From: "Mark M." <markm@voicenet.com>
Date: Fri, 20 Sep 1996 07:31:18 +0800
To: stuart johnson <sjohnson@packetengines.com>
Subject: Re: a simple cypher scheme
In-Reply-To: <9609180005.AA14720@ns.tsinet.com>
Message-ID: <Pine.LNX.3.95.960918173456.1272B-100000@gak>
MIME-Version: 1.0
Content-Type: text/plain


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On Tue, 17 Sep 1996, stuart johnson wrote:

>    i've been on cypherpunks for about a year a half now, and have wacthed
> many interesting threads pass by but i've never posited anything.  what has
> brought me out into the open is this : i work for an engineering firm doing
> asic design, i use pgp ( as do all rational persons ), a co-worker here has
> come up with a 'cypher' scheme that he would like to use to send code to our
> clients.  the scheme is this : he would take the file of code and pad all
> lines to the length of the longest line, he would then preform column swaps,
> and then row swaps, to 'mix up' the file. the person receiving the file
> would then preform the opposite functions to recover the file.  it seems so
> simple that it can't be good. i've convenced him to use pgp, but i would
> like some input if possible on why his cypher scheme is not a good one.

This is a transposition cipher.  The problem with this cipher is that it does
not obscure patterns in the plaintext.  There is no substitution function in
this algorithm.  Secure ciphers have several rounds with both a substitution
and transposition function.  In other words, there is no way this could be
secure.  Tell your co-worker to stick with triple-DES.

- -- Mark

PGP encrypted mail prefered.
Key fingerprint = d61734f2800486ae6f79bfeb70f95348
http://www.voicenet.com/~markm/

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