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.
+
+  // Here, check that checksum size is not greater than the hash
+  // algorithm's digest length.
+  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