Add Common Encryption support to BMFF, including subsample decryption.

media/mp4/cenc.cc

Index: media/mp4/cenc.cc
diff --git a/media/mp4/cenc.cc b/media/mp4/cenc.cc
index 996ebb12c28c50fe3b3c520ff78e0f6ae47854f5..ee2721d293e23de91a7dab66cbc4307076f80eb5 100644
--- a/media/mp4/cenc.cc
+++ b/media/mp4/cenc.cc
@@ -4,6 +4,8 @@
 
 #include "media/mp4/cenc.h"
 
+#include <string.h>

[ddorwin, 2012/06/26 06:09:19]

Why is this needed?

[strobe_, 2012/06/27 02:01:21]

memset().

+
 #include "media/mp4/box_reader.h"
 #include "media/mp4/rcheck.h"
 
@@ -15,20 +17,27 @@ FrameCENCInfo::~FrameCENCInfo() {}
 
 bool FrameCENCInfo::Parse(int iv_size, BufferReader* reader) {
   const int kEntrySize = 6;
-
   // Mandated by CENC spec
   RCHECK(iv_size == 8 || iv_size == 16);
-  iv.resize(iv_size);
+
+  memset(iv, 0, sizeof(iv));
+  for (int i = 0; i < iv_size; i++)
+    RCHECK(reader->Read1(&iv[i]));

[ddorwin, 2012/06/26 06:09:19]

Reading one byte at a time isn't the fastest solution, especially when the spec
only allows multiples of 8. Why not read in muliples of 8 (or 4 if 8 is not
supported)?

[strobe_, 2012/06/27 02:01:21]

Given the buffer's representation as a uint8 string elsewhere, I felt it was
simpler to avoid issues such as endianness conversion that would come with a
more complicated read scheme. If you'd like, we could store IVs as one or two
uint64s in the DecryptConfig, perform host-to-network conversion on reads, and
then perform network-to-host conversion when handling the data in AesDecryptor,
but this seems like it wouldn't result in much of a performance benefit overall.

+
+  if (!reader->HasBytes(1)) return true;
 
   uint16 subsample_count;
-  RCHECK(reader->ReadVec(&iv, iv_size) &&
-         reader->Read2(&subsample_count) &&
+  RCHECK(reader->Read2(&subsample_count) &&
          reader->HasBytes(subsample_count * kEntrySize));
-  subsamples.resize(subsample_count);
 
+  subsamples.resize(subsample_count);
   for (int i = 0; i < subsample_count; i++) {
-    RCHECK(reader->Read2(&subsamples[i].clear_size) &&
-           reader->Read4(&subsamples[i].encrypted_size));
+    uint16 clear_bytes;
+    uint32 cypher_bytes;
+    RCHECK(reader->Read2(&clear_bytes) &&

[ddorwin, 2012/06/26 06:09:19]

Why not eliminate 4 lines by reading into the vector element directly?

[strobe_, 2012/06/27 02:01:21]

I modified the vector element to use the definition of SubsampleEntry in
decrypt_config.h, which has different types from those used in the BMFF file.

+           reader->Read4(&cypher_bytes));
+    subsamples[i].clear_bytes = clear_bytes;
+    subsamples[i].cypher_bytes = cypher_bytes;
   }
   return true;
 }
@@ -36,7 +45,7 @@ bool FrameCENCInfo::Parse(int iv_size, BufferReader* reader) {
 size_t FrameCENCInfo::GetTotalSize() const {

[ddorwin, 2012/06/26 06:09:19]

"Total" appears to not include the iv. Maybe the function name can be more
specific.

[strobe_, 2012/06/27 02:01:21]

Done.

   size_t size = 0;
   for (size_t i = 0; i < subsamples.size(); i++) {
-    size += subsamples[i].clear_size + subsamples[i].encrypted_size;
+    size += subsamples[i].clear_bytes + subsamples[i].cypher_bytes;
   }
   return size;
 }