Add constant-time comparison operators for cryptographic uses.

crypto/hmac.cc

 // 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 "crypto/hmac.h"
 
 #include <algorithm>
 
 #include "base/logging.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
-static bool SecureMemcmp(const void* s1, const void* s2, size_t n) {
+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);
 }
 
 size_t HMAC::DigestLength() const {
   switch (hash_alg_) {
@@ skipping 22 matching lines @@
   scoped_array<unsigned char> computed_digest(
       new unsigned char[digest_length]);
   if (!Sign(data, computed_digest.get(), static_cast<int>(digest_length)))
     return false;
 
   return SecureMemcmp(digest.data(), computed_digest.get(),
                       std::min(digest.size(), digest_length));
 }
 
 }  // namespace crypto