From: "Ben Davis" <ben AT vjpoole DOT freeserve DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Some dodgy FILE hackery
Date: Thu, 16 Mar 2000 18:03:08 -0000
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Damian Yerrick wrote in message ...
>On Tue, 14 Mar 2000 22:48:21 -0000, "Ben Davis"
><ben AT vjpoole DOT freeserve DOT co DOT uk> wrote:
>
>>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.
>
>Why?

Do I detect a note of resent for the secretive?

>>I have to do the encryption manually; the built-in password function
>>isn't secure enough.
>
>I know; it's xor.

And it contains an error in that most of the data are not encrypted!
Obviously fixing it would mess up all the encrypted data everywhere that had
already been saved by Allegro. Of course I could correct it myself, but XOR
encryption is "subject to analysis" (as was discussed once on the Allegro
mailing list).

>>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.
>
>Why not?  Allegro for DJGPP is static linked.

But the source isn't. I consider Allegro to be a general-purpose library for
use in all programs. I'm not going to tailor it to any one program.

>>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.
>
>The Allegro source code is stored in /.../allegro/src so you can
>access it readily.

Yes, but I don't have to modify the PACKFILE structure; just read it.
There's no point in writing when I know how to read :-)

Ben Davis