media/webm/webm_crypto_helpers.cc

   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 bool WebMCreateDecryptConfig(const uint8* data, int data_size,
  28                              const uint8* key_id, int key_id_size,
  29                              scoped_ptr<DecryptConfig>* decrypt_config,
  30                              int* data_offset) {
  31   if (data_size < kWebMSignalByteSize) {
  32     DVLOG(1) << "Got a block from an encrypted stream with no data.";
  33     return false;
  34   }
  35
  36   uint8 signal_byte = data[0];
  37   int frame_offset = sizeof(signal_byte);
  38
  39   // Setting the DecryptConfig object of the buffer while leaving the
  40   // initialization vector empty will tell the decryptor that the frame is
  41   // unencrypted.
  42   std::string counter_block;
  43
  44   if (signal_byte & kWebMFlagEncryptedFrame) {
  45     if (data_size < kWebMSignalByteSize + kWebMIvSize) {
  46       DVLOG(1) << "Got an encrypted block with not enough data " << data_size;
  47       return false;
  48     }
  49     counter_block = GenerateWebMCounterBlock(data + frame_offset, kWebMIvSize);
  50     frame_offset += kWebMIvSize;
  51   }
  52
  53   decrypt_config->reset(new DecryptConfig(
  54       std::string(reinterpret_cast<const char*>(key_id), key_id_size),
  55       counter_block,
  56       std::vector<SubsampleEntry>()));
  57   *data_offset = frame_offset;
  58
  59   return true;
  60 }
  61
  62 }  // namespace media