media/webm/webm_crypto_helpers.cc

// Copyright (c) 2013 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/webm/webm_crypto_helpers.h"

#include "base/logging.h"
#include "base/sys_byteorder.h"
#include "media/base/decrypt_config.h"
#include "media/webm/webm_constants.h"

namespace media {
namespace {

// Generates a 16 byte CTR counter block. The CTR counter block format is a
// CTR IV appended with a CTR block counter. |iv| is an 8 byte CTR IV.
// |iv_size| is the size of |iv| in btyes. Returns a string of
// kDecryptionKeySize bytes.
std::string GenerateWebMCounterBlock(const uint8* iv, int iv_size) {
  std::string counter_block(reinterpret_cast<const char*>(iv), iv_size);
  counter_block.append(DecryptConfig::kDecryptionKeySize - iv_size, 0);
  return counter_block;
}

}  // namespace anonymous

scoped_ptr<DecryptConfig> WebMCreateDecryptConfig(
    const uint8* data, int data_size,
    const uint8* key_id, int key_id_size) {
  if (data_size < kWebMSignalByteSize) {
    DVLOG(1) << "Got a block from an encrypted stream with no data.";
    return scoped_ptr<DecryptConfig>();
  }

  uint8 signal_byte = data[0];
  int frame_offset = sizeof(signal_byte);

  // Setting the DecryptConfig object of the buffer while leaving the
  // initialization vector empty will tell the decryptor that the frame is
  // unencrypted.
  std::string counter_block;

  if (signal_byte & kWebMFlagEncryptedFrame) {
    if (data_size < kWebMSignalByteSize + kWebMIvSize) {
      DVLOG(1) << "Got an encrypted block with not enough data " << data_size;
      return scoped_ptr<DecryptConfig>();
    }
    counter_block = GenerateWebMCounterBlock(data + frame_offset, kWebMIvSize);
    frame_offset += kWebMIvSize;
  }

  scoped_ptr<DecryptConfig> config(new DecryptConfig(
      std::string(reinterpret_cast<const char*>(key_id), key_id_size),
      counter_block,
      frame_offset,
      std::vector<SubsampleEntry>()));
  return config.Pass();
}

}  // namespace media