Define SampleEncryption 'senc' box

Issue #41

Change-Id: Ib28c079209f3a58ecd8d7b86d720a1c8c774b669

packager/media/formats/mp4/box.cc

@@ -13,7 +13,7 @@ namespace edash_packager {
 namespace media {
 namespace mp4 {
 
-Box::Box() : atom_size(0) {}
+Box::Box() : box_size_(0) {}
 Box::~Box() {}
 
 bool Box::Parse(BoxReader* reader) {
@@ -24,21 +24,21 @@ bool Box::Parse(BoxReader* reader) {
 
 void Box::Write(BufferWriter* writer) {
   DCHECK(writer);
-  // Compute and update atom_size.
+  // Compute and update box size.
   uint32_t size = ComputeSize();
-  DCHECK_EQ(size, this->atom_size);
+  DCHECK_EQ(size, box_size_);
 
   size_t buffer_size_before_write = writer->Size();
   BoxBuffer buffer(writer);
   CHECK(ReadWriteInternal(&buffer));
-  DCHECK_EQ(this->atom_size, writer->Size() - buffer_size_before_write);
+  DCHECK_EQ(box_size_, writer->Size() - buffer_size_before_write);
 }
 
 void Box::WriteHeader(BufferWriter* writer) {
   DCHECK(writer);
-  // Compute and update atom_size.
+  // Compute and update box size.
   uint32_t size = ComputeSize();
-  DCHECK_EQ(size, this->atom_size);
+  DCHECK_EQ(size, box_size_);
 
   size_t buffer_size_before_write = writer->Size();
   BoxBuffer buffer(writer);
@@ -47,8 +47,8 @@ void Box::WriteHeader(BufferWriter* writer) {
 }
 
 uint32_t Box::ComputeSize() {
-  this->atom_size = ComputeSizeInternal();
+  box_size_ = ComputeSizeInternal();
-  return this->atom_size;
+  return box_size_;
 }
 
 uint32_t Box::HeaderSize() const {
@@ -61,7 +61,7 @@ bool Box::ReadWriteHeaderInternal(BoxBuffer* buffer) {
   if (buffer->Reading()) {
     // Skip for read mode, which is handled already in BoxReader.
   } else {
-    CHECK(buffer->ReadWriteUInt32(&this->atom_size));
+    CHECK(buffer->ReadWriteUInt32(&box_size_));
     FourCC fourcc = BoxType();
     CHECK(buffer->ReadWriteFourCC(&fourcc));
   }

packager/media/formats/mp4/box.h

@@ -50,24 +50,28 @@ struct Box {
   /// @return box type.
   virtual FourCC BoxType() const = 0;
 
+  /// @return The size of result box including child boxes. Note that this
+  //          function expects that ComputeSize has been invoked already.
+  uint32_t box_size() { return box_size_; }
+
  protected:
-  /// Read/write mp4 box header. Note that this function expects box size
+  /// Read/write mp4 box header. Note that this function expects that
-  /// updated already.
+  /// ComputeSize has been invoked already.
   /// @return true on success, false otherwise.
   virtual bool ReadWriteHeaderInternal(BoxBuffer* buffer);
 
  private:
   friend class BoxBuffer;
   // Read/write the mp4 box from/to BoxBuffer. Note that this function expects
-  // box size updated already.
+  // that ComputeSize has been invoked already.
   virtual bool ReadWriteInternal(BoxBuffer* buffer) = 0;
   // Compute the size of this box. A value of 0 should be returned if the box
   // should not be written. Note that this function won't update box size.
   virtual uint32_t ComputeSizeInternal() = 0;
 
-  // We don't support 64-bit atom sizes. 32-bit should be large enough for our
+  // We don't support 64-bit box sizes. 32-bit should be large enough for our
   // current needs.
-  uint32_t atom_size;
+  uint32_t box_size_;
 
   // Not using DISALLOW_COPY_AND_ASSIGN here intentionally to allow the compiler
   // generated copy constructor and assignment operator.

packager/media/formats/mp4/box_buffer.h

@@ -167,7 +167,7 @@ class BoxBuffer {
     if (reader_)
       return reader_->ReadChild(box);
     // The box is mandatory, i.e. the box size should not be 0.
-    DCHECK_NE(0u, box->atom_size);
+    DCHECK_NE(0u, box->box_size());
     CHECK(box->ReadWriteInternal(this));
     return true;
   }
@@ -178,7 +178,7 @@ class BoxBuffer {
     if (reader_)
       return reader_->TryReadChild(box);
     // The box is optional, i.e. it can be skipped if the box size is 0.
-    if (box->atom_size != 0)
+    if (box->box_size() != 0)
       CHECK(box->ReadWriteInternal(this));
     return true;
   }

packager/media/formats/mp4/box_definitions.cc

@@ -41,6 +41,12 @@ const char kVpcCompressorName[] = "\012VPC Coding";
 // at once.
 const int kCueSourceIdNotSet = -1;
 
+// According to ISO/IEC FDIS 23001-7: CENC spec, IV should be either
+// 64-bit (8-byte) or 128-bit (16-byte).
+bool IsIvSizeValid(size_t iv_size) {
+  return iv_size == 8 || iv_size == 16;
+}
+
 // Utility functions to check if the 64bit integers can fit in 32bit integer.
 bool IsFitIn32Bits(uint64_t a) {
   return a <= std::numeric_limits<uint32_t>::max();
@@ -183,6 +189,143 @@ uint32_t SampleAuxiliaryInformationSize::ComputeSizeInternal() {
          (default_sample_info_size == 0 ? sample_info_sizes.size() : 0);
 }
 
+SampleEncryptionEntry::SampleEncryptionEntry() {}
+SampleEncryptionEntry::~SampleEncryptionEntry() {}
+
+bool SampleEncryptionEntry::ReadWrite(uint8_t iv_size,
+                                      bool has_subsamples,
+                                      BoxBuffer* buffer) {
+  DCHECK(IsIvSizeValid(iv_size));
+  DCHECK(buffer);
+
+  RCHECK(buffer->ReadWriteVector(&initialization_vector, iv_size));
+
+  if (!has_subsamples) {
+    subsamples.clear();
+    return true;
+  }
+
+  uint16_t subsample_count = subsamples.size();
+  RCHECK(buffer->ReadWriteUInt16(&subsample_count));
+  RCHECK(subsample_count > 0);
+  subsamples.resize(subsample_count);
+  for (auto& subsample : subsamples) {
+    RCHECK(buffer->ReadWriteUInt16(&subsample.clear_bytes) &&
+           buffer->ReadWriteUInt32(&subsample.cipher_bytes));
+  }
+  return true;
+}
+
+bool SampleEncryptionEntry::ParseFromBuffer(uint8_t iv_size,
+                                            bool has_subsamples,
+                                            BufferReader* reader) {
+  DCHECK(IsIvSizeValid(iv_size));
+  DCHECK(reader);
+
+  initialization_vector.resize(iv_size);
+  RCHECK(reader->ReadToVector(&initialization_vector, iv_size));
+
+  if (!has_subsamples) {
+    subsamples.clear();
+    return true;
+  }
+
+  uint16_t subsample_count;
+  RCHECK(reader->Read2(&subsample_count));
+  RCHECK(subsample_count > 0);
+  subsamples.resize(subsample_count);
+  for (auto& subsample : subsamples) {
+    RCHECK(reader->Read2(&subsample.clear_bytes) &&
+           reader->Read4(&subsample.cipher_bytes));
+  }
+  return true;
+}
+
+uint32_t SampleEncryptionEntry::ComputeSize() const {
+  const uint32_t subsample_entry_size = sizeof(uint16_t) + sizeof(uint32_t);
+  const uint16_t subsample_count = subsamples.size();
+  return initialization_vector.size() +
+         (subsample_count > 0 ? (sizeof(subsample_count) +
+                                 subsample_entry_size * subsample_count)
+                              : 0);
+}
+
+uint32_t SampleEncryptionEntry::GetTotalSizeOfSubsamples() const {
+  uint32_t size = 0;
+  for (uint32_t i = 0; i < subsamples.size(); ++i)
+    size += subsamples[i].clear_bytes + subsamples[i].cipher_bytes;
+  return size;
+}
+
+SampleEncryption::SampleEncryption() : iv_size(0) {}
+SampleEncryption::~SampleEncryption() {}
+FourCC SampleEncryption::BoxType() const { return FOURCC_SENC; }
+
+bool SampleEncryption::ReadWriteInternal(BoxBuffer* buffer) {
+  RCHECK(ReadWriteHeaderInternal(buffer));
+
+  // If we don't know |iv_size|, store sample encryption data to parse later
+  // after we know iv_size.
+  if (buffer->Reading() && iv_size == 0) {
+    RCHECK(buffer->ReadWriteVector(&sample_encryption_data,
+                                   buffer->Size() - buffer->Pos()));
+    return true;
+  }
+
+  if (!IsIvSizeValid(iv_size)) {
+    LOG(ERROR) << "IV_size can only be 8 or 16, but seeing " << iv_size;
+    return false;
+  }
+
+  uint32_t sample_count = sample_encryption_entries.size();
+  RCHECK(buffer->ReadWriteUInt32(&sample_count));
+
+  sample_encryption_entries.resize(sample_count);
+  for (auto& sample_encryption_entry : sample_encryption_entries) {
+    RCHECK(sample_encryption_entry.ReadWrite(
+        iv_size, flags & kUseSubsampleEncryption, buffer));
+  }
+  return true;
+}
+
+uint32_t SampleEncryption::ComputeSizeInternal() {
+  const uint32_t sample_count = sample_encryption_entries.size();
+  if (sample_count == 0) {
+    // Sample encryption box is optional. Skip it if it is empty.
+    return 0;
+  }
+
+  DCHECK(IsIvSizeValid(iv_size));
+  uint32_t box_size = HeaderSize() + sizeof(sample_count);
+  if (flags & kUseSubsampleEncryption) {
+    for (const SampleEncryptionEntry& sample_encryption_entry :
+         sample_encryption_entries) {
+      box_size += sample_encryption_entry.ComputeSize();
+    }
+  } else {
+    box_size += sample_count * iv_size;
+  }
+  return box_size;
+}
+
+bool SampleEncryption::ParseFromSampleEncryptionData(
+    size_t iv_size,
+    std::vector<SampleEncryptionEntry>* sample_encryption_entries) const {
+  DCHECK(IsIvSizeValid(iv_size));
+
+  BufferReader reader(vector_as_array(&sample_encryption_data),
+                      sample_encryption_data.size());
+  uint32_t sample_count = 0;
+  RCHECK(reader.Read4(&sample_count));
+
+  sample_encryption_entries->resize(sample_count);
+  for (auto& sample_encryption_entry : *sample_encryption_entries) {
+    RCHECK(sample_encryption_entry.ParseFromBuffer(
+        iv_size, flags & kUseSubsampleEncryption, &reader));
+  }
+  return true;
+}
+
 OriginalFormat::OriginalFormat() : format(FOURCC_NULL) {}
 OriginalFormat::~OriginalFormat() {}
 FourCC OriginalFormat::BoxType() const { return FOURCC_FRMA; }
@@ -1440,7 +1583,8 @@ bool Track::ReadWriteInternal(BoxBuffer* buffer) {
          buffer->PrepareChildren() &&
          buffer->ReadWriteChild(&header) &&
          buffer->ReadWriteChild(&media) &&
-         buffer->TryReadWriteChild(&edit));
+         buffer->TryReadWriteChild(&edit) &&
+         buffer->TryReadWriteChild(&sample_encryption));
   return true;
 }
 
@@ -1921,14 +2065,16 @@ bool TrackFragment::ReadWriteInternal(BoxBuffer* buffer) {
            buffer->TryReadWriteChild(&sample_group_description));
   }
   return buffer->TryReadWriteChild(&auxiliary_size) &&
-         buffer->TryReadWriteChild(&auxiliary_offset);
+         buffer->TryReadWriteChild(&auxiliary_offset) &&
+         buffer->TryReadWriteChild(&sample_encryption);
 }
 
 uint32_t TrackFragment::ComputeSizeInternal() {
   uint32_t box_size =
       HeaderSize() + header.ComputeSize() + decode_time.ComputeSize() +
       sample_to_group.ComputeSize() + sample_group_description.ComputeSize() +
-      auxiliary_size.ComputeSize() + auxiliary_offset.ComputeSize();
+      auxiliary_size.ComputeSize() + auxiliary_offset.ComputeSize() +
+      sample_encryption.ComputeSize();
   for (uint32_t i = 0; i < runs.size(); ++i)
     box_size += runs[i].ComputeSize();
   return box_size;

packager/media/formats/mp4/box_definitions.h

@@ -7,6 +7,7 @@
 
 #include <vector>
 
+#include "packager/media/base/decrypt_config.h"
 #include "packager/media/formats/mp4/aac_audio_specific_config.h"
 #include "packager/media/formats/mp4/box.h"
 #include "packager/media/formats/mp4/es_descriptor.h"
@@ -75,6 +76,60 @@ struct SampleAuxiliaryInformationSize : FullBox {
   std::vector<uint8_t> sample_info_sizes;
 };
 
+struct SampleEncryptionEntry {
+  SampleEncryptionEntry();
+  ~SampleEncryptionEntry();
+  /// Read/Write SampleEncryptionEntry.
+  /// @param iv_size specifies the size of initialization vector.
+  /// @param has_subsamples indicates whether this sample encryption entry
+  ///        constains subsamples.
+  /// @param buffer points to the box buffer for reading or writing.
+  /// @return true on success, false otherwise.
+  bool ReadWrite(uint8_t iv_size,
+                 bool has_subsamples,
+                 BoxBuffer* buffer);
+  /// Parse SampleEncryptionEntry from buffer.
+  /// @param iv_size specifies the size of initialization vector.
+  /// @param has_subsamples indicates whether this sample encryption entry
+  ///        constains subsamples.
+  /// @param reader points to the buffer reader. Cannot be NULL.
+  /// @return true on success, false otherwise.
+  bool ParseFromBuffer(uint8_t iv_size,
+                       bool has_subsamples,
+                       BufferReader* reader);
+  /// @return The size of the structure in bytes when it is stored.
+  uint32_t ComputeSize() const;
+  /// @return The accumulated size of subsamples. Returns 0 if there is no
+  ///         subsamples.
+  uint32_t GetTotalSizeOfSubsamples() const;
+
+  std::vector<uint8_t> initialization_vector;
+  std::vector<SubsampleEntry> subsamples;
+};
+
+struct SampleEncryption : FullBox {
+  enum SampleEncryptionFlags {
+    kUseSubsampleEncryption = 2,
+  };
+
+  DECLARE_BOX_METHODS(SampleEncryption);
+  /// Parse from @a sample_encryption_data.
+  /// @param iv_size specifies the size of initialization vector.
+  /// @param[out] sample_encryption_entries receives parsed sample encryption
+  ///             entries.
+  /// @return true on success, false otherwise.
+  bool ParseFromSampleEncryptionData(
+      size_t iv_size,
+      std::vector<SampleEncryptionEntry>* sample_encryption_entries) const;
+
+  /// We may not know @a iv_size before reading this box. In this case, we will
+  /// store sample encryption data for parsing later when @a iv_size is known.
+  std::vector<uint8_t> sample_encryption_data;
+
+  size_t iv_size;
+  std::vector<SampleEncryptionEntry> sample_encryption_entries;
+};
+
 struct OriginalFormat : Box {
   DECLARE_BOX_METHODS(OriginalFormat);
 
@@ -258,6 +313,8 @@ struct SampleDescription : FullBox {
   DECLARE_BOX_METHODS(SampleDescription);
 
   TrackType type;
+  // TODO(kqyang): Clean up the code to have one single member, e.g. by creating
+  // SampleEntry struct, std::vector<SampleEntry> sample_entries.
   std::vector<VideoSampleEntry> video_entries;
   std::vector<AudioSampleEntry> audio_entries;
   std::vector<WVTTSampleEntry> wvtt_entries;
@@ -428,6 +485,7 @@ struct Track : Box {
   TrackHeader header;
   Media media;
   Edit edit;
+  SampleEncryption sample_encryption;
 };
 
 struct MovieExtendsHeader : FullBox {
@@ -569,6 +627,7 @@ struct TrackFragment : Box {
   SampleGroupDescription sample_group_description;
   SampleAuxiliaryInformationSize auxiliary_size;
   SampleAuxiliaryInformationOffset auxiliary_offset;
+  SampleEncryption sample_encryption;
 };
 
 struct MovieFragment : Box {

packager/media/formats/mp4/box_definitions_comparison.h

@@ -13,6 +13,12 @@
 
 namespace edash_packager {
 namespace media {
+
+inline bool operator==(const SubsampleEntry& lhs, const SubsampleEntry& rhs) {
+  return lhs.clear_bytes == rhs.clear_bytes &&
+         lhs.cipher_bytes == rhs.cipher_bytes;
+}
+
 namespace mp4 {
 
 inline bool operator==(const FileType& lhs, const FileType& rhs) {
@@ -38,6 +44,18 @@ inline bool operator==(const SampleAuxiliaryInformationSize& lhs,
          lhs.sample_info_sizes == rhs.sample_info_sizes;
 }
 
+inline bool operator==(const SampleEncryptionEntry& lhs,
+                       const SampleEncryptionEntry& rhs) {
+  return lhs.initialization_vector == rhs.initialization_vector &&
+         lhs.subsamples == rhs.subsamples;
+}
+
+inline bool operator==(const SampleEncryption& lhs,
+                       const SampleEncryption& rhs) {
+  return lhs.iv_size == rhs.iv_size &&
+         lhs.sample_encryption_entries == rhs.sample_encryption_entries;
+}
+
 inline bool operator==(const OriginalFormat& lhs, const OriginalFormat& rhs) {
   return lhs.format == rhs.format;
 }
@@ -274,7 +292,7 @@ inline bool operator==(const Media& lhs, const Media& rhs) {
 
 inline bool operator==(const Track& lhs, const Track& rhs) {
   return lhs.header == rhs.header && lhs.media == rhs.media &&
-         lhs.edit == rhs.edit;
+         lhs.edit == rhs.edit && lhs.sample_encryption == rhs.sample_encryption;
 }
 
 inline bool operator==(const MovieExtendsHeader& lhs,
@@ -360,7 +378,8 @@ inline bool operator==(const TrackFragment& lhs, const TrackFragment& rhs) {
   return lhs.header == rhs.header && lhs.runs == rhs.runs &&
          lhs.decode_time == rhs.decode_time &&
          lhs.auxiliary_offset == rhs.auxiliary_offset &&
-         lhs.auxiliary_size == rhs.auxiliary_size;
+         lhs.auxiliary_size == rhs.auxiliary_size &&
+         lhs.sample_encryption == rhs.sample_encryption;
 }
 
 inline bool operator==(const MovieFragment& lhs, const MovieFragment& rhs) {

packager/media/formats/mp4/box_definitions_unittest.cc

@@ -18,6 +18,7 @@ namespace edash_packager {
 namespace media {
 namespace mp4 {
 namespace {
+const uint8_t kData8Bytes[] = {3, 4, 5, 6, 7, 8, 9, 0};
 const uint8_t kData16Bytes[] = {8, 7, 6, 5, 4, 3, 2, 1, 1, 2, 3, 4, 5, 6, 7, 8};
 const uint8_t kData4[] = {1, 5, 4, 3, 15};
 const uint8_t kData8[] = {1, 8, 42, 98, 156};
@@ -161,6 +162,30 @@ class BoxDefinitionsTestGeneral : public testing::Test {
     saiz->sample_info_sizes.clear();
   }
 
+  void Fill(SampleEncryption* senc) {
+    senc->iv_size = 8;
+    senc->flags = SampleEncryption::kUseSubsampleEncryption;
+    senc->sample_encryption_entries.resize(2);
+    senc->sample_encryption_entries[0].initialization_vector.assign(
+        kData8Bytes, kData8Bytes + arraysize(kData8Bytes));
+    senc->sample_encryption_entries[0].subsamples.resize(2);
+    senc->sample_encryption_entries[0].subsamples[0].clear_bytes = 17;
+    senc->sample_encryption_entries[0].subsamples[0].cipher_bytes = 3456;
+    senc->sample_encryption_entries[0].subsamples[1].clear_bytes = 1543;
+    senc->sample_encryption_entries[0].subsamples[1].cipher_bytes = 0;
+    senc->sample_encryption_entries[1] = senc->sample_encryption_entries[0];
+    senc->sample_encryption_entries[1].subsamples[0].clear_bytes = 0;
+    senc->sample_encryption_entries[1].subsamples[0].cipher_bytes = 15;
+    senc->sample_encryption_entries[1].subsamples[1].clear_bytes = 1988;
+    senc->sample_encryption_entries[1].subsamples[1].cipher_bytes = 8765;
+  }
+
+  void Modify(SampleEncryption* senc) {
+    senc->flags = 0;
+    senc->sample_encryption_entries.resize(1);
+    senc->sample_encryption_entries[0].subsamples.clear();
+  }
+
   void Fill(OriginalFormat* frma) { frma->format = FOURCC_AVC1; }
 
   void Modify(OriginalFormat* frma) { frma->format = FOURCC_MP4A; }
@@ -817,6 +842,7 @@ class BoxDefinitionsTestGeneral : public testing::Test {
 
   bool IsOptional(const SampleAuxiliaryInformationOffset* box) { return true; }
   bool IsOptional(const SampleAuxiliaryInformationSize* box) { return true; }
+  bool IsOptional(const SampleEncryption* box) { return true; }
   bool IsOptional(const ProtectionSchemeInfo* box) { return true; }
   bool IsOptional(const EditList* box) { return true; }
   bool IsOptional(const Edit* box) { return true; }
@@ -1066,6 +1092,48 @@ TEST_F(BoxDefinitionsTest, TrackFragmentRun_NoSampleSize) {
   ASSERT_EQ(trun, trun_readback);
 }
 
+TEST_F(BoxDefinitionsTest, SampleEncryptionIsOptional) {
+  SampleEncryption senc;
+  EXPECT_EQ(0u, senc.ComputeSize());
+}
+
+TEST_F(BoxDefinitionsTest, SampleEncryptionWithIvKnownWhenReading) {
+  SampleEncryption senc;
+  Fill(&senc);
+  senc.Write(buffer_.get());
+
+  SampleEncryption senc_readback;
+  senc_readback.iv_size = senc.iv_size;
+
+  ASSERT_TRUE(ReadBack(&senc_readback));
+  EXPECT_EQ(0u, senc_readback.sample_encryption_data.size());
+  EXPECT_NE(0u, senc_readback.sample_encryption_entries.size());
+  ASSERT_EQ(senc, senc_readback);
+
+  Modify(&senc);
+  senc.Write(buffer_.get());
+  ASSERT_TRUE(ReadBack(&senc_readback));
+  ASSERT_EQ(senc, senc_readback);
+}
+
+TEST_F(BoxDefinitionsTest, SampleEncryptionWithIvUnknownWhenReading) {
+  SampleEncryption senc;
+  Fill(&senc);
+  senc.Write(buffer_.get());
+
+  SampleEncryption senc_readback;
+  senc_readback.iv_size = 0;
+
+  ASSERT_TRUE(ReadBack(&senc_readback));
+  EXPECT_NE(0u, senc_readback.sample_encryption_data.size());
+  EXPECT_EQ(0u, senc_readback.sample_encryption_entries.size());
+
+  std::vector<SampleEncryptionEntry> sample_encryption_entries;
+  ASSERT_TRUE(senc_readback.ParseFromSampleEncryptionData(
+      senc.iv_size, &sample_encryption_entries));
+  ASSERT_EQ(senc.sample_encryption_entries, sample_encryption_entries);
+}
+
 }  // namespace mp4
 }  // namespace media
 }  // namespace edash_packager

packager/media/formats/mp4/fourccs.h

@@ -73,6 +73,7 @@ enum FourCC {
   FOURCC_SCHM = 0x7363686d,
   FOURCC_SDTP = 0x73647470,
   FOURCC_SEIG = 0x73656967,
+  FOURCC_SENC = 0x73656e63,
   FOURCC_SGPD = 0x73677064,
   FOURCC_SIDX = 0x73696478,
   FOURCC_SINF = 0x73696e66,

packager/media/formats/mp4/mp4_media_parser.h

@@ -74,8 +74,8 @@ class MP4MediaParser : public MediaParser {
                            uint8_t* buffer,
                            size_t buffer_size);
 
-  // To retain proper framing, each 'mdat' atom must be read; to limit memory
+  // To retain proper framing, each 'mdat' box must be read; to limit memory
-  // usage, the atom's data needs to be discarded incrementally as frames are
+  // usage, the box's data needs to be discarded incrementally as frames are
   // extracted from the stream. This function discards data from the stream up
   // to |offset|, updating the |mdat_tail_| value so that framing can be
   // retained after all 'mdat' information has been read.

packager/media/formats/mp4/mp4_muxer.h

@@ -42,7 +42,7 @@ class MP4Muxer : public Muxer {
   Status DoAddSample(const MediaStream* stream,
                      scoped_refptr<MediaSample> sample) override;
 
-  // Generate Audio/Video Track atom.
+  // Generate Audio/Video Track box.
   void InitializeTrak(const StreamInfo* info, Track* trak);
   void GenerateAudioTrak(const AudioStreamInfo* audio_info,
                          Track* trak,