RSA DES Challenge II-2 contest.
The key 3E CD A1 5E 70 4F B3 1C reveals: "The unknown message is: It's time for those 128-, 192-, and 256-bit keys."
We started searching within minutes of the contest's beginning at 9AM Pacific time, Monday July 13. We found the answer at 5:10PM Pacific time, Wednesday, July 15. It took us 56 hours to find a 56-bit key. (This is just a coincidence, but it's a nice sound bite.)
The key was found after searching almost exactly a quarter of the key space (24.8%).
We searched 17,902,806,669,197,312 keys, give or take a few, to find the answer. This averages out to a rate of 88,803,604,509 keys tested per second (88 billion).
When we started the search on Monday morning, we had 35868 search units running, on 26 boards. Each search unit examines 2.5 million keys per second. We stopped the search for a few minutes on Tuesday night to improve the software. We stopped it again for a few minutes on Wednesday to add a 27th board, speeding it up slightly (to 37050 search units). The machine was examining 92,625,000,000 keys per second when we found the answer.
The PC that controls the machine originally ran Windows 95, but we replaced that with Linux so we could operate it remotely over the Internet, rather than being forced to sit in front of it to make it do anything. Our DES Cracker control software runs fine on either Win95 or Linux. We have run it on a Linux laptop as well, using a PCMCIA interface card to attach it to the DES Cracker chassis.