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

media/crypto/hmac_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_HMAC_AES_DECRYPTOR_H_
+#define MEDIA_CRYPTO_HMAC_AES_DECRYPTOR_H_
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/hash_tables.h"
+#include "base/memory/scoped_ptr.h"
+#include "media/base/media_export.h"
+#include "media/crypto/decryptor.h"
+
+namespace crypto {
+class SymmetricKey;
+}
+
+namespace media {
+
+class DecoderBuffer;
+
+// Checks the integrity of the encrypted data and decrypts the AES encrypted
+// buffer into an unencrypted buffer.
+class MEDIA_EXPORT HmacAesDecryptor : public Decryptor {

[xhwang, 2012/06/14 19:42:27]

With the new HmacAesDecryptor, we don't need to keep the old AesDecryptor,
because nobody is going to use it anymore. In that case, we don't need this
Decryptor base class and can just keep Decryptor methods within
HmacAesDecryptor.

Also, it'll make it easier for review to keep the name AesDecryptor for now, and
rename it later, either before commit, or in a separate CL.

[ddorwin, 2012/06/14 21:41:24]

I think we should just keep the name AesDecryptor. Certainly for the code
review, but also because HMAC support should be an optional feature. We don't
want to have too much logic related to whether an HMAC is supported or not, and
the media stack shouldn't need to decide which to create. As an example, we will
create the same CDM regardless of the container.

[fgalligan1, 2012/07/03 22:00:15]

The media stack cannot create the same CDM for CENC and WebM  with the only
difference being CENC does not have an HMAC.  The problem is WebM derives the
encryption key and the HAMC key from the base secret (I.e. the key) with a very
specific algorithm.  This was the main reason I created this decryptor

+ public:
+  // The size is from the WebM encrypted specification. Current WebM
+  // encrypted request for comments specification is here
+  // http://wiki.webmproject.org/encryption/webm-encryption-rfc.
+  static const int kSha1DigestSize = 20;
+  static const char kHmacSeed[];
+  static const char kEncryptionSeed[];
+
+  HmacAesDecryptor();
+  virtual ~HmacAesDecryptor();
+
+  // Decryptor implementation.
+  // Add a |key_id| and |key| pair to the key system. The key is not limited to
+  // a decryption key. It can be any data that the key system accepts, such as
+  // a license. If multiple calls of this function set different keys for the
+  // same |key_id|, the older key will be replaced by the newer key.
+  virtual void AddKey(const uint8* key_id, int key_id_size,
+                      const uint8* key, int key_size) OVERRIDE;
+
+  // Check and Decrypt |input| buffer. The |input| should not be NULL.
+  // Return a DecoderBuffer with the decrypted data if the check and
+  // decryption succeeded. Return NULL if check or decryption failed.
+  // TODO(fgalligan): Do we need to differentiate between a check failure
+  // and a decryption failure?
+  virtual scoped_refptr<DecoderBuffer> Decrypt(
+      const scoped_refptr<DecoderBuffer>& input) OVERRIDE;
+
+ private:
+  // Helper class that manages the HMAC and encryption keys.
+  class HmacEncryptionKeys {
+   public:
+    explicit HmacEncryptionKeys(const std::string& secret);
+    ~HmacEncryptionKeys();
+
+    // Creates the HMAC and encryption key.
+    bool Init();
+
+    std::string hmac_key() { return hmac_key_; }
+    crypto::SymmetricKey* encryption_key() { return encryption_key_.get(); }
+
+   private:
+    // The base secret that is used to derive the HMAC and encryption keys.
+    const std::string secret_;
+
+    // The key used to perform the intergrity check.
+    std::string hmac_key_;
+
+    // The key used to decrypt the data.
+    scoped_ptr<crypto::SymmetricKey> encryption_key_;
+  };
+
+  // KeysMap owns the HmacEncryptionKeys* and must delete them when they are
+  // not needed any more.
+  typedef base::hash_map<std::string, HmacEncryptionKeys*> KeysMap;
+  KeysMap keys_map_;
+
+  DISALLOW_COPY_AND_ASSIGN(HmacAesDecryptor);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_CRYPTO_HMAC_AES_DECRYPTOR_H_