I was wondering...

I have a program that stores text files in the "App.Path".
The extension I have given these files are ".khj".

Double clicking on it, windows asks you what do you want to open it with. That is good, because I don't want my users opening these setting files. However, if they say to open the file with notepad, all the data is displayed, perfectly...

So my question is, how do I make a new extension, that can't just be opened by notepad or word. So only MY program can read it?

I'm guessing that it might have something to do with encryption?
Or am I wrong?

Thanks