[Gllug] entropykey: why did nobody ever mention this thing before?

[James Courtier-Dutton]

On 2 August 2010 11:32, Richard Jones <rich at annexia.org> wrote:
>
> Back when I was a hardware engineer we actually tried to build a
> theoretically perfect random number generator using a reverse-biased
> zener diode sampled at a fixed rate by an ADC (similar in design to
> how they describe the entropykey).  The diode bounces off and on "at
> random", but at quite a low average frequency.  The clock rate that
> you have to sample at must be some large multiple of this frequency in
> order to reduce correlation with the frequency of the diode (IIRC the
> clock rate finally chosen wasn't very much, 2 Hz or something, so you
> end up with 2 random bits per second).  Then there's the question of
> whether the diode is bouncing up and down with some external factor:
> power supply?  environmental radio sources?
>
> This was about the point where we stopped, but only after ascertaining
> that the way to do this was to put the whole device inside a half inch
> thick, earthed copper box.  Put a battery inside there, so there's no
> power supply interference.  Then run the output cable through a narrow
> hole in the metal box.  The problem _then_ is that the output cable
> carries environmental RF back into the box, and really the only way
> around that would be some sort of metalized window with an opto
> coupler.  I guess you can probably see why we didn't build the final
> device in the end.
>

What other noise sources did you experiment with?
It should be relatively easy to get more than 2 random bits per second.
With any random number source, there is normally some whitening
function applied to it to reduce the problems one might observe if
there was an external RF source having influence on the output.
For example, most hardware random number generators I know of apply an
encryption function to the hardware generated random number, and the
final encrypted random number is the one actually used for the key.
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