Declare intent to use sample-aes later in the stream.

media/formats/mp2t/descriptors.cc

Index: media/formats/mp2t/descriptors.cc
diff --git a/media/formats/mp2t/descriptors.cc b/media/formats/mp2t/descriptors.cc
index ecfee57532f3dfac2a26669264fca4df38f28cc4..859ec84d4a9e04739a8f1d8737b9889aaaba6b3b 100644
--- a/media/formats/mp2t/descriptors.cc
+++ b/media/formats/mp2t/descriptors.cc
@@ -8,6 +8,7 @@
 
 #include "base/logging.h"
 #include "media/base/bit_reader.h"
+#include "media/base/encryption_scheme.h"
 #include "media/formats/mp2t/mp2t_common.h"
 
 namespace media {
@@ -24,6 +25,7 @@ enum DescriptorTag {
 };
 
 const int kCASystemIdCenc = 0x6365;  // 'ce'
+const uint32_t kFourccCbcs = 0x63626373;  // 'cbcs'
 
 class StringBitReader : public BitReader {
  public:
@@ -110,7 +112,9 @@ bool Descriptors::HasCADescriptor(int* system_id,
   return true;
 }
 
-bool Descriptors::HasCADescriptorCenc(int* ca_pid, int* pssh_pid) const {
+bool Descriptors::HasCADescriptorCenc(int* ca_pid,
+                                      int* pssh_pid,
+                                      EncryptionScheme* scheme) const {
   DCHECK(ca_pid);
   DCHECK(pssh_pid);
   int system_id;
@@ -125,10 +129,12 @@ bool Descriptors::HasCADescriptorCenc(int* ca_pid, int* pssh_pid) const {
   int num_systems;
   int encryption_algorithm;
   char pssh_system_id[16];
-  // TODO(dougsteed). Currently we don't check many of the following values.
+  // TODO(dougsteed). Currently we don't check many of the following values,
+  // and we only support the 'cbcs' scheme (which involves AES-CBC encryption).
   // When we flesh out this implementation to cover all of ISO/IEC 23001-9 we
-  // will need to use and check these values.
+  // will need to use and check these values more comprehensively.
   RCHECK(reader.ReadBits(32, &scheme_type));
+  RCHECK(scheme_type == kFourccCbcs);
   RCHECK(reader.ReadBits(32, &scheme_version));
   RCHECK(reader.ReadBits(8, &num_systems));
   RCHECK(num_systems == 1);
@@ -137,6 +143,10 @@ bool Descriptors::HasCADescriptorCenc(int* ca_pid, int* pssh_pid) const {
     RCHECK(reader.ReadBits(8, &pssh_system_id[i]));
   }
   RCHECK(reader.ReadBits(13, pssh_pid));

[kqyang, 2017/04/07 23:33:43]

Are you expecting the inserted ca descriptor to contain a valid pssh pid? If
not, how are you going to deal with it? If yes, how do you ensure that?

[dougsteed, 2017/04/26 20:34:14]

The parser expects a 13-bit field which is interpreted as a PID, and will fail
if it's not present. In the scenario captured by this CL, the PSSH PID is not
actually used, i.e. no packet is actually received on that PID. But we still
require the CA descriptor because of other fields we do use.

+  // The pattern is actually set differently for audio and video, so OK not to
+  // set it here. Important thing is to set the cipher mode.
+  *scheme = EncryptionScheme(EncryptionScheme::CIPHER_MODE_AES_CBC,
+                             EncryptionScheme::Pattern());
   return true;
 }