NetNews Usenet Archive 1992 #23/NN_1992_23.iso/spool/sci/crypt/3807

Text File | 1992-10-15 | 1.6 KB | 38 lines

Newsgroups: sci.crypt
Path: sparky!uunet!hela.iti.org!usc!venice!reuter
From: [e-mail] (Joseph Reuter)
Subject: Re: DES Encryption/ Encrypting more than once.
Message-ID: <[e-mail]>
Sender: Joseph A. Reuter
Organization: TRW Systems Engineering & Development Division, Carson, CA
References: <[e-mail]>
Date: Thu, 15 Oct 1992 19:47:09 GMT
Lines: 26

In article <[e-mail]> [e-mail] writes:
>[stuff deleted]
>The simplest way to show this is to do Simple Substitution
>ciphers.  One can re-arrange the standard two alphabet key as
>chains of substitutions.  For instance, consider (for brevity) the following
>simple substitution key
>
>ABCDEFGHIJK...   for plaintext and
>REPUBLICAND...   for ciphertext.
>
>This has B->E->B (and hence, is B<->E), but G->I->A->R->morestuff->G.  This
>sort of irregular value for n is commonplace in ciphers.
>

This argument shows that the MINIMUM value of n is different for each plaintext
character.  However, the number of plaintext characters is finite (even in
DEX it is 2^64, which is finite) and the least common multiple of the various
n's has the property that repeating the substitution that many times will
transform each plaintext character into itself.  Far from disproving the
original assertion, you have proved it!

--
Joseph A. Reuter, Wizard-in-Training
Speaking for myself from [e-mail]
"Olorin I was in my youth in the West that is forgotten." -- J.R.R. Tolkien
"You can't win, you can't break even, and it's the only game in town."