Protecting files from being shared
===

I'm making video files for my students sharing these files with them via dropbox. What I want to do is, to prevent illegal copy of these files. In other words my service is paid, and currently one person can pay for it then share with others for free. So I want to make my files so that, only the person who paid for it, can view these files. No one else

I have no idea what can I do? Any suggestions? I'm searching for an good solution. I don't need any code. Just idea. Thx in advance