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Subject: Re: Descrambler for multimedia
From: Laurent Giroud (laurent.giroud_at_libertysurf.fr)
Date: 2004-01-09


On Thu, 8 Jan 2004 02:08:23 +0100, Björn Stenberg <bjorn_at_haxx.se> wrote:

> Hi.
>
> My little christmas project has paid off.
> I just committed an update to 'descrambler' which
> supports descrambling Multimedia firmware files.
> I'll commit a new 'scrambler' tomorrow.

Hey, this is something I hadn't thought to do. I'll try to do the same for the gmini when I get my hands back on it (no more on hollydays since last monday...).

> The Multimedia firmware files are compressed using
> a custom token packing algorithm, and then XORed
> with a 32-byte string. The descrambler tool finds
> this string automagically, simply by assuming that
> 0 is the most common byte in the plaintext.
> A simple statistical analysis then produces the string.

This looks very much like what is used on the gmini. Except that 0 is not common enough to exactly produce the key (some characters are invalid), however other techniques can be used.

> The compression was trickier, but thanks to it not
> being very efficient (gzip compresses the firmware
> to about 60% of the .ajz size) there was plenty of
> plaintext to look at in the compressed data.

I haven't had time yet to check if the unXORed data is compressed but I assume it is with a similar system since there is a data field who contains values approximately equal to twice the .aos file size.
I'll have a look at it this week end and will commit a scrambler/descrambler too, this at least is legal in France according to the law's texts I've read and especially more if the tools are generic.

I'll also take more time to reply to your post regarding legal issues as your point clearly seems a good objection to how I understood french copyright law. I haven't had time yet to contact a lawyer but I am going to prepare everything this week end in order to do in on monday as I don't want to wait eons before knowing if I can post what I've found.

Regards,

Laurent



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