Challenge-Response Spam Blocking Challenges Patience

By Larry Seltzer  |  Posted 2003-05-08 Print this article Print

In a way, it's the perfect spam blocking technique. But Security Supersite Editor Larry Seltzer has some concerns about challenge-response spam blocking. And you'll never know how badly it's really working.

Spam filtering is an exercise in trade-offs. There are many different technological approaches to the problem, and some are implemented better than others. But the different approaches also have their own inherent problems, and your tolerance of those problems is part of the equation in deciding whether the approach is a good one.

One of the more interesting approaches to spam filtering is challenge-response. The idea is that other people need permission to send you email and the product maintains a whitelist of such people. When someone not on the list sends you mail, the mail is held (at least for a while) and the sender is sent a message asking them to fill out a form, answer a question, and so on. In some cases, the user need only fill out the form, thereby proving they are not a bot sending out a zillion solicitations for increasing the size of certain body parts. In other cases, the email recipient may actually approve or decline specific lists of users.

When I reviewed a bunch of personal antispam tools for PC Magazine many months ago we only ended up reviewing one challenge-response product, Matador from MailFrontier Inc. Its a cute implementation and theres a lot to like technically about it, but too many challenges were sent out blindly to mailing lists and other bots and to people with whom we wished not to communicate.

And once again, just the other day, challenge-response got a shot in the arm when the Washington Post reported that Earthlink will offer it to their subscribers. Earthlink is already a leader in blocking spam. We had a high opinion of their filters, based on BrightMail. But as the Earthlink VP quoted in the story says, its really hard to write a good filter and hard to keep it working well.

The Post story also quotes other vendors in the challenge-response field, including an executive of MailBlocks, Inc., about the concept. That same day, MailBlocks put up a press release annoucing that they were suing Earthlink for patent infringement. No news is good news I guess.

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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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