Use constant-time HMAC comparison in Nigori.

Cryptography is timing-sensitive. MACs should never be compared with usual
string comparison functions, only constant-time checks. Use HMAC::Verify.

Bug: none
Change-Id: Ic1b164441396e4a4a853f1a50d91bc8f5c7fa18e
Reviewed-on: https://chromium-review.googlesource.com/1246323Reviewed-by: vitaliii <vitaliii@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594737}

components/sync/base/nigori.cc

@@ -334,12 +334,7 @@ bool Nigori::Decrypt(const std::string& encrypted, std::string* value) const {
   if (!hmac.Init(keys_.mac_key->key()))
     return false;
 
-  std::vector<unsigned char> expected(kHashSize);
-  if (!hmac.Sign(ciphertext, &expected[0], expected.size()))
-    return false;
-
-  if (hash.compare(0, hash.size(), reinterpret_cast<char*>(&expected[0]),
-                   expected.size()))
+  if (!hmac.Verify(ciphertext, hash))
     return false;
 
   crypto::Encryptor encryptor;