>>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.
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>Instead of doing this, I will suggest you to use ready-made softwares available for that..(like cipher, cryptor etc.)


Agree with Vinod. Tampering with file headers, etc., is risky but works OK with stand-alone systems. Changing bits at the beginning of a file creates a problem with mult-user systems. "Last one out, change the bit."