Hallo Manob,
the API is a reference of the PDF Encryption Library. All the functions and classes are implemented there. The two classes /GRUBE/CL_PDF_ENCRYPTION and /GRUBE/CL_PDF_ENCRYPTION_AES are part of a hierarchy and can be casted into the base class /GRUBE/CL_PDF_ENCRYPTION. It seems that you have tried to implement the classes by yourself and so you have also implement the rest of the functionality which is provided by the PDF Encryption Library.
Cheers,
Steffen