Number of rounds needed for perfect Feistel?

[Tim Dierks]

Barney Wolff wrote:
> On Fri, Aug 12, 2005 at 11:47:26AM -0400, Tim Dierks wrote:
>> I'm attempting to design a block cipher with an "odd" block size (34
>> bits). I'm planning to use a balanced Feistel structure with AES as the
>> function f(), padding the 17-bit input blocks to 128 bits with a pad
>> dependent on the round number, encrypting with a key, and extracting the
>> low 17 bits as the output of f().
>
> Pardon a dumb question, but how do you plan on avoiding collisions in
> the encrypted values, independent of the number of rounds?  Seems to me
> that even if the 128-bit encryption is guaranteed to be 1-to-1 with the
> plaintext, there is no such guarantee on any subset of the 128 bits.

A Feistel network doesn't depend on lack of collision in f(). The Handbook
of Applied Cryptography,
http://www.cacr.math.uwaterloo.ca/hac/about/chap7.pdf describes it pretty
well.

 - Tim

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