[Cryptography] Based on Even Mansour

[Jonathan Katz]

On Thu, Sep 25, 2014 at 9:09 PM, Ryan Carboni <ryacko at gmail.com> wrote:

> Based on Even Mansour, wouldn't even the most basic block ciphers be
> secure as long as the key size was half of the block size?
>

There seems to be a misunderstanding about what Even-Mansour show.

The Even-Mansour analysis shows that given access to a public (unkeyed),
truly random permutation, it is possible to construct a secure (keyed)
block cipher.

In practice, however, we don't have public random permutations. Instead, we
build block ciphers from "somewhat random-looking" permutations that I will
call round functions. Because these are far from random, block ciphers in
practice do not use a single round function, but instead use many rounds.
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