Add verify method to HMAC instances to have a comparison utility that does not leak information thr…

sdk/lib/crypto/hmac.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class _HMAC implements HMAC {
   _HMAC(Hash this._hash, List<int> this._key) : _message = [];
 
   HMAC update(List<int> data) {
     _message.addAll(data);
     return this;
@@ skipping 31 matching lines @@
     // Compute outer padding.
     for (var i = 0; i < blockSize; i++) {
       padding[i] = 0x5c ^ _key[i];
     }
 
     // Outer hash computation which is the result.
     _hash = _hash.newInstance();
     return _hash.update(padding).update(innerHash).digest();
   }
 
+  bool verify(List<int> digest) {
+    var computedDigest = this.digest();
+    if (digest.length != computedDigest.length) {
+      throw new ArgumentError(
+          'Invalid digest size: ${digest.length} in HMAC.verify. '
+          'Expected: ${_hash.blockSize}.');
+    }
+    int result = 0;
+    for (var i = 0; i < digest.length; i++) {
+      result |= digest[i] ^ computedDigest[i];
+    }
+    return result == 0;
+  }
+
   // HMAC internal state.
   Hash _hash;
   List<int> _key;
   List<int> _message;
 }