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Message
From: Wesley J. Landaker<wjl@i...>
Date: Thu Jan 5 21:14:04 CET 2006
Subject: [oc] Cryptographic hardware
On Friday 23 December 2005 08:16, Nicolas Boulay wrote:
> Describe a way to study a black box that is supposed to make a good AES
> (for example) encryption but how to be sure that :
> 1) the encryption is a good quality (key generation ?)
> 2) There is no "undocumented" feature
>
> Maybe it's impossible to do it. But maybe a kind of design could expose
> those internal in a certain manner that permit external observer to
> garantie what the hardware does.
I think this is by definition an impossible task if it's really a "black
box". Given a "black box" takes inputs I and gives outputs O, given over
time t:
1. There are an infinite number of (I,t) pairs.
2. Every possible sequence of inputs (I,ti) must be tested and compared
against all possible outputs (O,to) where to >= ti.
3. It would take infinite time/space to test this.
QED
For example, what about black box that you suspect stores that last 1024
encryption keys used and spits them out instead of the normal expected
encrypted data when a certain sequence of data is input? How can you prove
that this is NOT the case? Only by testing every possible input sequence,
or by breaking open the black box and checking some other way.
If you're even slightly worried about it, it's better to not even think
about using a black box. =)
--
Wesley J. Landaker <attachment.pgp
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