feat+doc: expose toPublic and document wipePrivateData

Author: coolaj86

README.md

@@ -239,6 +239,8 @@ However, production code will look more like this:
       RECEIVE, CHANGE
   async fromSeed(seedBytes, opts)   // depth-0 hdkey (Wallet)
   async fromXKey(xprv||xpub, opts)  // depth-4 hdkey (XKey)
+  async toPublic(xKey)
+  async wipePrivateData(xKey)
   async toWif(privBytes, opts)
   async toAddr(pubBytes, opts)
   async toXPrv(xprvKey, opts)
@@ -415,6 +417,22 @@ let xkey = await DashHd.fromXKey(xprvOrXPub, options);
 }
 ```
 
+### `toPublic(xkey)`
+
+Creates a copy of the HD Key with `privateKey` set to `null`.
+
+```js
+let xpubKey = await DashHd.toPublic(xprvKey);
+```
+
+### `wipePrivateData(xkey)`
+
+Performs an in-place secure erase of the private key memory.
+
+```js
+await DashHd.wipePrivateData(xprvKey);
+```
+
 ### `toWif(privBytes, opts)`
 
 Wrapper around `DashKeys.encodeKey(keyBytes, options)` to Base58Check-encode a

dashhd.js

@@ -14,6 +14,7 @@
  * @prop {HDToXKeyBytes} toXPubBytes
  * @prop {HDUtils} utils
  * @prop {HDWipePrivates} wipePrivateData - randomizes private key buffer in-place
+ * @prop {HDToPublic} toPublic - returns public key
  * @prop {Number} HARDENED_OFFSET - 0x80000000
  * @prop {HDVersions} MAINNET - 'xprv' & 'xpub'
  * @prop {HDVersions} TESTNET - 'tprv' & 'tpub'
@@ -651,6 +652,12 @@ var DashHd = ("object" === typeof module && exports) || {};
     return hdkey;
   };
 
+  DashHd.toPublic = function (_hdkey) {
+    let hdkey = Object.assign({}, _hdkey);
+    hdkey.privateKey = null;
+    return hdkey;
+  };
+
   DashHd.wipePrivateData = function (hdkey) {
     if (hdkey.privateKey) {
       Utils.secureErase(hdkey.privateKey);
@@ -908,6 +915,12 @@ if ("object" === typeof module) {
  * @param {Uint8Array} buf
  */
 
+/**
+ * @callback HDToPublic
+ * @param {HDKey} hdkey
+ * @returns {HDKey}
+ */
+
 /**
  * @callback HDWipePrivates
  * @param {HDKey} hdkey