>I want that my applications should seamlessly encrypt/decrypt all the files (DBFs, CDXs, FPTs. One of the ways i think is to replace the DOS interrupt functions at the start of the application and reinitialise them to the origional state when the application quits.. I will welcome advice if anybody has tried this option.

I tried it with Visual dBASE and it worked: just use a low-level i/o functions (fWRITE for one) and replace the first byte with something like chr(2). This will make the file invisible even if the user tries to "type" it at the DOS level. Of course, they WILL see everything they want if they go into "debug" or open the file with some other binary editor.