tests/integration: add HDNode private key recovery example

src/hdnode.js

@@ -37,6 +37,7 @@ function HDNode(K, chainCode, network) {
   this.chainCode = chainCode
   this.depth = 0
   this.index = 0
+  this.parentFingerprint = 0x00000000
   this.network = network
 
   if (K instanceof BigInteger) {
@@ -196,8 +197,7 @@ HDNode.prototype.toBuffer = function(isPrivate, __ignoreDeprecation) {
   buffer.writeUInt8(this.depth, 4)
 
   // 4 bytes: the fingerprint of the parent's key (0x00000000 if master key)
-  var fingerprint = (this.depth === 0) ? 0x00000000 : this.parentFingerprint
-  buffer.writeUInt32BE(fingerprint, 5)
+  buffer.writeUInt32BE(this.parentFingerprint, 5)
 
   // 4 bytes: child number. This is the number i in xi = xpar/i, with xi the key being serialized.
   // This is encoded in Big endian. (0x00000000 if master key)

test/integration/crypto.js

@@ -0,0 +1,52 @@
+var assert = require('assert')
+var bigi = require('bigi')
+var bitcoin = require('../../')
+var crypto = require('crypto')
+
+describe('bitcoinjs-lib (crypto)', function() {
+  it('can recover a parent private key from the parent\'s public key and a derived non-hardened child private key', function() {
+    function recoverParent(master, child) {
+      assert(!master.privKey, 'You already have the parent private key')
+      assert(child.privKey, 'Missing child private key')
+
+      var curve = bitcoin.ECKey.curve
+      var QP = master.pubKey.toBuffer()
+      var QP64 = QP.toString('base64')
+      var d1 = child.privKey.d
+      var d2
+      var indexBuffer = new Buffer(4)
+
+      // search index space until we find it
+      for (var i = 0; i < bitcoin.HDNode.HIGHEST_BIT; ++i) {
+        indexBuffer.writeUInt32BE(i, 0)
+
+        // calculate I
+        var data = Buffer.concat([QP, indexBuffer])
+        var I = crypto.createHmac('sha512', master.chainCode).update(data).digest()
+        var IL = I.slice(0, 32)
+        var pIL = bigi.fromBuffer(IL)
+
+        // See hdnode.js:273 to understand
+        d2 = d1.subtract(pIL).mod(curve.n)
+
+        var Qp = new bitcoin.ECKey(d2, true).pub.toBuffer()
+        if (Qp.toString('base64') === QP64) break
+      }
+
+      var node = new bitcoin.HDNode(d2, master.chainCode, master.network)
+      node.depth = master.depth
+      node.index = master.index
+      node.masterFingerprint = master.masterFingerprint
+      return node
+    }
+
+    var seed = crypto.randomBytes(32)
+    var master = bitcoin.HDNode.fromSeedBuffer(seed)
+    var child = master.derive(6) // m/6
+
+    // now for the recovery
+    var neuteredMaster = master.neutered()
+    var recovered = recoverParent(neuteredMaster, child)
+    assert.equal(recovered.toBase58(), master.toBase58())
+  })
+})