# Tripping up spammers

|  Posted 2004-03-21 Print

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.)

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.

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.

Larry Seltzer has been writing software for and English about computers ever since—,much to his own amazement—,he graduated from the University of Pennsylvania in 1983.

He was one of the authors of NPL and NPL-R, fourth-generation languages for microcomputers by the now-defunct DeskTop Software Corporation. (Larry is sad to find absolutely no hits on any of these +products on Google.) His work at Desktop Software included programming the UCSD p-System, a virtual machine-based operating system with portable binaries that pre-dated Java by more than 10 years.

For several years, he wrote corporate software for Mathematica Policy Research (they're still in business!) and Chase Econometrics (not so lucky) before being forcibly thrown into the consulting market. He bummed around the Philadelphia consulting and contract-programming scenes for a year or two before taking a job at NSTL (National Software Testing Labs) developing product tests and managing contract testing for the computer industry, governments and publication.

In 1991 Larry moved to Massachusetts to become Technical Director of PC Week Labs (now eWeek Labs). He moved within Ziff Davis to New York in 1994 to run testing at Windows Sources. In 1995, he became Technical Director for Internet product testing at PC Magazine and stayed there till 1998.

Since then, he has been writing for numerous other publications, including Fortune Small Business, Windows 2000 Magazine (now Windows and .NET Magazine), ZDNet and Sam Whitmore's Media Survey.

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