Add a Verify routine for HMAC

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 "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) {
+  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:
       return 20;
     case SHA256:
       return 32;
     default:
       NOTREACHED();
       return 0;
   }
 }
 
+bool HMAC::Verify(const base::StringPiece& data,
+                  const base::StringPiece& digest) const {
+  if (digest.size() != DigestLength())
+    return false;
+  scoped_array<unsigned char> computed_digest(
+      new unsigned char[digest.size()]);
+  if (!Sign(data, computed_digest.get(), static_cast<int>(digest.size())))
+    return false;
+
+  return SecureMemcmp(digest.data(), computed_digest.get(), digest.size());
+}
+
 }  // namespace crypto