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The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:20:07 ADT
Ben Laurie (ben@algroup.co.uk)
Mon, 06 Jul 1998 21:45:27 +0100
William H. Geiger III wrote:
>
> -----BEGIN PGP SIGNED MESSAGE-----
>
> In <35A12436.90537B7B@algroup.co.uk>, on 07/06/98
> at 08:23 PM, Ben Laurie <ben@algroup.co.uk> said:
>
> >Perry E. Metzger wrote:
> >>
> >> "William H. Geiger III" writes:
> >> > I am looking at information on pulling random data from a Geiger counter
> >> > for use in crypto aplications.
> >> >
> >> > I plan on using a RM-60 from Aware Electronics:
> >> >
> >> > http://www.aw-el.com/index.htm
> >>
> >> John Walker has a cool page on a similar gadget he built and wrote
> >> software for. Check out
> >>
> >> http://www.fourmilab.ch/hotbits/
>
> >Seems to me like he's being very conservative about the entropy of the
> >data source - 1 bit for two (detected) decays. Surely we can get a bit
> >(or two) more out of it than that?
>
> I though it said that he was using 3 hits/bit?
Sorry, brain went on strike momentarily. Yes, 3 hits per bit. Though you
could amortize them (if that's the right word in these circs) to get ~1
hit per bit, of course.
While we're talking about it, although inverting every second bit may
eliminate systematic bias (if it happens to fit the 3 hits/bit system,
of course - if it happens to every second hit it may reinforce it :-), I
don't really buy it as a value add. OTOH, I can't fault it (other than
its lack of effect): XORing with any fixed stream of bits can hardly
hurt a random stream.
> Anyway this is why I am looking for information on this. I have done a
> couple of searches on the web but have not found any detailed study on
> using a Geiger-Muler Detector as a random source for crypto programs.
Well, it seems to me that the standard deviation of the interval between
hits should be predictable (a fixed fraction of the mean interval -
anyone remember enough stats to know what that fraction is?) and from
that and the timer resolution you can guesstimate a reasonable number of
bits to allow per hit, which can simply be derived from the timing of
the particular hit minus the mean.
Hmmm ... I can see that this train of thought is going to lead to
someone demanding that I back it up with some maths, so perhaps I'd
better shut up.
Cheers,
Ben.
-- Ben Laurie |Phone: +44 (181) 735 0686| Apache Group member Freelance Consultant |Fax: +44 (181) 735 0689|http://www.apache.org/ and Technical Director|Email: ben@algroup.co.uk | A.L. Digital Ltd, |Apache-SSL author http://www.apache-ssl.org/ London, England. |"Apache: TDG" http://www.ora.com/catalog/apache/WE'RE RECRUITING! http://www.aldigital.co.uk/recruit/
The following archive was created by hippie-mail 7.98617-22 on Fri Aug 21 1998 - 17:20:07 ADT