NetNews Usenet Archive 1992 #23/NN_1992_23.iso/spool/sci/crypt/3766

Text File | 1992-10-14 | 1.6 KB | 38 lines

Newsgroups: sci.crypt
Path: sparky!uunet!mcsun!fuug!funic!news.cs.hut.fi!cs.hut.fi!Ari.Huttunen
From: [e-mail] (Ari Huttunen)
Subject: Re: DES Encryption/ Encrypting more than once.
In-Reply-To: [e-mail]'s message of 12 Oct 92 21:45:43 GMT
Message-ID: <[e-mail]>
Lines: 25
Sender: [e-mail] (Uutis Ankka)
Organization: Helsinki University of Technology, Finland
References: <[e-mail]>
Date: Tue, 13 Oct 1992 18:38:20 GMT

In article <[e-mail]> [e-mail] (Dave Ferret) writes:

! Just a sidenote to 'Hackers' words...

!   There are also encryption algorithms that when used to encrypt the
! plaintext over and over and over, will yield the un-encrypted text. (Ie:
! Its a circular encryption -- Sorry, I don't know the correct term here)

Any encryption scheme that has a fixed block length *must* do this. Think
of a series of encryptions:
    x_1 -> x_2 -> x_3 -> ... -> x_k -> ... -> x_n -> ...
If 'n' is greater than the possible number of messages that can be encoded
in the fixed length block, there must be some blocks in the chain that
are the same (pigeonhole principle). Let x_k and x_n be the same blocks.
Then by encrypting x_k (n-k)-times yields x_k.

(n-k) might be quite large, though. ;-)

ps. I'm not sure, but it would seem that this applies to every
    block x_n.....?
--
...............................................................................
Ari Huttunen                      Any similarity to other alien life forms
                          is purely coincidental.
                          <Alien 3 misquote>