Support for parsing encrypted WebM streams by src.

media/webm/webm_crypto_helpers.cc

Index: media/webm/webm_crypto_helpers.cc
diff --git a/media/webm/webm_crypto_helpers.cc b/media/webm/webm_crypto_helpers.cc
new file mode 100644
index 0000000000000000000000000000000000000000..ea63f87d182d06b20367f90616a96b017bcf615a
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+++ b/media/webm/webm_crypto_helpers.cc
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+// Copyright (c) 2013 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/webm/webm_crypto_helpers.h"
+
+#include "base/logging.h"
+#include "base/sys_byteorder.h"
+#include "media/base/decrypt_config.h"
+#include "media/webm/webm_constants.h"
+
+namespace media {
+namespace {
+
+// Generates a 16 byte CTR counter block. The CTR counter block format is a
+// CTR IV appended with a CTR block counter. |iv| is an 8 byte CTR IV.
+// |iv_size| is the size of |iv| in btyes. Returns a string of
+// kDecryptionKeySize bytes.
+std::string GenerateWebMCounterBlock(const uint8* iv, int iv_size) {
+  std::string counter_block(reinterpret_cast<const char*>(iv), iv_size);
+  counter_block.append(DecryptConfig::kDecryptionKeySize - iv_size, 0);
+  return counter_block;
+}
+
+}  // namespace anonymous
+
+scoped_ptr<DecryptConfig> WebMCreateDecryptConfig(
+    const uint8* data, int data_size,
+    const uint8* key_id, int key_id_size) {
+  if (data_size < kWebMSignalByteSize) {
+    DVLOG(1) << "Got a block from an encrypted stream with no data.";
+    return scoped_ptr<DecryptConfig>(NULL);
+  }
+
+  uint8 signal_byte = data[0];
+  int frame_offset = sizeof(signal_byte);
+
+  // Setting the DecryptConfig object of the buffer while leaving the
+  // initialization vector empty will tell the decryptor that the frame is
+  // unencrypted.
+  std::string counter_block;
+
+  if (signal_byte & kWebMFlagEncryptedFrame) {
+    if (data_size < kWebMSignalByteSize + kWebMIvSize) {
+      DVLOG(1) << "Got an encrypted block with not enough data " << data_size;
+      return scoped_ptr<DecryptConfig>(NULL);
+    }
+    counter_block = GenerateWebMCounterBlock(data + frame_offset, kWebMIvSize);
+    frame_offset += kWebMIvSize;
+  }
+
+  scoped_ptr<DecryptConfig> config(new DecryptConfig(
+      std::string(reinterpret_cast<const char*>(key_id), key_id_size),
+      counter_block,
+      frame_offset,
+      std::vector<SubsampleEntry>()));
+  return config.Pass();
+}
+
+}  // namespace media