Replace memcmp() with HMAC.VerifyTruncated() in aes_decryptor.cc

media/crypto/aes_decryptor.cc

 // Copyright (c) 2012 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 "media/crypto/aes_decryptor.h"
 
 #include "base/logging.h"
 #include "base/stl_util.h"
 #include "base/string_number_conversions.h"
 #include "crypto/encryptor.h"
@@ skipping 48 matching lines @@
                       const base::StringPiece& hmac_key) {
   CHECK(input.GetDataSize());
   CHECK(input.GetDecryptConfig());
   CHECK_GT(input.GetDecryptConfig()->checksum_size(), 0);
   CHECK(!hmac_key.empty());
 
   crypto::HMAC hmac(crypto::HMAC::SHA1);
   if (!hmac.Init(hmac_key))
     return false;
 
-  // The HMAC covers the IV and the frame data.
+  // The component that initializes |input.GetDecryptConfig()| is responsible
+  // for checking that |input.GetDecryptConfig()->checksum_size()| matches
+  // what is defined by the format.

[ddorwin, 2012/07/24 18:09:40]

Thanks. One might assume this is related to the next line. To avoid that, you
might say something like, "Here, just confirm that the size is not bigger than
the cypher's digest length."

[fgalligan1, 2012/07/24 18:38:33]

Done. PTAL.

+  DCHECK_LE(input.GetDecryptConfig()->checksum_size(),
+            static_cast<int>(hmac.DigestLength()));
+
   base::StringPiece data_to_check(
       reinterpret_cast<const char*>(input.GetData()), input.GetDataSize());
+  base::StringPiece digest(
+      reinterpret_cast<const char*>(input.GetDecryptConfig()->checksum()),
+      input.GetDecryptConfig()->checksum_size());
 
-  scoped_array<uint8> calculated_hmac(new uint8[hmac.DigestLength()]);
-  if (!hmac.Sign(data_to_check, calculated_hmac.get(), hmac.DigestLength()))
-    return false;
-
-  DCHECK(input.GetDecryptConfig()->checksum_size() <=
-         static_cast<int>(hmac.DigestLength()));
-  if (memcmp(input.GetDecryptConfig()->checksum(),
-             calculated_hmac.get(),
-             input.GetDecryptConfig()->checksum_size()) != 0)
-    return false;
-  return true;
+  return hmac.VerifyTruncated(data_to_check, digest);
 }
 
 // Decrypts |input| using |key|. |encrypted_data_offset| is the number of bytes
 // into |input| that the encrypted data starts.
 // Returns a DecoderBuffer with the decrypted data if decryption succeeded or
 // NULL if decryption failed.
 static scoped_refptr<DecoderBuffer> DecryptData(const DecoderBuffer& input,
                                                 crypto::SymmetricKey* key,
                                                 int encrypted_data_offset) {
   CHECK(input.GetDataSize());
@@ skipping 205 matching lines @@
 
   decryption_key_.reset(
       crypto::SymmetricKey::Import(crypto::SymmetricKey::AES, secret_));
   if (!decryption_key_.get()) {
     return false;
   }
   return true;
 }
 
 }  // namespace media