Having ACKs from the people cracking your SSL exchange is fun; it provides feedback on whether your code is working, allows the volunteers to see their name in lights, and gives you this nice warm feeling that progress is being made. In server-allocation schemes, it also provides an optimization: no need to hand out chunks that have been ACKed. Not having ACKs provides anonymity to those who are performing the crack. The only two agents who have issues of anonymity to consider are: the one presenting the challenge (and its prize), and the one that gets the solution (and its prize). Perhaps anonymity is unimportant for toy problems: so far, Hal has not complained that Agent 86's CCNs have been spread all over the Net. I can imagine a "real" challenge being a much more serious affair. Do you really want to be caught talking with www.brute.cam.cl.ac.uk for two days straight just before someone posts Louis Freeh's American Express number to alt.credit-cards.exploit? nathan