Add support for encrypted WebM files as defined in the RFC.

media/crypto/decryptor.cc

+// Copyright (c) 2012 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/crypto/decryptor.h"
+
+#include "base/logging.h"
+#include "crypto/encryptor.h"
+#include "media/base/decoder_buffer.h"
+#include "media/base/decrypt_config.h"
+
+namespace media {
+
+Decryptor::Decryptor() {}
+
+Decryptor::~Decryptor() {}
+
+scoped_refptr<DecoderBuffer> Decryptor::DecryptData(const DecoderBuffer& input,
+                                                    crypto::SymmetricKey* key,
+                                                    int offset) {
+  CHECK(input.GetDataSize());
+  CHECK(input.GetDecryptConfig());
+  CHECK(key);
+
+  // Initialize encryption data.
+  crypto::Encryptor encryptor;
+  if (!encryptor.Init(key, crypto::Encryptor::CTR, "")) {
+    DVLOG(1) << "Could not initialize encryptor.";
+    return NULL;
+  }
+
+  // Set the counter block.
+  std::string counter_block =
+      Decryptor::GenerateCounterBlock(input.GetDecryptConfig()->iv());
+  if (counter_block.empty()) {
+    DVLOG(1) << "Could not generate counter block.";
+    return NULL;
+  }
+  if (!encryptor.SetCounter(counter_block)) {
+    DVLOG(1) << "Could not set counter block.";
+    return NULL;
+  }
+
+  std::string decrypted_text;
+  const char* frame = reinterpret_cast<const char*>(input.GetData() + offset);
+  int frame_size = input.GetDataSize() - offset;
+  base::StringPiece encrypted_text(frame, frame_size);
+  if (!encryptor.Decrypt(encrypted_text, &decrypted_text)) {
+    DVLOG(1) << "Could not decrypt data.";
+    return NULL;
+  }
+
+  // TODO(xhwang): Find a way to avoid this data copy.
+  return DecoderBuffer::CopyFrom(
+      reinterpret_cast<const uint8*>(decrypted_text.data()),
+      decrypted_text.size());
+}
+
+std::string Decryptor::GenerateCounterBlock(uint64 iv) {
+  char counter_block_data[DecryptConfig::kKeySize];
+
+  // Set the IV.
+  memcpy(counter_block_data, &iv, sizeof(iv));
+
+  // Set block counter to all 0's.
+  memset(counter_block_data + sizeof(iv),
+         0,
+         DecryptConfig::kKeySize - sizeof(iv));
+
+  std::string counter_block(counter_block_data, DecryptConfig::kKeySize);
+  return counter_block;
+}
+
+}  // namespace media