Re: Composing large block cipher from smaller ones



Tom St Denis wrote:

No, see you don't get to invent terminology for things that already
exist. That makes you sound more like a crank than you already are.
Go read the damn DEAL paper.

And the reason people don't do recursive ciphers is that they're
normally horribly slow and inefficient. So stop barking up the wrong
tree, specially if you're not going to actually read the existing
literature on the subject.

I got yesterday this link:
http://www2.mat.dtu.dk/people/Lars.R.Knudsen/newblock.html
But my Adobe Reader claimed that the pdf-file was damaged and could not be opened. Further I failed to get stuff from:
http://www.ii.uib.no/~larsr/newblock.html That's why I wrote in the
previous post that I hadn't yet managed to read the document.

Just now I accessed http://www.schneier.com/paper-deal.pdf. I see that
DEAL has 128 bit block length and rests on 2 underlying DESs but with
all different keys in the different rounds, with these round keys
determined in a key schedule of DEAL's own.

O.k. using different keys in different rounds may indeed be better,
I don't know. In my post I use the same keys throughout the rounds and
rely on having a sufficient number of rounds to achieve satisfactory
result.

On the other hand DEAL deals only with 128 bit block length and not
larger block lengths. There seems to be no indication by DEAL's author
how one would do with, say, 256 bit block length.

To your last paragraph: Certainly, if one have and can spend enough
resources (time and man power), then one could develop a new cihper
with good efficiency. But, if for whatever reason, one wants to
have a block cipher of very big block length but can't afford to
design one from scratch, then using an already existing block cipher
to do the desired work is very well a reasonable way to go in my view.
Or what would you say to this?

M. K. Shen

.



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