How to prevent the user from getting decrypted files?

[_DICE_]

I have a little problem but in fact it is more coding in general then Qt. Here is no "general coding" subforum so I hope it is ok that I post it in Qt (because I develop my apps with Qt).

I have an third party app here that needs its files in there default state. But because of security I have to encrypt the files. This is not the problem. For this "issue" I developed a console app that decrypts the file the user wants to open and passes this file to the third party app. My console app also encrypts the file if it is no longer used by the third pary app.

Just imagine notepad. It can only display "normal" text files. My console app decrypts the files and runs notepad with the decrypted file. If the user closes notepad, the file will be encrypted again.
In fact we do not talk about text files but I think it is a good example.

Now my problem(s):

• How would you prevent the user from copying the decrypted files? He knows where the decrypted files have to be, so he could copy them if he wants to.
• How would you prevent the user from closing my console app? The only way I heard about is to start it as a Windows service because they cannot be killed using the taskmanager.

[petrib]

Is this for a PC-based Windows app?

If so, then try http://qt-project.org/forums