Reland "Cut down /crypto and switch what is left of it to boringssl".

crypto/secure_util.h

-// Copyright (c) 2012 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_UTIL_H_
-#define CRYPTO_SECURE_UTIL_H_
-
-#include <stddef.h>
-
-#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 SecureMemEqual(const void* s1, const void* s2, size_t n);
-
-}  // namespace crypto
-
-#endif  // CRYPTO_SECURE_UTIL_H_
-