Add xpub vanity seed key format

src/index.js

@@ -4,7 +4,8 @@ const ONE_SECOND = 1000;
 
 const keyFormats = {
 	wif: require('./key-formats/wif'),
-	bip39: require('./key-formats/bip39')
+	bip39: require('./key-formats/bip39'),
+	xpub: require('./key-formats/xpub')
 };
 
 const addressFormats = {
@@ -59,7 +60,7 @@ class Vain extends Emitter {
 
 			attempts++;
 
-			keyData = generateKey({addressFormat, options});
+			keyData = generateKey({addressFormat, options, attempts});
 			address = addressFormat.derive(keyData.publicKey);
 
 			if (address.startsWith(this.prefix)) {

src/key-formats/xpub.js

@@ -0,0 +1,19 @@
+const bitcoin = require('bitcoinjs-lib');
+
+const generateXpubKey = ({options, attempts}) => {
+	const change = 0;
+	const index = attempts - 1;
+	const derivationPath = `${change}/${index}`;
+
+	const node = bitcoin.bip32.fromBase58(options.xpub);
+	const {publicKey} = node.derivePath(derivationPath);
+
+	const format = () => ({
+		xpub: options.xpub,
+		derivationPath
+	});
+
+	return {publicKey, format};
+};
+
+module.exports = generateXpubKey;