Re: code cracking or how do you know you've got the correct key?

From: Ernst Lippe (ernstl-at-planet-dot-nl_at_ignore.this)
Date: 12/31/04 

Date: Fri, 31 Dec 2004 14:55:58 +0100

On Thu, 30 Dec 2004 07:58:09 +0100, Martin Bodenstedt wrote:

> So You're saying that testing for "randomness" is indeed a criterion for
> differentiating between a "plaintext" and its encrypted form.

Even very simple tests can give a very good result. An example is the test
that counts the number of bytes that have the highest bit set to zero. With a
'random' input the expected number is the total message size divided by
two. But for a very large group of message types you will find that the actual
numbers are much larger. There are several reasons: in most charactersets the
most commonly used characters have a zero high order bit. Also when the byte
is part of a binary number, small positive numbers are by far the most
frequent values, and in all normal binary representations these numbers also
have a zero high order bit.

The only serious exception are messages that have been compressed, but most
commonly used compression techniques have well known headers.

I tried this test on a set of files on my computer and for the large majority
of the files the results were highly significant. The only exception were
compressed files, such as zip files and image formats such as jpeg and png.

Ernst Lippe

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