From: "Ben Davis" <ben AT vjpoole DOT freeserve DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Some dodgy FILE hackery
Date: Tue, 14 Mar 2000 22:48:21 -0000
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

I use DJGPP and Allegro. Most of my files (eg. levels) are saved using the
packfile functions of Allegro. I need to encrypt some of these files without
making them bigger. I have to do the encryption manually; the built-in
password function isn't secure enough. I want to intercept all the low-level
file inputs/outputs (where DOS takes over from Allegro) so that I can modify
the bytes at this stage. But I don't want to modify the Allegro source.

The packfile functions of Allegro ultimately use read() and write(), and the
DOS file handle is stored in the PACKFILE structure so I can access it
readily.

So, using DJGPP, is there a way to intercept read() and write() in order to
encrypt/decrypt the data they are writing/reading? How do I do it?

Ben Davis