[rc5] Checking for 64 bit RC5 on the fly?

Joe Sunday void at null.net
Mon Jun 23 13:04:41 EDT 1997


Klaus Espenlaub wrote:

> Hi!
>
> someone please correct me if I'm wrong: The 56 bit keyspace is
> a subspace of the 64 bit one, so the odds of solving the 64bit
> RC5 challenge while doing the current challenge is 1 on 256.
>
> Would it be worth the time doing another decryption and check
> for the other pattern as well? The drawback is obvious: some loss
> of speed, but the gain would be the chance to solve the 64 bit RC5
> even before the 56 bit RC5 :)
>
> Or did I miss something essential in the RC5 specification? As
> far as I understood the algorithm, the words per key stay the
> same, thus the key schedule stays the same.
>
> Any comments on this?
>
> Klaus Espenlaub
>
You would more than double the time per keyblock, and it's most likely
that the MSB of the 64 bit key is not 00. Probably better to wait until
the 56 key is found, then you'll have a faster and larger network to try
the 64 bit challenge with. Not saying it can't be done, but it's
probably not worth it.

Joe



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