Android DRM Implementation

IDrmEngine

IDrmEngine is an interface with a set of APIs to suit DRM use cases. Plug-in developers must implement the interfaces specified in IDrmEngine and the listener interfaces specified below. This document assumes the plug-in developer has access to the Android source tree. The interface definition is available in the source tree at:
<<platform_root>/frameworks/base/drm/libdrmframework/plugins/common/include

DRM Info

DrmInfo is a wrapper class that wraps the protocol for communicating with the DRM server. Server registration, deregistration, license acquisition, or any other server-related transaction can be achieved by processing an instance of DrmInfo. The protocol should be described by the plug-in in XML format. Each DRM plug-in would accomplish the transaction by interpreting the protocol. The DRM framework defines an API to retrieve an instance of DrmInfo called acquireDrmInfo().

DrmInfo* acquireDrmInfo(int uniqueId, const DrmInfoRequest* drmInfoRequest);

Retrieves necessary information for registration, deregistration or rights acquisition information. See DrmInfoRequest for more information.

DrmInfoStatus* processDrmInfo(int uniqueId, const DrmInfo* drmInfo);

processDrmInfo() behaves asynchronously and the results of the transaction can be retrieved either from OnEventListener or OnErrorListener.

DRM rights

The association of DRM content and the license is required to allow playback of DRM content. Once the association has been made, the license will be handled in the DRM framework so the Media Player application is abstracted from the existence of license.

int checkRightsStatus(int uniqueId, const String8& path, int action);

Save DRM rights to the specified rights path and make association with content path. The input parameters are DrmRights to be saved, rights file path where rights are to be saved and content file path where content was saved.

status_t saveRights(int uniqueId, const DrmRights& drmRights, const String8& rightsPath, const String8& contentPath);

Save DRM rights to specified rights path and make association with content path.

License Metadata

License metadata such as license expiry time, repeatable count and etc., may be embedded inside the rights of the protected content. The Android DRM framework provides APIs to return constraints associated with input content. See DrmManagerClient for more information.

DrmConstraints* getConstraints(int uniqueId, const String path, int action);

The getConstraint function call returns key-value pairs of constraints embedded in protected content. To retrieve the constraints, the uniqueIds (the Unique identifier for a session and path of the protected content) are required. The action, defined as Action::DEFAULT, Action::PLAY, etc., is also required.

DrmMetadata* getMetadata(int uniqueId, const String path);

Get metadata information associated with input content for a given path of the protected content to return key-value pairs of metadata.

Decrypt session

To maintain the decryption session, the caller of the DRM framework has to invoke openDecryptSession() at the beginning of the decryption sequence. openDecryptSession() asks each DRM plug-in if it can handle input DRM content.

status_t openDecryptSession( int uniqueId, DecryptHandle* decryptHandle, int fd, off64_t offset, off64_t length);

The above call allows you to save DRM rights to specified rights path and make association with content path. DrmRights parameter is the rights to be saved, file path where rights should be and content file path where content should be saved.

DRM plug-in Listeners

Some APIs in DRM framework behave asynchronously in a DRM transaction. An application can register three listener classes to DRM framework.

• OnEventListener for results of asynchronous APIs
• OnErrorListener for recieving errors of asynchronous APIs
• OnInfoListener for any supplementary information during DRM transactions.

Source

The Android DRM framework includes a passthru plug-in as a sample plug-in. The implementation for passthru plug-in can be found in the Android source tree at:
<platform_root>/frameworks/base/drm/libdrmframework/plugins/passthru

Build and Integration

Add the following to the Android.mk of the plug-in implementation. The passthruplugin is used as a sample.

PRODUCT_COPY_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/<plugin_library>:system/lib/drm/plugins/native/<plugin_library> e.g.,
PRODUCT_COPY_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/ libdrmpassthruplugin.so:system/lib/drm/plugins/native/libdrmpassthruplugin.so

Plug-in developers must locate their respective plug-ins under this directory like so:
/system/lib/drm/plugins/native/libdrmpassthruplugin.so Read more...