derivekey: support xpubs in addition to xprvs

cmd/chantools/derivekey.go

@@ -107,7 +107,7 @@ func deriveKey(extendedKey *hdkeychain.ExtendedKey, path string,
 	}
 
 	privKey, xPriv := na, na
-	if !neuter {
+	if !neuter && wif != nil {
 		privKey, xPriv = wif.String(), child.String()
 	}
 

cmd/chantools/derivekey_test.go

@@ -82,3 +82,64 @@ func TestDeriveKeySeedBip39(t *testing.T) {
 
 	h.assertLogContains(keyContentBIP39)
 }
+
+func TestDeriveKeyXprv(t *testing.T) {
+	h := newHarness(t)
+
+	// Derive a specific key from xprv.
+	derive := &deriveKeyCommand{
+		Path: testPath,
+		rootKey: &rootKey{
+			RootKey: "xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nR" +
+				"k4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejM" +
+				"RNNU3TGtRBeJgk33yuGBxrMPHi",
+		},
+	}
+
+	err := derive.Execute(nil, nil)
+	require.NoError(t, err)
+
+	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")
+}
+
+func TestDeriveKeyXpubNoNeuter(t *testing.T) {
+	h := newHarness(t)
+
+	// Derive a specific key from xpub.
+	derive := &deriveKeyCommand{
+		Path: "m/5/6",
+		rootKey: &rootKey{
+			RootKey: "xpub661MyMwAqRbcFtXgS5sYJABqqG9YLmC4Q1Rdap" +
+				"9gSE8NqtwybGhePY2gZ29ESFjqJoCu1Rupje8YtGqse" +
+				"fD265TMg7usUDFdp6W1EGMcet8",
+		},
+	}
+
+	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 "+