Number of rounds needed for perfect Feistel?

[Alexander Klimov]

On Fri, 12 Aug 2005, 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().
>
> If I use this structure, how many rounds do I need to use to be secure (or
> can this structure be secure at all, aside from the obvious insecurity
> issues of the small block size itself)? I've been told that a small number
> of rounds is insecure (despite the fact that f() can be regarded as
> "perfect") due to collisions in the output of f(). However, I don't
> understand this attack precisely, so a reference would be appreciated.

IIRC the starting point was
 M. Luby and C. Rackoff,
 ``How to construct pseudorandom permutations from pseudorandom functions,''
 SIAM Journal on Computing, vol. 17, nb 2, pp. 373--386, April 1988.

Unfortunately, I was not able to quickly find it online, so you can
try any other paper which mentions Luby-Rackoff construction, e.g.,
 http://www.wisdom.weizmann.ac.il/%7Enaor/PAPERS/lr.ps

-- 
Regards,
ASK

---------------------------------------------------------------------
The Cryptography Mailing List
Unsubscribe by sending "unsubscribe cryptography" to majordomo at metzdowd.com