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

Colin L. Hildinger colin at ionet.net
Tue Jun 24 19:08:40 EDT 1997

On Mon, 23 Jun 1997 20:59:56 +0200, Remi Guyomarch wrote:

>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.
>We are trying to break *one* message, not the cryptosystem. When we will
>have the correct key, we will need the same amount of computing power to
>break another message encrypted with RC5-56 and 256 times more computing
>power to break a message encrypted with RC5-64.

I think you missed the entire point of his writing.  If it would take 1
month (let's just say) to check 50% of the 56bit keyspace, and 256
months to check 50% of the 64bit keyspace, then, by taking the 12% hit
on the 56-bit keyspace (so it would take 1.136 months) we would take
.88 months off of the 64bit keyspace.  With me?  OK, now add up the

Checked separately:
1 month + 256 months = 257 months

Checked simultaneously:
1.136 months + 255.12 months = 256.256 months

Can you see a difference???

It's not much, but it is a difference.

Colin L. Hildinger
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