Newsgroups: sci.crypt Path: cactus.org!cs.utexas.edu!uunet!olivea!decwrl!decwrl!netcomsv!netcom.com! + jktaber From: jktaber@netcom.com (John K. Taber) Subject: Re: Strong Block Ciphers from Weak Ones: NxM DES Message-ID:Organization: None, I'm disorganized X-Newsreader: TIN [version 1.2 PL1] References: <1994Feb2.071956.29014@cactus.org> Date: Thu, 3 Feb 1994 01:37:11 GMT Lines: 57 Stewart C. Strait (straits@netcom.com) wrote: : Terry Ritter (ritter@cactus.org) 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 : straits@netcom.com 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 jktaber@netcom.com =======================================================