prepare
Can be used to prepare the license acquisition message which will be sent to the license acquisition server.As many DRM provider expect different, vendor-specific message, this can be done using this user-definedfunction (optional / depending on the DRM server). The parameter is the key message event object as given bythe Widevine Content Decryption Module (CDM).
Return
The prepared key message. May be the same array as the input array.
Parameters
keyMessage
The key message as given by the content decryption module.