"Designing and implementing malicious hardware"

[Perry E. Metzger]

Ed Gerck <edgerck at nma.com> writes:
> Perry E. Metzger wrote:
>> Ed Gerck <edgerck at nma.com> writes:
>>> Each chip does not have to be 100% independent, and does not have to
>>> be used 100% of the time.
>>>
>>> Assuming a random selection of both outputs and chips for testing, and
>>> a finite set of possible outputs, it is possible to calculate what
>>> sampling ratio would provide an adequate confidence level -- a good
>>> guess is 5% sampling.
>>
>> Not likely.
>>
>> Sampling will not work. Sampling theory assumes statistical
>> independence and that the events that you're looking for are randomly
>> distributed. 
>
> Provided you have access to enough chip diversity so as to build a
> correction channel with sufficient capacity, Shannon's Tenth Theorem
> assures you that it is possible to reduce the effect of bad chips on
> the output to an error rate /as close to zero/ as you desire. There is
> no lower, limiting value but zero.

No. It really does not. Shannon's tenth theorem is about correcting
lossy channels with statistically random noise. This is about making
sure something bad doesn't happen to your computer like having someone
transmit blocks of your hard drive out on the network. I assure you
that Shannon's theorem doesn't speak about that possibility. The two
are not really related. It would be wonderful if they were, but they
aren't. Indeed, Shannon's tenth theorem doesn't even hold for error
correction if the noise on the channel is produced by an adversary
rather than being random.

I'm not particularly inclined to argue this at length.

Perry

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