Number of rounds needed for perfect Feistel?

[Tim Dierks]

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.

Thanks,
 - Tim


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