video/buffered_frame_decryptor.cc

/*
 *  Copyright (c) 2018 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#include "video/buffered_frame_decryptor.h"

#include <utility>

#include "rtc_base/logging.h"
#include "rtc_base/system/fallthrough.h"
#include "system_wrappers/include/field_trial.h"

namespace webrtc {

BufferedFrameDecryptor::BufferedFrameDecryptor(
    OnDecryptedFrameCallback* decrypted_frame_callback,
    rtc::scoped_refptr<FrameDecryptorInterface> frame_decryptor)
    : generic_descriptor_auth_experiment_(
          field_trial::IsEnabled("WebRTC-GenericDescriptorAuth")),
      frame_decryptor_(std::move(frame_decryptor)),
      decrypted_frame_callback_(decrypted_frame_callback) {}

BufferedFrameDecryptor::~BufferedFrameDecryptor() {}

void BufferedFrameDecryptor::ManageEncryptedFrame(
    std::unique_ptr<video_coding::RtpFrameObject> encrypted_frame) {
  switch (DecryptFrame(encrypted_frame.get())) {
    case FrameDecision::kStash:
      if (stashed_frames_.size() >= kMaxStashedFrames) {
        stashed_frames_.pop_front();
      }
      stashed_frames_.push_back(std::move(encrypted_frame));
      break;
    case FrameDecision::kDecrypted:
      RetryStashedFrames();
      decrypted_frame_callback_->OnDecryptedFrame(std::move(encrypted_frame));
      break;
    case FrameDecision::kDrop:
      break;
  }
}

BufferedFrameDecryptor::FrameDecision BufferedFrameDecryptor::DecryptFrame(
    video_coding::RtpFrameObject* frame) {
  // Optionally attempt to decrypt the raw video frame if it was provided.
  if (frame_decryptor_ == nullptr) {
    RTC_LOG(LS_WARNING) << "Frame decryption required but not attached to this "
                           "stream. Dropping frame.";
    return FrameDecision::kDrop;
  }
  // When using encryption we expect the frame to have the generic descriptor.
  absl::optional<RtpGenericFrameDescriptor> descriptor =
      frame->GetGenericFrameDescriptor();
  if (!descriptor) {
    RTC_LOG(LS_ERROR) << "No generic frame descriptor found dropping frame.";
    return FrameDecision::kDrop;
  }
  // Retrieve the bitstream of the encrypted video frame.
  rtc::ArrayView<const uint8_t> encrypted_frame_bitstream(frame->Buffer(),
                                                          frame->size());
  // Retrieve the maximum possible size of the decrypted payload.
  const size_t max_plaintext_byte_size =
      frame_decryptor_->GetMaxPlaintextByteSize(cricket::MEDIA_TYPE_VIDEO,
                                                frame->size());
  RTC_CHECK_LE(max_plaintext_byte_size, frame->size());
  // Place the decrypted frame inline into the existing frame.
  rtc::ArrayView<uint8_t> inline_decrypted_bitstream(frame->MutableBuffer(),
                                                     max_plaintext_byte_size);

  // Only enable authenticating the header if the field trial is enabled.
  rtc::ArrayView<const uint8_t> additional_data;
  if (generic_descriptor_auth_experiment_) {
    additional_data = descriptor->GetByteRepresentation();
  }

  // Attempt to decrypt the video frame.
  size_t bytes_written = 0;
  if (frame_decryptor_->Decrypt(cricket::MEDIA_TYPE_VIDEO, /*csrcs=*/{},
                                additional_data, encrypted_frame_bitstream,
                                inline_decrypted_bitstream,
                                &bytes_written) != 0) {
    // Only stash frames if we have never decrypted a frame before.
    return first_frame_decrypted_ ? FrameDecision::kDrop
                                  : FrameDecision::kStash;
  }
  RTC_CHECK_LE(bytes_written, max_plaintext_byte_size);
  // Update the frame to contain just the written bytes.
  frame->SetSize(bytes_written);

  // Indicate that all future fail to decrypt frames should be dropped.
  if (!first_frame_decrypted_) {
    first_frame_decrypted_ = true;
  }

  return FrameDecision::kDecrypted;
}

void BufferedFrameDecryptor::RetryStashedFrames() {
  for (auto& frame : stashed_frames_) {
    if (DecryptFrame(frame.get()) == FrameDecision::kDecrypted) {
      decrypted_frame_callback_->OnDecryptedFrame(std::move(frame));
    }
  }
  stashed_frames_.clear();
}

}  // namespace webrtc