Add derivekey command

README.md

@@ -51,6 +51,7 @@ Help Options:
   -h, --help             Show this help message
 
 Available commands:
+  derivekey      Derive a key with a specific derivation path from the BIP32 HD root key.
   dumpbackup     Dump the content of a channel.backup file.
   dumpchannels   Dump all channel information from lnd's channel database.
   forceclose     Force-close the last state that is in the channel.db provided.
@@ -62,6 +63,29 @@ Available commands:
 
 ## Commands
 
+### derivekey
+
+```text
+Usage:
+  chantools [OPTIONS] derivekey [derivekey-OPTIONS]
+
+[derivekey command options]
+          --rootkey=     BIP32 HD root key to derive the key from.
+          --path=        The BIP32 derivation path to derive. Must start with "m/".
+          --neuter       Do not output the private key, just the public key.
+```
+
+This command derives a single key with the given BIP32 derivation path from the
+root key and prints it to the console. Make sure to escape apostrophes in the
+derivation path.
+
+Example command:
+
+```bash
+chantools derivekey --rootkey xprvxxxxxxxxxx --path m/1017\'/0\'/5\'/0/0 \
+  --neuter
+```
+
 ### dumpbackup
 
 ```text

cmd_derivekey.go

@@ -0,0 +1,42 @@
+package chantools
+
+import (
+	"fmt"
+	"github.com/btcsuite/btcutil"
+	"github.com/btcsuite/btcutil/hdkeychain"
+)
+
+func deriveKey(extendedKey *hdkeychain.ExtendedKey, path string,
+	neuter bool) error {
+
+	fmt.Printf("Deriving path %s for network %s.\n", path, chainParams.Name)
+	parsedPath, err := parsePath(path)
+	if err != nil {
+		return fmt.Errorf("could not parse derivation path: %v", err)
+	}
+	derivedKey, err := deriveChildren(extendedKey, parsedPath)
+	if err != nil {
+		return fmt.Errorf("could not derive children: %v", err)
+	}
+	pubKey, err := derivedKey.ECPubKey()
+	if err != nil {
+		return fmt.Errorf("could not derive public key: %v", err)
+	}
+	fmt.Printf("Public key: %x\n", pubKey.SerializeCompressed())
+
+	if neuter {
+		return nil
+	}
+
+	privKey, err := derivedKey.ECPrivKey()
+	if err != nil {
+		return fmt.Errorf("could not derive private key: %v", err)
+	}
+	wif, err := btcutil.NewWIF(privKey, chainParams, true)
+	if err != nil {
+		return fmt.Errorf("could not encode WIF: %v", err)
+	}
+	fmt.Printf("Private key (WIF): %s\n", wif.String())
+
+	return nil
+}

hdkeychain.go

@@ -1,9 +1,12 @@
 package chantools
 
 import (
+	"fmt"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcutil/hdkeychain"
 	"github.com/lightningnetwork/lnd/keychain"
+	"strconv"
+	"strings"
 )
 
 const (
@@ -26,6 +29,33 @@ func deriveChildren(key *hdkeychain.ExtendedKey, path []uint32) (
 	return currentKey, nil
 }
 
+func parsePath(path string) ([]uint32, error) {
+	path = strings.TrimSpace(path)
+	if len(path) == 0 {
+		return nil, fmt.Errorf("path cannot be empty")
+	}
+	if !strings.HasPrefix(path, "m/") {
+		return nil, fmt.Errorf("path must start with m/")
+	}
+	parts := strings.Split(path, "/")
+	indices := make([]uint32, len(parts)-1)
+	for i := 1; i < len(parts); i++ {
+		index := uint32(0)
+		part := parts[i]
+		if strings.Contains(parts[i], "'") {
+			index += hardenedKeyStart
+			part = strings.TrimRight(parts[i], "'")
+		}
+		parsed, err := strconv.Atoi(part)
+		if err != nil {
+			return nil, fmt.Errorf("could not parse part \"%s\": "+
+				"%v", part, err)
+		}
+		indices[i-1] = index + uint32(parsed)
+	}
+	return indices, nil
+}
+
 type channelBackupEncryptionRing struct {
 	extendedKey *hdkeychain.ExtendedKey
 	chainParams *chaincfg.Params

main.go

@@ -74,6 +74,10 @@ func Main() error {
 		"dumpbackup", "Dump the content of a channel.backup file.", "",
 		&dumpBackupCommand{},
 	)
+	_, _ = parser.AddCommand(
+		"derivekey", "Derive a key with a specific derivation path "+
+			"from the BIP32 HD root key.", "", &deriveKeyCommand{},
+	)
 
 	_, err := parser.Parse()
 	return err
@@ -293,6 +297,27 @@ func (c *dumpBackupCommand) Execute(_ []string) error {
 	return dumpChannelBackup(multi)
 }
 
+type deriveKeyCommand struct {
+	RootKey string `long:"rootkey" description:"BIP32 HD root key to derive the key from."`
+	Path    string `long:"path" description:"The BIP32 derivation path to derive. Must start with \"m/\"."`
+	Neuter  bool   `long:"neuter" description:"Do not output the private key, just the public key."`
+}
+
+func (c *deriveKeyCommand) Execute(_ []string) error {
+	setupChainParams(cfg)
+
+	// Check that root key is valid.
+	if c.RootKey == "" {
+		return fmt.Errorf("root key is required")
+	}
+	extendedKey, err := hdkeychain.NewKeyFromString(c.RootKey)
+	if err != nil {
+		return fmt.Errorf("error parsing root key: %v", err)
+	}
+
+	return deriveKey(extendedKey, c.Path, c.Neuter)
+}
+
 func setupChainParams(cfg *config) {
 	if cfg.Testnet {
 		chainParams = &chaincfg.TestNet3Params