[rc5] rc5 crack schemes

Richard Freeman rfreeman at netaxs.com
Mon Aug 25 13:05:48 EDT 1997


On Mon, 25 Aug 1997, Pete Lai wrote:

> how about a variation on brute-force?
> We shouldn't be doing a purely sequential scan, but using a bisection
> technique.
> At the very least, I think that we should assign blocks from both ends
> of the keyspace, and go sequentially from there, instead of starting at
> the beginning.
> 

There is absolutely no value in this sort of tactic - it may only make
server software more complicated.  Since we know that the key was randomly
generated it is equally likely to be anywhere in the keyspace.  If
DESchall had done this it would have actually taken twice as long to find
the key since it was in the first half of the keyspace...

> Also, there are some other schemes. The military uses a scheme which can
> crack a lesser generation 128-bit crypt in approx. 25 minutes. (the gov
> thinks only the US mil has this technology so it likes the DES-56 for
> exportation)
> I've heard that it uses some sort of neural net + fuzzy logic in
> determining key probabilities, and testing the keys that are calculated
> to have a high probability of being correct.
> Am I right or dead wrong?

People have talked about better methods of cracking keys.  The best method
(basically what you are suggesting), relies on dedicated hardware, which
makes it useless for Bovine - we are using spare CPU on existing machines.
If everyone chipped in about $1000 we could have it cracked in a couple of
minutes with dedicated hardware, but I doubt that you'll get too many
takers...  

Non-brute force schemes like the one you outlined above also require
statistical information (ie if you encrypt a 100 page document and email
it to me, there are patterns that might be visible in the ciphertext).
This is what is needed to assign a probability to a particular key, and we
do not have this here...  There is presently no known alternative to
brute-force in this situation...

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Richard T. Freeman <rfreeman at netaxs.com> - finger for pgp key
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