[rc5] RC64 keyspace

Thompson, Christopher CThompson at suncor.com
Mon Oct 27 11:09:04 EST 1997


<snip discussion on OTPs in the digest...>

>Date: Mon, 27 Oct 1997 10:46:45 -0600
>From: Mike Lingk <mlingk at enteract.com>
>Subject: [rc5] RC64 keyspace
>
> What I was really wondering was whether we could use some analysis 
>techniques to check high probability key space segments first, instead of 
>just randomly checking the entire keyspace?  I read some information on 
>this early in the 56bit days, but don't remember the specifics.  Has this 
>already been ruled out?

It is my understanding that the keyspace in the RC64 (or 56 or whatever)
is basically totally flat.  That is to say, there is no section of the
key space that has any higher probability of holding the solution than
any other section of the keyspace.  That's what makes this form of
encryption so valuable.  This is also why you cannot use A-life methods
of cracking the encryption, there's no way to judge fitness.

So the best way we have to break the encryption is to use brute-force.
Unless you know something we all don't.  ;-)

IM decided, I think, to avoid checking keys with duplicate numbers in
them.  There's no statistical reason to do so and, in fact, it turned
out they were incorrect in making this guess.

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