Add constant-time comparison operators for cryptographic uses.

crypto/hmac.h

Index: crypto/hmac.h
===================================================================
--- crypto/hmac.h	(revision 103793)
+++ crypto/hmac.h	(working copy)
@@ -78,6 +78,8 @@
   DISALLOW_COPY_AND_ASSIGN(HMAC);
 };
 
+bool SecureMemcmp(const void* s1, const void* s2, size_t n);

[wtc, 2011/10/04 00:32:10]

This function is not related to HMAC.  We should create a new
header for this function, and perhaps a SecureMemset function
that won't be optimized away by the compiler.

+
 }  // namespace crypto
 
 #endif  // CRYPTO_HMAC_H_