Add Hmac.validate() and timingSafeEquals().

doc/api/crypto.markdown

@@ -605,6 +605,14 @@ encoding of `'binary'` is enforced. If `data` is a [`Buffer`][] then
 
 This can be called many times with new data as it is streamed.
 
+### hmac.validate(inputHmac)
+
+Return true if and only if the computed hmac matches the input hmac,
+provided as a [Buffer](buffer.html). This uses a timing-safe comparison.
+
+The `hmac` object can not be used after `validate()` method has been
+called.
+
 ## Class: Hmac
 
 The `Hmac` Class is a utility for creating cryptographic HMAC digests. It can
@@ -665,6 +673,13 @@ Calculates the HMAC digest of all of the data passed using `hmac.update()`. The
 `encoding` can be `'hex'`, `'binary'` or `'base64'`.  If `encoding` is provided
 a string is returned; otherwise a [`Buffer`][] is returned;
 
+Caution: Code that uses `digest()` directly for comparison with an input value
+is very likely to introduce a
+[timing attack](http://codahale.com/a-lesson-in-timing-attacks/).
+Such a timing attack would allow someone to construct an
+HMAC value for a message of their choosing without posessing the key.
+Prefer `validate()`, which does a timing-safe comparison.
+
 The `Hmac` object can not be used again after `hmac.digest()` has been
 called. Multiple calls to `hmac.digest()` will result in an error being thrown.
 
@@ -1153,6 +1168,15 @@ keys:
 
 All paddings are defined in the `constants` module.
 
+## crypto.timingSafeEqual(a, b)
+
+Returns true if a is equal to b, without leaking timing information that would
+help an attacker guess one of the values. This is suitable for comparing secret
+values like authentication cookies or
+[capability urls](http://www.w3.org/TR/capability-urls/).
+
+`a` and `b` can be strings or [Buffers](buffer.html).
+
 ### crypto.privateEncrypt(private_key, buffer)
 
 Encrypts `buffer` with `private_key`.

lib/crypto.js

@@ -83,6 +83,7 @@ exports.createHmac = exports.Hmac = Hmac;
 function Hmac(hmac, key, options) {
   if (!(this instanceof Hmac))
     return new Hmac(hmac, key, options);
+  this.key = key;
   this._handle = new binding.Hmac();
   this._handle.init(hmac, toBuf(key));
   LazyTransform.call(this, options);
@@ -95,6 +96,25 @@ Hmac.prototype.digest = Hash.prototype.digest;
 Hmac.prototype._flush = Hash.prototype._flush;
 Hmac.prototype._transform = Hash.prototype._transform;
 
+// This implements Brad Hill's Double HMAC pattern from
+// https://www.nccgroup.trust/us/about-us/
+//   newsroom-and-events/blog/2011/february/double-hmac-verification/.
+// In short, it's near-impossible to write a reliable constant-time compare in a
+// high level language like JS, because of the many layers that can optimize
+// away attempts at being constant time.
+//
+// Double HMAC avoids that problem by blinding the timing channel instead. After
+// running the inputs through a second round of HMAC, we are free to
+// short-circuit comparison, because the time it takes to reach the
+// short-circuit has no relation to the similarity between the guessed digest
+// and the correct one.
+//
+// Note: hmac object can not be used after validate() method has been called.
+Hmac.prototype.validate = function(inputBuffer) {
+  var ah = new Hmac('sha256', this.key).update(this.digest()).digest();
+  var bh = new Hmac('sha256', this.key).update(inputBuffer).digest();
+  return ah.equals(bh);
+};
 
 function getDecoder(decoder, encoding) {
   if (encoding === 'utf-8') encoding = 'utf8';  // Normalize encoding.
@@ -662,6 +682,14 @@ function filterDuplicates(names) {
   }).sort();
 }
 
+// timingSafeEqual reuses the timing-safe Hmac.equals function (see above) for
+// comparison of non-hash inputs, like capability URLs or authentication tokens.
+exports.timingSafeEqual = function(a, b) {
+  var key = randomBytes(32);
+  var ah = new Hmac('sha256', key).update(a);
+  return ah.validate(new Hmac('sha256', key).update(b).digest());
+};
+
 // Legacy API
 exports.__defineGetter__('createCredentials',
   internalUtil.deprecate(function() {

test/parallel/test-crypto-hmac.js

@@ -11,9 +11,24 @@ var crypto = require('crypto');
 // Test HMAC
 var h1 = crypto.createHmac('sha1', 'Node')
                .update('some data')
-               .update('to hmac')
-               .digest('hex');
-assert.equal(h1, '19fd6e1ba73d9ed2224dd5094a71babe85d9a892', 'test HMAC');
+               .update('to hmac');
+assert.equal(h1.digest('hex'),
+             '19fd6e1ba73d9ed2224dd5094a71babe85d9a892',
+             'test HMAC');
+
+var h2 = crypto.createHmac('sha1', 'Node')
+               .update('some data')
+               .update('to hmac');
+assert.ok(h2.validate(
+  new Buffer('19fd6e1ba73d9ed2224dd5094a71babe85d9a892', 'hex'),
+  'test HMAC valid'));
+
+var h3 = crypto.createHmac('sha1', 'Node')
+               .update('some data')
+               .update('to hmac');
+assert.ok(!h3.validate(
+  new Buffer('6bdee6ee47fb42c53a4f44c3e4bb97591c0c3635', 'hex'),
+  'test HMAC not valid'));
 
 // Test HMAC (Wikipedia Test Cases)
 var wikipedia = [

test/parallel/test-crypto-timing-safe-equal.js

@@ -0,0 +1,13 @@
+'use strict';
+var common = require('../common');
+var assert = require('assert');
+
+if (!common.hasCrypto) {
+  console.log('1..0 # Skipped: missing crypto');
+  return;
+}
+var crypto = require('crypto');
+
+assert.ok(crypto.timingSafeEqual('alpha', 'alpha'), 'equal strings not equal');
+assert.ok(!crypto.timingSafeEqual('alpha', 'beta'),
+  'inequal strings considered equal');