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 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