lnwallet+lnrpc: add derivation info for P2TR change addrs

In some situations (for example in Taproot Assets), we need to be able
to prove that an address is a bare BIP-0086 address that doesn't commit
to any script. We can do that by providing the BIP-0032 derivation info
and internal key.

lnrpc/walletrpc/walletkit_server.go

@@ -1758,6 +1758,34 @@ func (w *WalletKit) handleChange(packet *psbt.Packet, changeIndex int32,
 		return 0, fmt.Errorf("could not derive change script: %w", err)
 	}
 
+	// We need to add the derivation info for the change address in case it
+	// is a P2TR address. This is mostly to prove it's a bare BIP-0086
+	// address, which is required for some protocols (such as Taproot
+	// Assets).
+	pOut := psbt.POutput{}
+	_, isTaprootChangeAddr := changeAddr.(*btcutil.AddressTaproot)
+	if isTaprootChangeAddr {
+		changeAddrInfo, err := w.cfg.Wallet.AddressInfo(changeAddr)
+		if err != nil {
+			return 0, fmt.Errorf("could not get address info: %w",
+				err)
+		}
+
+		deriv, trDeriv, _, err := btcwallet.Bip32DerivationFromAddress(
+			changeAddrInfo,
+		)
+		if err != nil {
+			return 0, fmt.Errorf("could not get derivation info: "+
+				"%w", err)
+		}
+
+		pOut.TaprootInternalKey = trDeriv.XOnlyPubKey
+		pOut.Bip32Derivation = []*psbt.Bip32Derivation{deriv}
+		pOut.TaprootBip32Derivation = []*psbt.TaprootBip32Derivation{
+			trDeriv,
+		}
+	}
+
 	newChangeIndex := int32(len(packet.Outputs))
 	packet.UnsignedTx.TxOut = append(
 		packet.UnsignedTx.TxOut, &wire.TxOut{
@@ -1765,7 +1793,7 @@ func (w *WalletKit) handleChange(packet *psbt.Packet, changeIndex int32,
 			PkScript: changeScript,
 		},
 	)
-	packet.Outputs = append(packet.Outputs, psbt.POutput{})
+	packet.Outputs = append(packet.Outputs, pOut)
 
 	return newChangeIndex, nil
 }

lnwallet/btcwallet/psbt.go

@@ -9,12 +9,14 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcec/v2/schnorr"
 	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/btcutil/hdkeychain"
 	"github.com/btcsuite/btcd/btcutil/psbt"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcwallet/waddrmgr"
 	"github.com/btcsuite/btcwallet/wallet"
 	"github.com/lightningnetwork/lnd/input"
+	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 )
@@ -640,3 +642,78 @@ func (b *BtcWallet) lookupFirstCustomAccount(
 
 	return keyScope, account.AccountNumber, nil
 }
+
+// Bip32DerivationFromKeyDesc returns the default and Taproot BIP-0032 key
+// derivation information from the given key descriptor information.
+func Bip32DerivationFromKeyDesc(keyDesc keychain.KeyDescriptor,
+	coinType uint32) (*psbt.Bip32Derivation, *psbt.TaprootBip32Derivation,
+	string) {
+
+	bip32Derivation := &psbt.Bip32Derivation{
+		PubKey: keyDesc.PubKey.SerializeCompressed(),
+		Bip32Path: []uint32{
+			keychain.BIP0043Purpose + hdkeychain.HardenedKeyStart,
+			coinType + hdkeychain.HardenedKeyStart,
+			uint32(keyDesc.Family) +
+				uint32(hdkeychain.HardenedKeyStart),
+			0,
+			keyDesc.Index,
+		},
+	}
+
+	derivationPath := fmt.Sprintf(
+		"m/%d'/%d'/%d'/%d/%d", keychain.BIP0043Purpose, coinType,
+		keyDesc.Family, 0, keyDesc.Index,
+	)
+
+	return bip32Derivation, &psbt.TaprootBip32Derivation{
+		XOnlyPubKey:          bip32Derivation.PubKey[1:],
+		MasterKeyFingerprint: bip32Derivation.MasterKeyFingerprint,
+		Bip32Path:            bip32Derivation.Bip32Path,
+		LeafHashes:           make([][]byte, 0),
+	}, derivationPath
+}
+
+// Bip32DerivationFromAddress returns the default and Taproot BIP-0032 key
+// derivation information from the given managed address.
+func Bip32DerivationFromAddress(
+	addr waddrmgr.ManagedAddress) (*psbt.Bip32Derivation,
+	*psbt.TaprootBip32Derivation, string, error) {
+
+	pubKeyAddr, ok := addr.(waddrmgr.ManagedPubKeyAddress)
+	if !ok {
+		return nil, nil, "", fmt.Errorf("address is not a pubkey " +
+			"address")
+	}
+
+	scope, derivationInfo, haveInfo := pubKeyAddr.DerivationInfo()
+	if !haveInfo {
+		return nil, nil, "", fmt.Errorf("address is an imported " +
+			"public key, can't derive BIP32 path")
+	}
+
+	bip32Derivation := &psbt.Bip32Derivation{
+		PubKey: pubKeyAddr.PubKey().SerializeCompressed(),
+		Bip32Path: []uint32{
+			scope.Purpose + hdkeychain.HardenedKeyStart,
+			scope.Coin + hdkeychain.HardenedKeyStart,
+			derivationInfo.InternalAccount +
+				hdkeychain.HardenedKeyStart,
+			derivationInfo.Branch,
+			derivationInfo.Index,
+		},
+	}
+
+	derivationPath := fmt.Sprintf(
+		"m/%d'/%d'/%d'/%d/%d", scope.Purpose, scope.Coin,
+		derivationInfo.InternalAccount, derivationInfo.Branch,
+		derivationInfo.Index,
+	)
+
+	return bip32Derivation, &psbt.TaprootBip32Derivation{
+		XOnlyPubKey:          bip32Derivation.PubKey[1:],
+		MasterKeyFingerprint: bip32Derivation.MasterKeyFingerprint,
+		Bip32Path:            bip32Derivation.Bip32Path,
+		LeafHashes:           make([][]byte, 0),
+	}, derivationPath, nil
+}

lnwallet/btcwallet/psbt_test.go

@@ -17,6 +17,7 @@ import (
 	"github.com/btcsuite/btcwallet/waddrmgr"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
+	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/stretchr/testify/require"
 )
 
@@ -495,3 +496,136 @@ func TestEstimateInputWeight(t *testing.T) {
 		})
 	}
 }
+
+// TestBip32DerivationFromKeyDesc tests that we can correctly extract a BIP32
+// derivation path from a key descriptor.
+func TestBip32DerivationFromKeyDesc(t *testing.T) {
+	privKey, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+
+	testCases := []struct {
+		name              string
+		keyDesc           keychain.KeyDescriptor
+		coinType          uint32
+		expectedPath      string
+		expectedBip32Path []uint32
+	}{
+		{
+			name: "testnet multi-sig family",
+			keyDesc: keychain.KeyDescriptor{
+				PubKey: privKey.PubKey(),
+				KeyLocator: keychain.KeyLocator{
+					Family: keychain.KeyFamilyMultiSig,
+					Index:  123,
+				},
+			},
+			coinType:     chaincfg.TestNet3Params.HDCoinType,
+			expectedPath: "m/1017'/1'/0'/0/123",
+			expectedBip32Path: []uint32{
+				hardenedKey(keychain.BIP0043Purpose),
+				hardenedKey(chaincfg.TestNet3Params.HDCoinType),
+				hardenedKey(uint32(keychain.KeyFamilyMultiSig)),
+				0, 123,
+			},
+		},
+		{
+			name: "mainnet watchtower family",
+			keyDesc: keychain.KeyDescriptor{
+				PubKey: privKey.PubKey(),
+				KeyLocator: keychain.KeyLocator{
+					Family: keychain.KeyFamilyTowerSession,
+					Index:  456,
+				},
+			},
+			coinType:     chaincfg.MainNetParams.HDCoinType,
+			expectedPath: "m/1017'/0'/8'/0/456",
+			expectedBip32Path: []uint32{
+				hardenedKey(keychain.BIP0043Purpose),
+				hardenedKey(chaincfg.MainNetParams.HDCoinType),
+				hardenedKey(
+					uint32(keychain.KeyFamilyTowerSession),
+				),
+				0, 456,
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		tc := tc
+
+		t.Run(tc.name, func(tt *testing.T) {
+			d, trD, path := Bip32DerivationFromKeyDesc(
+				tc.keyDesc, tc.coinType,
+			)
+			require.NoError(tt, err)
+
+			require.Equal(tt, tc.expectedPath, path)
+			require.Equal(tt, tc.expectedBip32Path, d.Bip32Path)
+			require.Equal(tt, tc.expectedBip32Path, trD.Bip32Path)
+
+			serializedKey := tc.keyDesc.PubKey.SerializeCompressed()
+			require.Equal(tt, serializedKey, d.PubKey)
+			require.Equal(tt, serializedKey[1:], trD.XOnlyPubKey)
+		})
+	}
+}
+
+// TestBip32DerivationFromAddress tests that we can correctly extract a BIP32
+// derivation path from an address.
+func TestBip32DerivationFromAddress(t *testing.T) {
+	testCases := []struct {
+		name              string
+		addrType          lnwallet.AddressType
+		expectedAddr      string
+		expectedPath      string
+		expectedBip32Path []uint32
+		expectedPubKey    string
+	}{
+		{
+			name:         "p2wkh",
+			addrType:     lnwallet.WitnessPubKey,
+			expectedAddr: firstAddress,
+			expectedPath: "m/84'/0'/0'/0/0",
+			expectedBip32Path: []uint32{
+				hardenedKey(waddrmgr.KeyScopeBIP0084.Purpose),
+				hardenedKey(0), hardenedKey(0), 0, 0,
+			},
+			expectedPubKey: firstAddressPubKey,
+		},
+		{
+			name:         "p2tr",
+			addrType:     lnwallet.TaprootPubkey,
+			expectedAddr: firstAddressTaproot,
+			expectedPath: "m/86'/0'/0'/0/0",
+			expectedBip32Path: []uint32{
+				hardenedKey(waddrmgr.KeyScopeBIP0086.Purpose),
+				hardenedKey(0), hardenedKey(0), 0, 0,
+			},
+			expectedPubKey: firstAddressTaprootPubKey,
+		},
+	}
+
+	w, _ := newTestWallet(t, netParams, seedBytes)
+	for _, tc := range testCases {
+		tc := tc
+
+		addr, err := w.NewAddress(
+			tc.addrType, false, lnwallet.DefaultAccountName,
+		)
+		require.NoError(t, err)
+
+		require.Equal(t, tc.expectedAddr, addr.String())
+
+		addrInfo, err := w.AddressInfo(addr)
+		require.NoError(t, err)
+		managedAddr, ok := addrInfo.(waddrmgr.ManagedPubKeyAddress)
+		require.True(t, ok)
+
+		d, trD, path, err := Bip32DerivationFromAddress(managedAddr)
+		require.NoError(t, err)
+
+		require.Equal(t, tc.expectedPath, path)
+		require.Equal(t, tc.expectedBip32Path, d.Bip32Path)
+		require.Equal(t, tc.expectedBip32Path, trD.Bip32Path)
+	}
+}

lnwallet/btcwallet/signer_test.go

@@ -38,11 +38,21 @@ var (
 	// which is a special case for the BIP49/84 addresses in btcwallet).
 	firstAddress = "bcrt1qgdlgjc5ede7fjv350wcjqat80m0zsmfaswsj9p"
 
+	// firstAddressPubKey is the public key of the first address that we
+	// should get from the wallet.
+	firstAddressPubKey = "02b844aecf8250c29e46894147a7dae02de55a034a533b6" +
+		"0c6a6469294ee356ce4"
+
 	// firstAddressTaproot is the first address that we should get from the
 	// wallet when deriving a taproot address.
 	firstAddressTaproot = "bcrt1ps8c222fgysvnsj2m8hxk8khy6wthcrhv9va9z3t4" +
 		"h3qeyz65sh4qqwvdgc"
 
+	// firstAddressTaprootPubKey is the public key of the first address that
+	// we should get from the wallet when deriving a taproot address.
+	firstAddressTaprootPubKey = "03004113d6185c955d6e8f5922b50cc0ac3b64fa" +
+		"0979402604c5b887f07e3b5388"
+
 	testPubKeyBytes, _ = hex.DecodeString(
 		"037a67771635344641d4b56aac33cd5f7a265b59678dce3aec31b89125e3" +
 			"b8b9b2",