Add constant-time comparison operators for cryptographic uses.

crypto/hmac.cc

Index: crypto/hmac.cc
===================================================================
--- crypto/hmac.cc	(revision 103793)
+++ crypto/hmac.cc	(working copy)
@@ -7,28 +7,10 @@
 #include <algorithm>
 
 #include "base/logging.h"
+#include "crypto/secure_util.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) {
-  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_) {
     case SHA1: