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..7a7a1413558362d6893ab2321d6584c691472680
--- /dev/null
+++ b/media/webm/webm_crypto_helpers.cc
@@ -0,0 +1,51 @@
+// 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 {
+
+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;
+}
+
+scoped_ptr<DecryptConfig> WebMCreateDecryptConfig(
+    const uint8* data, int data_size,
+    const uint8* key_id, int key_id_size) {
+  CHECK(data_size >= kWebMSignalByteSize)

[ddorwin, 2013/03/10 21:29:39]

Is this a programming or media error?

[fgalligan1, 2013/03/12 00:42:42]

Could be either I guess. But most likely a media error. I added a unittest to
chunk_demuxer to test this code.

+        << "Got an encrypted block with not enough data "

[ddorwin, 2013/03/10 21:29:39]

I assume this is included in the prod binary. Should probably be short.
Something like "Invalid data size: "

[fgalligan1, 2013/03/12 00:42:42]

I copied this from media source. Should I change it?

[ddorwin, 2013/03/12 04:40:06]

It's only non-D CHECK strings that (might) remain in prod binary.

+        << data_size;
+  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) {

[ddorwin, 2013/03/10 21:29:39]

Should we DLOG_IF(unsupported bits set) somewhere? Is this the common location
for all paths for processing WebM files?

[fgalligan1, 2013/03/12 00:42:42]

I changed this up to return a NULL scoped_ptr so we could reuse this code with
MediaSource.

+    // IVs are only present on encrypted frames.
+    CHECK(data_size >= kWebMSignalByteSize + kWebMIvSize)

[ddorwin, 2013/03/10 21:29:39]

same

[fgalligan1, 2013/03/12 00:42:42]

ditto

+        << "Got an encrypted block with not enough data "
+        << data_size;
+    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