From: Eli Brandt <ebrandt@jarthur.Claremont.EDU>
Date: Tue, 5 Oct 93 20:00:29 PDT
To: cypherpunks@toad.com
Subject: Re: Need Suggestions for Random Numbers
In-Reply-To: <IggWg0W00Vp=NOUEhj@andrew.cmu.edu>
Message-ID: <9310060300.AA14566@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


> From: Matthew J Ghio <mg5n+@andrew.cmu.edu>
>      if a_n-2 < 195  then  a_n = a_n-4 + a_n-3   mod 256
>      if a_n-2 > 194  then  a_n = a_n-4 + a_n-3 + a_n-1   mod 256
> 
> This is considerably less easy to break.

True.  However, there are some fairly general attacks on
congruential PRNGs, and I wouldn't be willing to place much of a
prize on the unbreakability of schemes such as the above.  There
are simple techniques whose security is better tested.

> What PRNGs would you suggest using?

I'm hardly the person to ask, but here's a simple one:
  Given a cryptohash function hash(), and a key K,
  generate your series S as S_i = hash(K+i).
There are plenty of other games you can play with a secure hash
function.  They have a practical advantage over PRNGs built on
top of ciphers: there's no problem exporting them.  There are
also "pure" cryptographically-strong RNGs, but I don't know
anything about them.  (The name "Blum-Blum-Shub" springs to
mind, but how could it not?)

   Eli   ebrandt@jarthur.claremont.edu