Re: AES 256 based key derivation function.

Fabrice <fabrice.gautier@xxxxxxxxx> wrote:
How does one define security of the Key Derivation Function in this
case ?

It should be hard to distinguish a small number of derived keys from
a collection of random keys.

Your second proposal, dk=tk || AES(rk,tk), can only yield 2^128
different dk (because tk is 128 bits), whereas the first one would
produce 2^256 keys.

As you correctly conclude, this doesn't matter if an attacker cannot
use this fact to his advantage.

--
Kristian Gjøsteen
.

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