Add constant-time comparison operators for cryptographic uses.

crypto/secure_memcmp.h

Index: crypto/secure_memcmp.h
===================================================================
--- crypto/secure_memcmp.h	(revision 0)
+++ crypto/secure_memcmp.h	(revision 0)
@@ -0,0 +1,28 @@
+// 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.
+
+#ifndef CRYPTO_SECURE_MEMCMP_H_
+#define CRYPTO_SECURE_MEMCMP_H_
+#pragma once
+
+#include "crypto/crypto_export.h"
+
+namespace crypto {
+
+// Performs a constant-time comparison of two strings, returning true if the
+// strings are equal.
+//
+// For cryptographic operations, comparison functions such as memcmp() may
+// expose side-channel information about input, allowing an attacker to
+// perform timing analysis to determine what the expected bits should be. In
+// order to avoid such attacks, the comparison must execute in constant time,
+// so as to not to reveal to the attacker where the difference(s) are.
+// For an example attack, see
+// http://groups.google.com/group/keyczar-discuss/browse_thread/thread/5571eca0948b2a13
+CRYPTO_EXPORT bool SecureMemcmp(const void* s1, const void* s2, size_t n);
+
+} // namespace crypto
+
+#endif  // CRYPTO_HMAC_H_
+