Computerized OTP (was 5th AMENDMENT & DECRYPTION)

[Marc.Ringuette at GS80.SP.CS.CMU.EDU]

Thug writes,
> The other possibility is to find a truly random RF source that has all
> the properties you want, the more important being that the >average<
> length of a homogenous bit run (0's or 1's) is around 4 or 5 bits.

"All the properties you want?"  What you want is random, and nothing else!

Random isn't "average bit runs of 4 or 5 bits".  It isn't "nice white
noise".  It is TRULY RANDOM!  You need to understand that the absolutely
critical property for a one time pad bit-stream to have is this:  given 
all previous bits seen, the probability that the next bit seen will be
zero or one is exactly 0.5.  

What you need is a method for converting a biased random number stream
(say, one where after a run of zeroes, another zero has high probability)
into an unbiased one where the probability of the next bit being zero is
exactly 0.5.  Truncating runs to length 5 is an attempt at this, but a 
VERY BAD and cryptographically useless attempt.

Does anybody remember a good recipe for converting a biased RNG into an
unbiased one?  I can't think of one off the top of my head, and that's
what Thug's friend seems to need.  This has been discussed at length in
the literature.


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