Re: Hardware-random-bits interface

At 3:17 PM 11/15/95, David K. Merriman wrote:
>At 02:31 PM 11/14/95 -0800, you wrote:
>>I've been thinking about physically random bits recently, so
>>I'd like to offer a simple, easily implementable interface to a
>>source of such bits, along with a design sketch using a common
>>radio receiver chip as the noise source.
>
>It sounds like a fairly decent idea, _as long as you shield the bejeebers
>(technical term :-) out of it_. Otherwise, anybody with a signal generator
>could skew your numbers however they saw fit: any component lead can act as
>an antenna, no matter now small that lead is.

But if you "shield the bejeebers out of it," then all the radio receiver
generates is whatever signal manages to sneak through--which, ironically,
would make it _easier_ for an outside attacker to drive--and some amount of
internal receiver/amplifier noise, such as the Johnson noise talked about
here. (Every receiver has an "equivalent noise temperature," recall.)

And if one is left with only internal noise, why not simply use a nice
clean source like a Zener diode?


--Tim May

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Timothy C. May writes:
> But if you "shield the bejeebers out of it," then all the radio receiver
> generates is whatever signal manages to sneak through--which, ironically,
> would make it _easier_ for an outside attacker to drive--and some amount of
> internal receiver/amplifier noise, such as the Johnson noise talked about
> here. (Every receiver has an "equivalent noise temperature," recall.)
> 
> And if one is left with only internal noise, why not simply use a nice
> clean source like a Zener diode?

Quite solidly agreed. Internal noise beats external noise, and there
is no point in using a radio receiver when what you want is internal
noise.

.pm

> > And if one is left with only internal noise, why not simply use a nice
> > clean source like a Zener diode?
> 
> Quite solidly agreed. Internal noise beats external noise, and there
> is no point in using a radio receiver when what you want is internal
> noise.

It may have been misleading to refer to a radio receiver chip:  the
noise generated by the chip will be entirely internal.  It doesn't
depend on any external signal "sneaking in"; the noise comes from
the effective resistance of the first amplifier stage.  Nothing
is being "received", in some sense; it's just a big hunk 'o gain.

A Zener diode would be fine, but the design is a bit more complex,
especially for wide bandwidths---you need to mess with biasing
voltages, speedy op-amps, interface to CMOS, and all that jazz.
You'd need external power (or a DC-DC converter).  I wanted a
super-simple design that people could just plonk down on a PC
board.  Two chips and some bypass capacitors.

Peter