Add 'cbcs' encryption scheme support in Android media.

media/base/encryption_scheme.h

 // Copyright 2015 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.
 
 #ifndef MEDIA_BASE_ENCRYPTION_SCHEME_H_
 #define MEDIA_BASE_ENCRYPTION_SCHEME_H_
 
 #include <stdint.h>
 
 #include "media/base/media_export.h"
@@ skipping 19 matching lines @@
   // actual encryption applied through a repeating pattern of (encrypt:skip)
   // 16 byte blocks. For example, in a (1:9) pattern, the first block is
   // encrypted, and the next nine are skipped. This pattern is applied
   // repeatedly until the end of the last 16-byte block in the subsample.
   // Any remaining bytes are left clear.
   // If either of encrypt_blocks or skip_blocks is 0, pattern encryption is
   // disabled.
   class MEDIA_EXPORT Pattern {
    public:
     Pattern();
     Pattern(uint32_t encrypt_blocks, uint32_t skip_blocks);

[xhwang, 2016/10/20 23:52:07]

the member variables are using uint8_t but the input here are uint32_t

[dougsteed, 2016/10/21 17:15:05]

See my comment below.

     ~Pattern();
 
     bool Matches(const Pattern& other) const;
 
-    uint32_t encrypt_blocks() const { return encrypt_blocks_; }
-    uint32_t skip_blocks() const { return skip_blocks_; }
+    uint8_t encrypt_blocks() const { return encrypt_blocks_; }
+    uint8_t skip_blocks() const { return skip_blocks_; }
 
     bool IsInEffect() const;
 
    private:
-    uint32_t encrypt_blocks_ = 0;
-    uint32_t skip_blocks_ = 0;
+    uint8_t encrypt_blocks_ = 0;
+    uint8_t skip_blocks_ = 0;

[xhwang, 2016/10/20 23:52:07]

why do we make this change? Does it make sense to just use a simple type like
"int"?

Also, in general we don't use unsigned integers only because they can't be
negative:
https://google.github.io/styleguide/cppguide.html#Integer_Types

[dougsteed, 2016/10/21 17:15:05]

I am reverting this for this CL. The uint32_t originated from an error in my
reading of the spec. These numbers are allocated 4 bits (0-15) in the standard
whereas I had originally read that as 4 bytes. When I realized this error I
changed it to uint8_t (inconsistently). In any case I agree that there is
probably no reason not to just use int. But I think this probably for a separate
CL, so will leave it unchanged for now.

 
     // Allow copy and assignment.
   };
 
   // The default constructor makes an instance that indicates no encryption.
   EncryptionScheme();
 
   // This constructor allows specification of the cipher mode and the pattern.
   EncryptionScheme(CipherMode mode, const Pattern& pattern);
   ~EncryptionScheme();
 
   bool Matches(const EncryptionScheme& other) const;
 
   bool is_encrypted() const { return mode_ != CIPHER_MODE_UNENCRYPTED; }
   CipherMode mode() const { return mode_; }
   const Pattern& pattern() const { return pattern_; }
 
  private:
   CipherMode mode_ = CIPHER_MODE_UNENCRYPTED;
   Pattern pattern_;
 
   // Allow copy and assignment.
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_ENCRYPTION_SCHEME_H_