media config: expand is_encrypted to a struct.

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>
+
+namespace media {
+
+// Specification of whether and how the stream is encrypted (in whole or part).
+class EncryptionScheme {
+ public:
+  // Algorithm and mode used for encryption.
+  enum CipherMode {
+    kCipherModeNone,
+    kCipherModeAesCtr,
+    kCipherModeAesCbc,
+    kCipherModeMax = kCipherModeAesCbc
+  };
+
+  // V3 of the CENC standard adds pattern encryption, through two new
+  // protection schemes 'cens' (with AES-CTR) and 'cbcs' (with AES-CBC).
+  // The pattern applies only to the 'encrypted' part of the frame (as
+  // defined by the relevant subsample entries), and reduces further the
+  // 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 or both of encrypt_blocks or skip_blocks is 0, pattern
+  // encryption is disabled.
+  class PatternSpec {
+   public:
+    PatternSpec();
+    PatternSpec(uint32_t encrypt_blocks, uint32_t skip_blocks);
+    ~PatternSpec();
+
+    bool Matches(const PatternSpec& other) const;
+
+    uint32_t encrypt_blocks() const { return encrypt_blocks_; }
+    uint32_t skip_blocks() const { return skip_blocks_; }
+
+    bool IsInEffect() const;
+
+   private:
+    uint32_t encrypt_blocks_;
+    uint32_t skip_blocks_;
+
+    // Allow copy and assignment.
+  };
+
+  EncryptionScheme();
+  explicit EncryptionScheme(bool is_encrypted);

[halliwell, 2016/01/13 03:29:42]

As mentioned in downstream CLs, I don't like callsites that look like this:

EncryptionScheme(false);

as it's not immediately clear what 'false' refers to.  I'm not clear why we need
this constructor anyway if 'none' means unencrypted?

[dougsteed, 2016/02/09 22:58:54]

Following our discussion, I added a static function "unencrypted" that returns
an EncryptionScheme object with mode of none. This covers the vast majority of
cases sprinkled around chromium.

+  explicit EncryptionScheme(CipherMode mode);
+  EncryptionScheme(CipherMode mode, const PatternSpec& pattern);
+  ~EncryptionScheme();
+
+  void Initialize(CipherMode mode, const PatternSpec& pattern);
+  bool Matches(const EncryptionScheme& other) const;
+
+  bool is_encrypted() const;
+  CipherMode mode() const { return mode_; }
+  const PatternSpec& pattern() const { return pattern_; }
+
+ private:
+  CipherMode mode_;
+  PatternSpec pattern_;
+
+  // Allow copy and assignment.
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_ENCRYPTION_SCHEME_H_