From: Peter Monta <pmonta@qualcomm.com>
Date: Thu, 18 Jan 1996 03:31:54 +0800
To: cypherpunks@toad.com
Subject: Re: Random Number Generators
In-Reply-To: <0kzHl6200bky0_dkQ0@andrew.cmu.edu>
Message-ID: <199601171909.LAA05247@mage.qualcomm.com>
MIME-Version: 1.0
Content-Type: text/plain


Timothy Nali writes:

> [ CMOS RNG chip ]
> ...  The most promising design I've seen so far (that I can actually
> do) is based on clocking a D flip-flop in the following way:
> ...
> The slow clock has enough random variation in it's period  for the Dff
> to generate random numbers.

While a scheme like this will work, one of the needs in a design like
this is convincing yourself of how much entropy is available from the
noisy clock and where it comes from.  It's nontrivial to evaluate
the phase noise of a CMOS relaxation oscillator, for example.
Also, at what rate do you want random bits?

> Can anyone give me pointers or references to other types of true random
> number generators and to ways of correcting the biases and other
> problems in the resulting random bitstream?

The references in Applied Cryptography are pretty useful; the only other
ones I know of are a tech report by Gifford at MIT/LCS and a thesis
by Sridhar Vembu (who also works here at Qualcomm) on optimal extraction
of entropy from biased sources.

> One thing I'm concerned about is making sure the random bitstream is
> uniformly random.  What effects, if any, will things like thermal noise,
> power comsumption (what if there is a sudden rise in power comsumption
> in another part of the circuit), etc. have on the randomness of the
> bitstream?  

I'd say thermal noise is your friend; the other systematics, as you
say, are a slight issue, but their effect on the entropy is very small
and they'll be taken out by the postprocessing (hash function, etc.).

> I'd also appreciate any other suggestions or advice you have on RNGs.

I plan to make a simple board-level RNG design available to the net Real
Soon Now.  I'd be interested to see your CMOS design when it's finished.
(By the way, try searching the cypherpunks and sci.crypt archives on the
subject. There's lots of good discussion.)

Cheers,
Peter Monta   pmonta@qualcomm.com
Qualcomm, Inc./Globalstar