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

media/crypto/aes_decryptor.h

 // Copyright (c) 2012 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_CRYPTO_AES_DECRYPTOR_H_
 #define MEDIA_CRYPTO_AES_DECRYPTOR_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/hash_tables.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/string_piece.h"
 #include "base/synchronization/lock.h"
 #include "media/base/decryptor.h"
 #include "media/base/media_export.h"
 
 namespace crypto {
 class SymmetricKey;
 }
 
 namespace media {
 
 class DecryptorClient;
 
-// Decryptor implementation that decrypts AES-encrypted buffer.
+// Decrypts an AES encrypted buffer into an unencrypted buffer. The AES
+// encryption must be CTR with a key size of 128bits. Optionally checks the
+// integrity of the encrypted data.
 class MEDIA_EXPORT AesDecryptor : public Decryptor {
  public:
   // The AesDecryptor does not take ownership of the |client|. The |client|
   // must be valid throughout the lifetime of the AesDecryptor.
   explicit AesDecryptor(DecryptorClient* client);
   virtual ~AesDecryptor();
 
   // Decryptor implementation.
   virtual void GenerateKeyRequest(const std::string& key_system,
                                   const uint8* init_data,
                                   int init_data_length) OVERRIDE;
   virtual void AddKey(const std::string& key_system,
                       const uint8* key,
                       int key_length,
                       const uint8* init_data,
                       int init_data_length,
                       const std::string& session_id) OVERRIDE;
   virtual void CancelKeyRequest(const std::string& key_system,
                                 const std::string& session_id) OVERRIDE;
+
+  // Decrypts |input| buffer. |input| should not be NULL. |input| will signal
+  // if an integrity check must be performed before decryption. Return a
+  // DecoderBuffer with the decrypted data if the decryption succeeded. Return
+  // NULL if the integrity check or decryption failed.
   virtual scoped_refptr<DecoderBuffer> Decrypt(
       const scoped_refptr<DecoderBuffer>& input) OVERRIDE;
 
  private:
-  // KeyMap owns the crypto::SymmetricKey* and must delete them when they are
+  // Helper class that manages the decryption key and HMAC key. The HMAC key
+  // may be NULL.
+  class DecryptionKey {
+   public:
+    explicit DecryptionKey(const std::string& secret);
+    ~DecryptionKey();
+
+    // Creates the encryption key and HMAC. If |derive_webm_keys| is true then
+    // the object will derive the decryption key and the HMAC key from
+    // |secret_|.
+    bool Init(bool derive_webm_keys);
+
+    crypto::SymmetricKey* decryption_key() { return decryption_key_.get(); }
+    base::StringPiece hmac_key() { return base::StringPiece(hmac_key_); }
+
+   private:
+    // The base secret that is used to derive the decryption key and optionally
+    // the HMAC key.
+    const std::string secret_;
+
+    // The key used to decrypt the data.
+    scoped_ptr<crypto::SymmetricKey> decryption_key_;
+
+    // The key used to perform the integrity check.  Currently the HMAC key is
+    // defined by the WebM encrypted specification. Current encrypted WebM
+    // request for comments specification is here
+    // http://wiki.webmproject.org/encryption/webm-encryption-rfc
+    std::string hmac_key_;
+
+    DISALLOW_COPY_AND_ASSIGN(DecryptionKey);
+  };
+
+  // KeyMap owns the DecryptionKey* and must delete them when they are
   // not needed any more.
-  typedef base::hash_map<std::string, crypto::SymmetricKey*> KeyMap;
+  typedef base::hash_map<std::string, DecryptionKey*> KeyMap;
 
   // Since only Decrypt() is called off the renderer thread, we only need to
   // protect |key_map_|, the only member variable that is shared between
   // Decrypt() and other methods.
   KeyMap key_map_;  // Protected by the |key_map_lock_|.
   base::Lock key_map_lock_;  // Protects the |key_map_|.
 
   // Make session ID unique per renderer by making it static.
   // TODO(xhwang): Make session ID more strictly defined if needed:
   // https://www.w3.org/Bugs/Public/show_bug.cgi?id=16739#c0
   static uint32 next_session_id_;
 
   DecryptorClient* const client_;
 
   DISALLOW_COPY_AND_ASSIGN(AesDecryptor);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_CRYPTO_AES_DECRYPTOR_H_