Date: Tue, 01 Aug 2000 09:29:44 +0100 From: Matthew Seaman <m.seaman@inpharmatica.co.uk> To: Poul-Henning Kamp <phk@FreeBSD.ORG> Cc: hackers@FreeBSD.ORG Subject: Re: How to make *real* random bits. Message-ID: <39868A78.B6B11393@inpharmatica.co.uk> References: <5924.965073661@critter.freebsd.dk>
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Poul-Henning Kamp wrote:
>
> Ok, some people just can't leave an open end dangling (people like
> me for instance :-)
>
> I located a surplus german geiger counter cheaply [1], I have always
> wanted to have one anyway, and in my junkbox I already had an old
> smoke alarm [2]. The Geiger counter has a thin-walled tube which
> takes about 15 events per second from the Am-241 source in the
> smoke alarm.
>
Nice. If you're thinking about this a possible commercial product, I'd be a
bit dubious though. Even if Am-241 is just an alpha emitter, I'd still be a
bit worried about having it built into the guts of a PC.
Perhaps there is a cheaper alternative as a good source of random bits. As a
former NMR spectroscopist, I know that if you take an Inductive - Capacitive
resonant tuned circuit (typically somewhere in the range 5MHz -- 1GHz for
NMR), carefully sheilded from any rf interference and amplify the bejezus out
of the (non)-output, feed the result into a heterodyne radio receiver tuned to
the same frequency as the circuit and then digitise the audio frequency
result, and you should end up with a pretty perfect white noise signal. That
signal is principally due to the random thermal motion of electrons in the
circuitry.
What's more, if you choose the operating frequencies wisely, such a circuit
can be put together from off-the-shelf components cheaply. Standard audio
ADC's should give you about 20,000 samples per second. Efficiently converting
the normally distributed white noise samples to the evenly distributed random
numbers most computer uses require is left as an exercise for the student.
Matthew
--
Certe, Toto, sentio nos in Kansate non iam adesse.
Dr. Matthew Seaman, Inpharmatica Ltd, 60 Charlotte St, London, W1T 2NU
Tel: +44 20 7631 4644 x229 Fax: +44 20 7631 4844
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