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      1 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #include "media/webm/webm_crypto_helpers.h"
      6 
      7 #include "base/logging.h"
      8 #include "base/sys_byteorder.h"
      9 #include "media/base/decrypt_config.h"
     10 #include "media/webm/webm_constants.h"
     11 
     12 namespace media {
     13 namespace {
     14 
     15 // Generates a 16 byte CTR counter block. The CTR counter block format is a
     16 // CTR IV appended with a CTR block counter. |iv| is an 8 byte CTR IV.
     17 // |iv_size| is the size of |iv| in btyes. Returns a string of
     18 // kDecryptionKeySize bytes.
     19 std::string GenerateWebMCounterBlock(const uint8* iv, int iv_size) {
     20   std::string counter_block(reinterpret_cast<const char*>(iv), iv_size);
     21   counter_block.append(DecryptConfig::kDecryptionKeySize - iv_size, 0);
     22   return counter_block;
     23 }
     24 
     25 }  // namespace anonymous
     26 
     27 scoped_ptr<DecryptConfig> WebMCreateDecryptConfig(
     28     const uint8* data, int data_size,
     29     const uint8* key_id, int key_id_size) {
     30   if (data_size < kWebMSignalByteSize) {
     31     DVLOG(1) << "Got a block from an encrypted stream with no data.";
     32     return scoped_ptr<DecryptConfig>();
     33   }
     34 
     35   uint8 signal_byte = data[0];
     36   int frame_offset = sizeof(signal_byte);
     37 
     38   // Setting the DecryptConfig object of the buffer while leaving the
     39   // initialization vector empty will tell the decryptor that the frame is
     40   // unencrypted.
     41   std::string counter_block;
     42 
     43   if (signal_byte & kWebMFlagEncryptedFrame) {
     44     if (data_size < kWebMSignalByteSize + kWebMIvSize) {
     45       DVLOG(1) << "Got an encrypted block with not enough data " << data_size;
     46       return scoped_ptr<DecryptConfig>();
     47     }
     48     counter_block = GenerateWebMCounterBlock(data + frame_offset, kWebMIvSize);
     49     frame_offset += kWebMIvSize;
     50   }
     51 
     52   scoped_ptr<DecryptConfig> config(new DecryptConfig(
     53       std::string(reinterpret_cast<const char*>(key_id), key_id_size),
     54       counter_block,
     55       frame_offset,
     56       std::vector<SubsampleEntry>()));
     57   return config.Pass();
     58 }
     59 
     60 }  // namespace media
     61