Add constant-time comparison operators for cryptographic uses.

crypto/secure_memcmp.cc

Index: crypto/secure_memcmp.cc
===================================================================
--- crypto/secure_memcmp.cc	(revision 0)
+++ crypto/secure_memcmp.cc	(revision 0)
@@ -0,0 +1,15 @@
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "secure_memcmp.h"
+

[Ryan Sleevi, 2011/10/04 00:38:28]

missing namespace crypto {} here

[Chris Palmer, 2011/10/04 00:40:30]

Done.

+bool SecureMemcmp(const void* s1, const void* s2, size_t n) {
+  const unsigned char* s1_ptr = reinterpret_cast<const unsigned char*>(s1);
+  const unsigned char* s2_ptr = reinterpret_cast<const unsigned char*>(s2);
+  unsigned char tmp = 0;
+  for (size_t i = 0; i < n; ++i, ++s1_ptr, ++s2_ptr)
+    tmp |= *s1_ptr ^ *s2_ptr;
+  return (tmp == 0);
+}
+