I hate to rain on your parade, but...

the RC5-72 contest is pointless, unwinnable and wasteful.

Pointless

The contest is scientifically pointless. Cracking a single key by brute force is not rocket science, it's been done before. Successfully cracking that one particular key will not weaken RC5-72 as a cryptographic algorithm one bit, brute force is the lowest common denominator attack against all algorithms. And unlike the previous DES and RC5-56 challenges, this contest cannot even serve to point out how insecure RC5-72 is, because nobody uses RC5-72 and because...

Unwinnable

The contest is not winnable in a reasonable amount of time. As of today, distributed.net expects that keyspace exhaustion will take place in 788,747 days. For the mathematically impaired, that's 2161 years. During the RC5-64 contest, it took on average 261 days for processing speed to double; if this doubling keeps up for 5 years (that's 7 doublings), you're only looking at another 17 years of number crunching after those 5 years. Do you seriously expect to keep going until 2025?

Another way to look at the situation: buying a lottery coupon for $1, with a possible yield of $1,000,000 if you get 7 right, has an expectation of $1 * 6.5e-8 * 1e6 = $0.01. However, the odds of the key being found today are 0.000127%, and of the 1392922 blocks completed 8744 (0.63%) were done by the E2 team. The odds of E2 receiving the price money tomorrow are thus 1.01e-12, or 0.000000001%. That's a lotta zeros.

But hey, you never know. With a prize money of $10000 in the offering, we need to have a 'bet' of less than $0.000000001 ($1e-10) per day to beat the odds of the lottery. But processor power is free, right?

Wasteful

Wrong. Back in the halcyon days of RC5-56 and the DES Challenges, computers didn't make a distinction between idling and crunching, so it was a great idea to use those spare cycles for something (remotely) productive. But this is no longer true: modern-day power-sucking CPUs do have circuitry that lets them idle and cool off when the processor is just running NOPs. Thus, keeping a number cruncher running 24 hours a day will stress your processor, requiring full ventilation and running up your power bill.

To illustrate, a standard Pentium III, without the requisite array of cooling fans, sucks down around 30W when running at full speed. Run that sucker for 24 hours, and you've run up a bill of 0.72 kilowatt-hours, or about 3.6¢ assuming 5¢/kW/H.

Conclusions

If you want to earn money for E2...
...it would be 36,000,000,000 times more efficient to buy a lottery ticket each day. Then again, 3.6¢ a day adds up to $13.14 a year, so why not donate the sum directly?
If you want to advance science through distributed computing...
...you might want to try folding proteins (folding.stanford.edu), finding a cure for cancer (www.ud.com), looking for Sierpinski numbers (www.seventeenorbust.com), or even optimal Golomb rulers (www.distributed.net/ogr/).

References

My elementary statistics course. Calculations done on the back of a virtual napkin, so corrections welcome. Lottery probability based on the Finnish model, ie. 39 balls with 7 picked and 7 needed for the grand prize.

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