The Spam Diaries

News and musings about the fight against spam.
 by Edward Falk

Monday, February 06, 2006

Yet Another Final Ultimate Solution to the Spam Problemtm

Microsoft is proposing yet another solution to the spam problem. They call it Penny Black. The proposal is similar to various e-postage proposals (in which the sender is required to bundle a small amount of e-cash in the header of the email; the recipient can choose to refund the e-cash to friends and other non-spammers, while spammers wind up spending real money on each message they send.)

In the Microsoft proposal, the sender's expenditure is measured in cpu cycles instead of money. When a recipient does not trust the sender, it sends the sender a computationally-expensive puzzle to solve (typically breaking a one-way hash). If there are 86,000 seconds in a day, and the puzzle takes ten cpu-seconds to solve, a spammer would be limited to sending 8600 spams per day.

The idea is actually not original to Microsoft; see the Hashcash web page and corresponding Wikipedia entry.

Excersize for the reader: find two things wrong with this proposal. See the FUSSP web page for hints.


Anonymous Anonymous said...

Penny Black has been around forever. Microsoft got all excited about it a few years ago when they hired Cynthia Dwork away from IBM. When I talked to her about it at the CEAS conference a few years ago, she agreed that zombie farms completely defeat any sort of hashcash, since the bad guys have more computing resources than the good guys.

8:29 PM  

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