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

media/webm/webm_cluster_parser.cc

Index: media/webm/webm_cluster_parser.cc
diff --git a/media/webm/webm_cluster_parser.cc b/media/webm/webm_cluster_parser.cc
index be5e724c14af8d4f0dc23c4f1d98b7e401a031be..b884b53c0a747c9a5ea5294f69bbb1d13c75bf86 100644
--- a/media/webm/webm_cluster_parser.cc
+++ b/media/webm/webm_cluster_parser.cc
@@ -5,12 +5,34 @@
 #include "media/webm/webm_cluster_parser.h"
 
 #include "base/logging.h"
+#include "base/sys_byteorder.h"
 #include "media/base/data_buffer.h"
 #include "media/base/decrypt_config.h"
 #include "media/webm/webm_constants.h"
 
 namespace media {
 
+// Generates a 16 byte CTR counter block. The CTR counter block format is a
+// CTR IV appended with a CTR block counter. |iv| is a CTR IV. |iv_size| is
+// the size of |iv| in bytes. Returns counter block on success. Returns empty
+// string on failure.
+static std::string GenerateCounterBlock(const uint8* iv, int iv_size) {
+  if (iv_size != WebMClusterParser::kIvSize)
+    return std::string();
+
+  char counter_block_data[WebMClusterParser::kDecryptionKeySize];
+
+  // Set the IV.
+  memcpy(counter_block_data, iv, iv_size);
+
+  // Set block counter to all 0's.
+  memset(counter_block_data + iv_size,
+         0,
+         WebMClusterParser::kDecryptionKeySize - iv_size);
+
+  return std::string(counter_block_data, WebMClusterParser::kDecryptionKeySize);
+}
+
 WebMClusterParser::WebMClusterParser(int64 timecode_scale,
                                      int audio_track_num,
                                      int video_track_num,
@@ -193,15 +215,36 @@ bool WebMClusterParser::OnBlock(int track_num, int timecode,
   base::TimeDelta timestamp = base::TimeDelta::FromMicroseconds(
       (cluster_timecode_ + timecode) * timecode_multiplier_);
 
+  // Every encrypted Block has an HMAC and IV prepended to it. Current encrypted
+  // WebM request for comments specification is here
+  // http://wiki.webmproject.org/encryption/webm-encryption-rfc
+  bool encrypted = track_num == video_.track_num() &&
+                   video_encryption_key_id_.get();

[ddorwin, 2012/07/14 00:50:31]

any idea why this is "video_"?

[fgalligan1, 2012/07/16 23:51:42]

No, besides the usual private member to the class.

+  // If encrypted skip past the HMAC. Encrypted buffers must include the IV and
+  // the encrypted frame because the decryptor will verify this data before
+  // decryption. The HMAC and IV will be copied into DecryptConfig.
+  int offset = (encrypted) ? kWebMHmacSize : 0;
+
   // The first bit of the flags is set when the block contains only keyframes.
   // http://www.matroska.org/technical/specs/index.html
   bool is_keyframe = (flags & 0x80) != 0;
   scoped_refptr<StreamParserBuffer> buffer =
-      StreamParserBuffer::CopyFrom(data, size, is_keyframe);
+      StreamParserBuffer::CopyFrom(data + offset, size - offset, is_keyframe);
+
+  if (encrypted) {
+    uint64 network_iv;
+    memcpy(&network_iv, data + kWebMHmacSize, sizeof(network_iv));
+    const uint64 iv = base::NetToHost64(network_iv);
 
-  if (track_num == video_.track_num() && video_encryption_key_id_.get()) {
+    std::string counter_block =
+        GenerateCounterBlock(reinterpret_cast<const uint8*>(&iv), sizeof(iv));
+    const uint8* counter_block_data =
+        reinterpret_cast<const uint8*>(counter_block.data());
     buffer->SetDecryptConfig(scoped_ptr<DecryptConfig>(new DecryptConfig(
-        video_encryption_key_id_.get(), video_encryption_key_id_size_)));
+        video_encryption_key_id_.get(), video_encryption_key_id_size_,
+        counter_block_data, counter_block.size(),
+        data, kWebMHmacSize,
+        sizeof(iv))));
   }
 
   buffer->SetTimestamp(timestamp);