[RC5] After RC5-64?

Gavan Schneider gavan at magna.com.au
Fri Sep 10 13:47:59 EDT 1999


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On 1999-09-08 at 15:09, Carl Johnstone <rc5 at fadetoblack.demon.co.uk>
wrote:

>> We have approx. completed 12% of the total keyspace. What will
>> happen if we find the correct key real soon? Is there a "backup
>> project"? 
>
>RC5-72 ??
>
What would be the point? We know it will be 256 times more difficult.
We have seen how the RC5-56 to RC5-64 has meant an enormous increase in
computational effort. We are certain to get the 64-bit key. This means
we have already proven it is a difficult (but not impossible)
proposition to brute force such a small key. I see little point in
doing more of the same.

For all practical purposes a 40-bit key exceeds the "keyspace" of
current credit cards, a 56-bit key exceeds individual and reasonable
commercial computing resources, a 64-bit probably needs to have
management approval at NSA because it will use more than a day's time
on their best hardware array. The rest is speculation... we are going
to use 128 bit for serious stuff, and 40 bit when it is not that
serious and it is all we have. The politics of allowable/exportable
encryption key length will not be influenced much by a >5 year effort
to crack a 72 bit key.

The biggest achievement of the d.net project is that we collectively
form a huge computer resource. Together we have shown some limits on
the effort needed to brute force crack limited length encryption keys.
Now we can move onto something else and give the cow a little rest
maybe.

As I see it the ideal projects are those that have a heavy
computational component, split naturally into smaller parallel
problems, don't need billions of large files to be transported between
machines and serve a useful purpose. Personally I prefer a project that
can be shown to be complete, ie., there is an answer to the question
posed if enough effort has been made (this is where SETI fails for me).

I am certain all these points have been made by others over time so I
cannot claim the going rate of AUD$0.02 :-)

Regards
Gavan Schneider

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