From: jbugden@smtplink.alis.ca
Date: Sat, 5 Oct 1996 04:31:09 +0800
To: "geeman@best.com>
Subject: RE: DESCrack keyspace partitioning
Message-ID: <9609048444.AA844448332@smtplink.alis.ca>
MIME-Version: 1.0
Content-Type: text/plain


"geeman@best.com" <geeman@best.com> wrote:
>(P)RNG's just aren't that likely to produce a key of 010101010.....
nor 001100110011... etc etc and I have been thinking about how one might
formalize and exploit this randomness property to increase the probability of
finding the key sooner.
...
>Any thoughts here?


Suppose we have a function f() that randomly chooses an integer between 1 and
100 such that each integer in the range is equally likely to be selected on each
call. If we call f() 6 times, each time noting whether the result is even or
odd, which of the following sequences is more likely?

Even Even Even Even Even Even

Even Odd Odd Even Even Odd

If you picked the second sequence as more likely, you are like most people who
responded to similar questions in a classic study of cognitive biases [1]. The
two sequences actually have an identical probability of occuring, and, in fact,
all sequences of six outcome are EQUALLY probable.

This example illustrates the representativeness bias, in which we mistakenly
suspect the global characteristics of a process to be represented locally in a
specific sequence[2].

Ciao,
James

1. Kahnman, D., and Tversky, A. Subjective probability: A judgement or
representativeness. Cognit. Psych. 3 (1972) 430-454

2. Stacy, Webb and MacMillian, Jean, Cognitive Bias in Software Development.
Communications of the ACM June 1995, 57-63