From: Eli Brandt <ebrandt@jarthur.Claremont.EDU>
Date: Tue, 5 Oct 93 18:15:07 PDT
To: cypherpunks@toad.com
Subject: Re: Need Suggestions for Random Numbers
In-Reply-To: <oggTBZu00awJIEOqNB@andrew.cmu.edu>
Message-ID: <9310060114.AA13172@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


> From: Matthew J Ghio <mg5n+@andrew.cmu.edu>
> 1,1,2,3,5,8,13,21,34,55,89,144,233...
> 
> Taking modulo 10, we get:
> 
> 1,1,2,3,5,8,3,1,4,5,9,4,3,7,0,7,7,4,1,5,6,1,7,8,5,3,8,1,9,0,9,9,8...
> 
> Which gives a fairly random distribution of numbers from 0 to 9.

This is a very simple linear congruential generator:
	a_n = a_n-1 + a_n-2	mod 10
It is decidedly *not* suitable for "producing an `acceptable' random
file to be xor'd with the plaintext."  It's not a cryptographically
strong PRNG (it's not even a particularly good PRNG).  To break such
a system, try Boyar's paper, "Inferring Sequences Produced by
PRNGs", in JACM 36(1): 129-141.  I believe it takes time logarithmic
in the modulus, which is not a recipe for security.

   Eli   ebrandt@jarthur.claremont.edu