Re: Unbreakable Encryption - Scenarios - What encryption method would be best?

From: Tom St Denis (tomstdenis_at_yahoo.com)
Date: 03/16/04 

Date: 15 Mar 2004 18:56:17 -0800

"Bartosz Zoltak" <QPbzoltak@vmpcfunction.com*(without "QP")> wrote in message news:<c34m7n$9qc$1@atlantis.news.tpi.pl>...
> Tom St Denis wrote:
> [snip]
>
> The main point was my question :
>
> Is there really no known method of combining cryptographic algorithms
> in a way to build a provably unbreakable symmetric encryption? Certain
> modes of operation can be proved
> as-secure-as-the-underlying-block-cipher, but IS THERE no way to get
> "a level lower" with the proof - that the very cipher can be proved
> secure if some basic assumptions are satisfied?
>
> As far as I know there is no such scheme but I may be wrong. Is there?

You can prove security against most attacks. The problem is not all
attacks are known.

However, yes, you can get some "quantifyable" level of security. In
fact there was a recent Crypto'03 paper on the subject. The problem
is these schemes require a HUGE amount of ram [e.g. totally random
round functions]. IIRC 7 rounds are enough for CPA and 10 rounds for
CPCA.

So let's put numbers to that. For a 64-bit block cipher this amounts
to 2^34 bytes per round for a total of around 2^38 [or 1/4 of a TB of
ram] for 10 rounds.

Of course you may be able to extend that into heuristics... E.g.
turtle style. Make a 16-bit Feistel [which would only require 2.5KB
of ram] and use it over and over [e.g. 10 times for the 32-bit feistel
giving 25KB of ram then 10 times for the 64-bit feistel giving 250KB
of ram]. Such a design wouldn't have the provable chars but would
most likely be secure [and definitely very slow].

Tom

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