(It lacks a bit the academic research aspect - use rather as "Studienarbeit" or internship.)
High definition video streams are usually encoded with H.264 codec. This codec allows for layered video coding. Hence with each layer the number of details respectively the resolution increases. This encoded stream can be encrypted with standard encryption mechanisms like AES. It is supposed to encrypt every layer with a separate key. On the decoding side it shall only decrypt and decode the layers that a key exists for. The result shall be the source video stream with reduced resolution. The following resolution should be considered and reflected by the layers and respective keys: 1920x1080, 1366x768, 720x576 (letterboxed?) or 352x288(letterboxed).