I need to have controlled usage over STL files, using a desktop software.( I shall develop the software). It can be either in C++ or C#.
Here STL files refers to STereoLithography files, used for 3D printing.
Controlled usage , refers to usage specified by the distributor. So it can be 1 day, 2 hour, or whatever the distributor deems fit . The files shall self expire after the user has received it.
Any ideas shall be appreciated.
Files do not expire on their own (unless we're talking about faulty storage media) and access to them needs to be restricted by software or a combination of software and hardware.
If you plan to make the STL files openly available at any single point (e.g. when the user tries to open them in the viewer or editor), their content cannot be hidden or prevented from copying.
And even if you bundle them with a program that would extract them from itself or obtain them from your website when the editor starts and delete them when it exits (automatically), the editor may still be able to save a copy as a different file (it may even save a temporary/backup copy automatically).
One way to protect those files from copying is to make them available within a program of yours and never outside of it, which may render the files totally useless if your program doesn't let the user determine if they're good (I'm imagining, 1 day, 2 hour, or whatever implies some sort of trial version). But even then they may still be extracted from it at run time by skillful hackers.
If the OS supports DRM for arbitrary files and in ways of interest to you, you might be able to use the OS DRM functionality to control file copying and lifetime. Unfortunately, I do not have practical knowledge of this to point in the direction of such a solution.
Another option is to distribute the files in the open, but embed into them some kind of watermarks, unique for each user/license and able to survive a certain amount of editing. This won't solve every problem, but if a copy starts circulating online, you will be able to tell who "leaked" it and go after them.
At any rate, all protection can be circumvented, given enough time and skills. If you can't break it, it doesn't mean someone else won't be able to.