Re: A89: Linux Port for 89/92
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Re: A89: Linux Port for 89/92
On Wed, Jul 14, 1999 at 11:45:04 -0400, Adam Davis wrote:
>
> Ok, let's put it this way:
>
> We have, or can have, full and complete access to the contents of the flash rom,
> where the boot loader is, I am told, located.
Right, but pointers to the functions for encrypting, decrypting and
calculating MD5 checksums exists in the ROM table also. (Of course you need
to supply a key to the enc/dec funcs...)
> It would be trivial to make a program that would run tests on that code within a
> certian range, and have everyone who owns a ti-89 who uses ticalc.org, these
> mailing lists, and the newsgroup bit.listserv.calc-ti run the program on various
> ranges. I think we could amass a large enough amount of computing power to give
> us a good chance at breaking it soon.
But exactly what are you going to test?
> Now, I completely understand that it still a matter of chance that we could find
> it in under 5 years (unless we could get one of the larger internet distributed
> processing sites on our side...), but that chance is much *much* greater than
> finding it by having us sitting around shooting other people's ideas down
> without really thinking about them. Hmm?
As long as I live to see the results, I'm in ;)
> But it's really irrelevent, because we can tack the same encrypted checksum on
> the end of whatever code we introduce, and add a few bytes as necessary to match
> the checksum it will be checking for.
That could work, but the checksum is not a simple "sum" of bytes, it's based
on a complex mathematic formula (or whatever) that is called MD5.
Theoretically, it would always be enough to add 16 bytes at the end of the
code, but which 16 bytes? There are 2^128 combinations... and we would have
to find another 16 bytes for the next piece of code that we want to
install... it simply won't pay off, IMHO.
> Secondly, we'll have a second checksum at the end of this year. Should narrow
> the field of search.
Yeah... It seems like TI89 and TI92+ uses different decryption keys :(
//Johan
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