Xref: utzoo sci.crypt:2592 sci.electronics:9395
Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!mailrus!ames!pacbell!osc!jgk
From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.crypt,sci.electronics
Subject: Re: A TRUE (not pseudo) random number Chip by AT&T (T7001)
Summary: Why use oscillators?
Keywords: noise, random numbers
Message-ID: <1833@osc.COM>
Date: 10 Jan 90 00:45:49 GMT
References: <50497@bbn.COM> <25718@cup.portal.com>
Reply-To: jgk@osc.COM (Joe Keane)
Followup-To: sci.crypt
Organization: Object Sciences Corp., Menlo Park, CA
Lines: 25

In article <25718@cup.portal.com> mmm@cup.portal.com (Mark Robert Thorson)
writes:
>One pitfall to consider is the possibility that the free-running
>oscillators might get coupled to each other by the noise they inject
>into the power supply rails.  Obviously, this would prevent them from
>being truly free-running, hence you might end up with less randomness
>than you would otherwise expect.

Not just the power supply rails, either.  They need some big capacitors in a
bunch of places, and better not use any inductors.  It's not clear to me why
you'd want to use free-running oscillators.  How do you model the phase
jitter?  You can't possibly get more than about a bit per oscillator cycle.

It seems to me that you could get a lot more bandwidth out of amplified
Johnson noise or shot noise.  If you know the spectrum of the source, you can
filter it to get flat energy per frequency (white noise) across a known
frequency band, and do the appropriate sampling to get independent bits.  It's
a bit more complicated to do it right, but then you have some assurance about
the correlations, since you know the frequency spectrum and how it may vary.

Of course, no matter how good you think your source is, at least some
post-processing is a good idea.  With a good enough hashing function, you can
probably keep most of the bits, but if you're cheap about it some patterns are
going to come through.  Remember, you should assume the spooks have built a
circuit just like yours and have figured out what radio stations it picks up.