HDKeys: add toBuffer/fromBuffer to HDPrivateKey

lib/hdprivatekey.js

@@ -484,6 +484,14 @@ HDPrivateKey.prototype.toJSON = function toJSON() {
   return JSON.stringify(this.toObject());
 };
 
+HDPrivateKey.fromBuffer = function(arg) {
+  return new HDPrivateKey(arg);
+};
+
+HDPrivateKey.prototype.toBuffer = function() {
+  return this._buffers.xprivkey;
+};
+
 HDPrivateKey.DefaultDepth = 0;
 HDPrivateKey.DefaultFingerprint = 0;
 HDPrivateKey.DefaultChildIndex = 0;

test/hdprivatekey.js

@@ -262,6 +262,17 @@ describe('HDPrivate key interface', function() {
     });
   });
 
+  describe.only('conversion to/from buffer', function() {
+    var str = 'xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi';
+    it('should roundtrip to/from a buffer', function() {
+      var priv = new HDPrivateKey(str);
+      var toBuffer = priv.toBuffer();
+      var fromBuffer = HDPrivateKey.fromBuffer(toBuffer);
+      var roundTrip = new HDPrivateKey(fromBuffer.toBuffer());
+      roundTrip.xprivkey.should.equal(str);
+    });
+  });
+
   describe('conversion to plain object/json', function() {
     var plainObject = {
       'network':'livenet',