[14052] in cryptography@c2.net mail archive
lopsided Feistel (was: cryptographic ergodic sequence generators)
daemon@ATHENA.MIT.EDU (John S. Denker)
Sun Sep 7 00:03:54 2003
X-Original-To: cryptography@metzdowd.com
X-Original-To: cryptography@metzdowd.com
Date: Sat, 06 Sep 2003 20:28:31 -0400
From: "John S. Denker" <jsd@av8n.com>
To: Tim Dierks <tim@dierks.org>, cryptography@metzdowd.com
In-Reply-To: <6.0.0.21.2.20030906142617.04598b68@127.0.0.1>
On 09/06/2003 02:33 PM, Tim Dierks wrote:
> I'm sure that it would be possible to design a Feistel-based block
> cipher with variable block size, supporting some range of even values
> of n.
There's no need to exclude odd n.
I know the typical superficial textbook describes
the Feistel trick in terms of splitting each block
exactly in half, but if you understand the trick
you see that it works just fine for other splits.
It doesn't need to be anywhere near half. It
doesn't even need to be a two-way split.
You could process a 21-bit word as:
-- three groups of seven, or
-- seven groups of three, or
-- one group of twelve and one group of nine, or
-- whatever.
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