crypto: dh secret should be left-padded

DH_compute_secret() may return key that is smaller than input buffer,
in such cases key should be left-padded because it is a BN (big number).

fix #5239

src/node_crypto.cc

@@ -3586,7 +3586,8 @@ class DiffieHellman : public ObjectWrap {
     // allocated buffer.
     if (size != dataSize) {
       assert(dataSize > size);
-      memset(data + size, 0, dataSize - size);
+      memmove(data + dataSize - size, data, size);
+      memset(data, 0, dataSize - size);
     }
 
     Local<Value> outString;

test/simple/test-dh-padding.js

@@ -0,0 +1,46 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+
+try {
+  var crypto = require('crypto');
+} catch (e) {
+  console.log('Not compiled with OPENSSL support.');
+  process.exit();
+}
+
+var prime = 'c51f7bf8f0e1cf899243cdf408b1bc7c09c010e33ef7f3fbe5bd5feaf906113b';
+var apub = '6fe9f37037d8d017f908378c1ee04fe60e1cd3668bfc5075fac55c2f7153dd84';
+var bpub = '31d83e167fdf956c9dae6b980140577a9f8868acbfcbdc19113e58bfb9223abc';
+var apriv = '4fbfd4661f9181bbf574537b1a78adf473e8e771eef13c605e963c0f3094b697';
+var secret = '25616eed33f1af7975bbd0a8071d98a014f538b243bef90d76c08e81a0b3c500';
+
+var p = crypto.createDiffieHellman(prime, 'hex');
+
+p.setPublicKey(apub, 'hex');
+p.setPrivateKey(apriv, 'hex');
+
+assert.equal(
+  p.computeSecret(bpub, 'hex', 'hex'),
+  '0025616eed33f1af7975bbd0a8071d98a014f538b243bef90d76c08e81a0b3c5'
+);