Tripping up spammers
But, ignoring those (ahem) minor issues, Penny Black is really a slick idea. The compute payment would only apply to senders you dont know, so it should not bother you or your regular correspondents. When you, the recipient, receive a message from me, the sender, and Im not on your whitelist, you send me a computational puzzle to solve. Theres enough randomization involved that I really do have to solve the puzzle on a case-by-case basis. Only when I send you the correct result will you accept the message from me. If the computation is complex enough, it will take far longer to send large numbers of unsolicited messages than it does now, throwing a monkey wrench into the economics of spam. The nature of the problem that the sender has to solve is central to the idea of Penny Black. The problem isnt a classic problem-solving computation; it is a problem designed to take a particular amount of time, no matter the speed of the CPU. So when they say there is a cost in computing time, they mean it. Microsoft is specifically proposing "about 10 seconds" of compute power. That would mean a 10-second delay for the sender on his or her system. (Im assuming that with task prioritization this neednt be a system-modal 10 seconds in which nothing else is happening but that it will consume 10 seconds of the CPU over some period of time.)Lets think about it another way: Right now the cost to send 1 million e-mails is between trivial and nothing. 1,000,000 divided by 8,640 equals just less than 116, so the cost under a Penny Black system is more than 115 CPU days. Thats nontrivial. Next page: Microsoft Research chimes in.
There are 60*60*24=86,400 seconds in a day. Divided by 10 seconds, that means that one CPU could send no more than 8,640 messages per day. Of course, the real number is less than that because the CPU will have more to do than just Penny Black problems. Microsoft says that spammers would have to invest heavily in CPU resources, and the company is betting they cant. By the way, this 10-second figure is only a proposal and would certainly have to be agreed upon by the community at large.