I presume you know this was one of the first step in breaking the
original Xbox security. Even the guy who did that original work
emphasized the take home message that a high speed bus is not a secure
system (I would guess it would not be to difficult to access if you had
a LVDS receiver).
Purely speculating I would guess that some sort of encryption/scambling
may have be used but at a 10.8 GB/s transfer rate [1] the complexity of
the encryption/scamling would have minimal to be feasible.
George
[1] http://www.cdrinfo.com/images/uploaded/Xbox_Architecture_large.gif
Bruce Boettjer wrote:
It's probably worth mentioning that since the interface between the CPU and
GPU is running so fast (>1GHz), that it is probably 8b10b encoded, in some
form. What this means is that anything that goes out on the bus first goes
through a look-up table, of sorts, and the results are what is put on the
bus. This is done for the simple reason of trying to improve signal
integrity at those speeds and not for any protection/encryption
requirements. Formal 8b10b encoding ensures that there are always the same
number of '1' and '0' signals across a bus... this helps keep down the
current draw between subsequent transactions (at 1GHz, the period is
1nSec... imagine the amount of current needed to drive all zeros to all
ones, at 1nSec, with an adequate eye-diagram opening to tell the
difference... That would need drivers that were faster than PECL... i.e...
LOTS of current)...
If anyone got the idea to physically probe the bus, one would first need to
R.E. the 8b10b 'like' scheme before getting to the actual data, which might,
in itself, be pre-encrypted... One would be able to tell, from early
accesses, if the transactions looked like setup/config accesses, or garbage.
Regards,
Bruce Boettjer
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