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)
@@ -20,7 +20,7 @@
 // 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;