lnd/hdkeychain: add support for xpubs in DeriveKey

If the extendedKey is an xpub, then private key is not generated and the
returned WIF will be nil.

This enables passing an xpub to "chantools derivekey --rootkey".
Added a test for this new feature.

Author: starius

cmd/chantools/derivekey_test.go

@@ -102,3 +102,24 @@ func TestDeriveKeyXprv(t *testing.T) {
 	h.assertLogContains("cQcdieZy2d1TAdCsa5MjmHJs2gdHcD7x22nDbhJyVTUa3Ax" +
 		"5KB3w")
 }
+
+func TestDeriveKeyXpub(t *testing.T) {
+	h := newHarness(t)
+
+	// Derive a specific key from xpub.
+	derive := &deriveKeyCommand{
+		Path: "m/5/6",
+		rootKey: &rootKey{
+			RootKey: "xpub661MyMwAqRbcFtXgS5sYJABqqG9YLmC4Q1Rdap" +
+				"9gSE8NqtwybGhePY2gZ29ESFjqJoCu1Rupje8YtGqse" +
+				"fD265TMg7usUDFdp6W1EGMcet8",
+		},
+		Neuter: true,
+	}
+
+	err := derive.Execute(nil, nil)
+	require.NoError(t, err)
+
+	h.assertLogContains("03dc8655d58bd4fd4326863fe34bd5cdddbefaa3b042571" +
+		"05eb1ab99aa05e01c2a")
+}

lnd/hdkeychain.go

@@ -110,7 +110,8 @@ func HardenedKey(key uint32) uint32 {
 }
 
 // DeriveKey derives the public key and private key in the WIF format for a
-// given key path of the extended key.
+// given key path from the extended key. If the extendedKey is an xpub, then
+// private key is not generated and the returned WIF will be nil.
 func DeriveKey(extendedKey *hdkeychain.ExtendedKey, path string,
 	params *chaincfg.Params) (*hdkeychain.ExtendedKey, *btcec.PublicKey,
 	*btcutil.WIF, error) {
@@ -131,6 +132,11 @@ func DeriveKey(extendedKey *hdkeychain.ExtendedKey, path string,
 			"key: %w", err)
 	}
 
+	// If the extended key is xpub, we can't generate the private key.
+	if !derivedKey.IsPrivate() {
+		return derivedKey, pubKey, nil, nil
+	}
+
 	privKey, err := derivedKey.ECPrivKey()
 	if err != nil {
 		return nil, nil, nil, fmt.Errorf("could not derive private "+