# [rc5] Mail failure

Tue Jun 24 17:13:00 EDT 1997

```TO: Colin L. Hildinger                                         DATE: 06-24-97
TIME: 16:13
SUBJECT: Mail failure
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User mail failed mailing to mailbag (file does not exist):

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From: Colin L. Hildinger [SMTP:rc5 at llamas.net]
To: rc5 at llamas.net [SMTP:rc5 at llamas.net]
Date: 1997-06-24 18:08
Subject:  Re: [rc5] Checking for 64 bit RC5 on the fly?

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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
two:

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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