Newsgroups: sci.crypt
+     jktaber
From: (John K. Taber)

Subject: Re: Strong Block Ciphers from Weak Ones: NxM DES
Organization: None, I'm disorganized
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Date: Thu, 3 Feb 1994 01:37:11 GMT
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Stewart C. Strait ( wrote:
: Terry Ritter ( presented a long-overdue idea.

: It is analogous in some sense to Playfair, which can
: be though of as 2x2 simple substitution.

: Start with plaintext--convert to a string of mod-25 (usually) numbers
: by deleting non-alpha characters, forcing case to upper, replacing
: J by I, and replacing letters by numbers in order (A=0, B=1, ..., I=8,
: K=9, L=10, ... Z=24).

: Now do simple substitution, swap high-order digits base 5, and
: substitute again using the inverse of the original substitution.

: Replace numbers by letters and you have the ciphertext.  

: [Real Playfair introduces special rules for the cases where the
: interchanged digits or the noninterchanged digits are the same.
: Leaving out the special rules makes an easy puzzle, in which the
: degenerate cases show bits of plaintext, sometimes with letter
: pairs reversed, so the solver has many hints.  I think that using
: the inverse of a previous block encryption might require special
: rules even with DES, but Terry Ritter does not fall into this
: pitfall.]

: Another idea that comes to mind is using a block cipher in 
: what might be called "overlapping block mode".  This would be encrypting
: the first block, overwriting the plaintext (or better a copy of it),
: moving on by part of a block, and then encrypting and overwriting again.
: A disadvantage is that decryption must work backwards through the text.
: A crude example:

:   Block cipher:  multiply by 3 mod 100
:   Overlap:  one decimal digit
:   Plaintext:  3141592
:   Step 1:     9341592
:   Step 2:     9021592
:   Step 3:     9063592
:   Step 4:     9060592
:   Step 5:     9060772
:   Step 6:     9060716=ciphertext

: Of course, this example is not a strong cipher.  Repeating the whole
: process twice would turn the whole plaintext into one block, but
: I don't think its avalanch properties are as good as the NxM idea.

: There must be many other block cipher modes that lengthen the effective
: block size.  Perhaps there are more of special interest.
: -- 
: Stewart C. Strait

I'm not sure I understand all this.  The DES itself is a simple 
substitution cipher on an "alphabet" consisting of 2^64 characters.
John K. Taber