It's a 126 byte public/private key thingie. Bruce Tomlin has the full scoop
on it.
The code/data is not actually encrypted, it is just used to generate the
encryption key which is processed with the 126 byte key to determine that
the cart is valid. That is what it is doing when it shows the Atari logo on
power up.
The Encryption does not have to cover the entire cart. A Byte in the top of
the cart indicates what portion of the cart to use to generate the
encryption check. it can be as little as 4K, I think the 6502 vectors have
to point into the encryption key area. Bank Switched data is not used for
encryption of course. Harry Dodgson's monitor cart (believe it or not, I
got one of these from cart hunting in the wild) only "encrypts" the top 4K
and the vectors immediately jump out to point in the "unencrypted" space.
-Matt Pritchard
Graphics Engine and Optimization Specialist
MS Age of Empires & Age of Empires ][
> -----Original Message-----
> From: Teknovore [SMTP:tek@xxxxxxxxxxxxxxx]
> Sent: Thursday, November 12, 1998 2:40 PM
> To: stella@xxxxxxxxxxx
> Subject: Re: [stella] Atari 7800
>
> On Thu, Nov 12, 1998 at 11:53:00AM -0500, bwmott@xxxxxxxxxxxx wrote:
> >
> > If we knew enough about the algorithm (is it DES in some form) then
> > maybe we could get a massive internet search going to recover the 7800
> > encryption key :-) As I recall there's some information about the
> > encryption in one of Atari's patents.
>
> With knowledge of the algorithm we could use much more productive
> cryptanalysis attacks. If it's based on DES, differential cryptanalysis
> comes to mind (IIRC). Brute force is so passe ;-)
>
> Also, I doubt it could be anything more than like 40-bit keys, if that.
>
> --
> -/
> |/| Teknovore <tek@xxxxxxx>
> /-
>
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