
		if !pubKeysEqual {
			log.Warnf("SignPsbt: Skipping input %d, derived "+
				"public key %x does not match bip32 "+
				"derivation info public key %x", idx,
				privKey.PubKey().SerializeCompressed(),
				derivationInfo.PubKey)
			continue
		}

		// Do we need to tweak anything? Single or double tweaks are
		// sent as custom/proprietary fields in the PSBT input section.
		privKey = maybeTweakPrivKeyPsbt(in.Unknowns, privKey)
		pubKeyBytes := privKey.PubKey().SerializeCompressed()

		// Extract the correct witness and/or legacy scripts now,
		// depending on the type of input we sign. The txscript package
		// has the peculiar requirement that the PkScript of a P2PKH
		// must be given as the witness script in order for it to arrive
		// at the correct sighash. That's why we call it subScript here
		// instead of witness script.
		subScript, scriptSig, err := prepareScripts(in)
		if err != nil {
			// We derived the correct key so we _are_ expected to
			// sign this. Not being able to sign at this point means
			// there's something wrong.
			return fmt.Errorf("error deriving script for input "+
				"%d: %v", idx, err)
		}

		// We have everything we need for signing the input now.
		sig, err := txscript.RawTxInWitnessSignature(
			tx, sigHashes, idx, in.WitnessUtxo.Value, subScript,
			in.SighashType, privKey,
		)
		if err != nil {
			return fmt.Errorf("error signing input %d: %v", idx,
				err)
		}
		packet.Inputs[idx].FinalScriptSig = scriptSig
		packet.Inputs[idx].PartialSigs = append(
			packet.Inputs[idx].PartialSigs, &psbt.PartialSig{
				PubKey:    pubKeyBytes,
				Signature: sig,
			},
		)
	}

	return nil
}

// prepareScripts returns the appropriate witness and/or legacy scripts,
// depending on the type of input that should be signed.
func prepareScripts(in psbt.PInput) ([]byte, []byte, error) {
	switch {
	// It's a NP2WKH input:
	case len(in.RedeemScript) > 0:
		builder := txscript.NewScriptBuilder()
		builder.AddData(in.RedeemScript)
		sigScript, err := builder.Script()
		if err != nil {
			return nil, nil, fmt.Errorf("error building np2wkh "+
				"script: %v", err)
		}

		return in.RedeemScript, sigScript, nil

	// It's a P2WSH input:
	case len(in.WitnessScript) > 0:
		return in.WitnessScript, nil, nil

	// It's a P2WKH input:
	default:
		return in.WitnessUtxo.PkScript, nil, nil
	}
}

// maybeTweakPrivKeyPsbt examines if there are any tweak parameters given in the
// custom/proprietary PSBT fields and may perform a mapping on the passed
// private key in order to utilize the tweaks, if populated.
func maybeTweakPrivKeyPsbt(unknowns []*psbt.Unknown,
	privKey *btcec.PrivateKey) *btcec.PrivateKey {

	// There can be other custom/unknown keys in a PSBT that we just ignore.
	// Key tweaking is optional and only one tweak (single _or_ double) can
	// ever be applied (at least for any use cases described in the BOLT
	// spec).
	for _, u := range unknowns {
		if bytes.Equal(u.Key, PsbtKeyTypeInputSignatureTweakSingle) {
			return input.TweakPrivKey(privKey, u.Value)
		}

		if bytes.Equal(u.Key, PsbtKeyTypeInputSignatureTweakDouble) {
			doubleTweakKey, _ := btcec.PrivKeyFromBytes(
				btcec.S256(), u.Value,
			)
			return input.DeriveRevocationPrivKey(
				privKey, doubleTweakKey,
			)
		}
	}

	return privKey
}

// FinalizePsbt expects a partial transaction with all inputs and outputs fully
// declared and tries to sign all inputs that belong to the specified account.
// Lnd must be the last signer of the transaction. That means, if there are any
// unsigned non-witness inputs or inputs without UTXO information attached or
// inputs without witness data that do not belong to lnd's wallet, this method
// will fail. If no error is returned, the PSBT is ready to be extracted and the
// final TX within to be broadcast.
//
// NOTE: This method does NOT publish the transaction after it's been
// finalized successfully.
//
// This is a part of the WalletController interface.
func (b *BtcWallet) FinalizePsbt(packet *psbt.Packet, accountName string) error {
	var (
		keyScope   *waddrmgr.KeyScope
		accountNum uint32
	)
	switch accountName {
	// If the default/imported account name was specified, we'll provide a
	// nil key scope to FundPsbt, allowing it to sign inputs from both key
	// scopes (NP2WKH, P2WKH).
	case lnwallet.DefaultAccountName:
		accountNum = defaultAccount

	case waddrmgr.ImportedAddrAccountName:
		accountNum = importedAccount

	// Otherwise, map the account name to its key scope and internal account
	// number to determine if the inputs belonging to this account should be
	// signed.
	default:
		scope, account, err := b.wallet.LookupAccount(accountName)
		if err != nil {
			return err
		}
		keyScope = &scope
		accountNum = account
	}

	return b.wallet.FinalizePsbt(keyScope, accountNum, packet)
}