From: Marc.Ringuette@GS80.SP.CS.CMU.EDU
Date: Fri, 29 Jan 93 13:19:44 PST
To: cypherpunks@toad.com
Subject: Re: OTP Generators
Message-ID: <9301292119.AA12633@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


Hal> Also, relying on a half-life calculation in which we wait a certain time 
Hal> interval, and see if there is a decay or not, won't be that accurate.  
Hal> If your time is off a little, it could bias the results.
Hal> Tim May posted the best (IMO) fix for this.  You collect bits in pairs; 
Hal> discard 00 and 11; for each 01 output a 0, for each 10 output a 1.

This is better than nothing, but it doesn't completely fix the biased
bit stream.  For instance, if your detector typically has some runs of
zeroes, then after a 10 sequence, a 01 sequence is more likely than
another 10.

I think that all schemes which rely on _single_ random events from a
radioactive source are going to be very sensitive to tuning errors
which will make their random bit streams biased and thus useless.

Better is the following:  select a time interval in which 100-1000
random events will occur.  Count events in one of these time intervals
and output the parity of the count.  Repeat.  If you ever detect fewer
than 10 events in an interval, quit with an error.

This method has the advantage that no "tuning" of the randomness source
is necessary; you must only ensure that your time interval contains a
lot of random events, so that there is no chance of a small drift in the
random number source causing a corresponding failure in randomness.

Another useful technique, if you're willing to trust crypto technology,
is to compute MD5 hashes or DES encryptions of the bit stream.  This
will do a lot, actually.  If your bits are already random, then applying
a pseudo-random permutation can't hurt; but if you've been brain-dead
somehow, it's a great insurance policy to apply a well-known scrambling
algorithm to your bits.

I wish we weren't just discussing hacks, though.  I think I'll hunt for
some theoretical results to make this more solid.


-- Marc Ringuette (mnr@cs.cmu.edu)