A Better Way To Squelch Spam?

Continued from page 1

By Eric S. Johansson and Keith Dawson

March 26, 2004

Strangers Pay, Friends Fly Free

The Camram project has coined the term "hybrid sender-pays" to describe a system in which work stamps are combined with other anti-spam techniques in a "cocktail" that stops unwanted e-mail from reaching your inbox while enhancing your ability to communicate with people you know. Mail that arrives without a stamp has the same chance of getting through to your inbox as ordinary mail does in the current anti-spam environment.

The Camram project has learned that the most effective anti-spam cocktail contains at minimum three filters: a stamp filter, a smart "white list," and a content filter. The white list is a roster of those with whom you exchange e-mail; it is used to let this friendly mail in unchallenged. The content filter looks at the content of the message and makes a probabilistic assessment as to whether the message is spam. Taken together, these three measures implement the principle of "strangers pay, friends fly free." In other words, strangers who stamp their mail, and friends with whom you regularly communicate, have easy access to your inbox. All others go through the content filter.

To understand hybrid sender-pays techniques by analogy to the real world, imagine a postal system that delivers anything to anybody-for no cost. The Camram filters would function something like an administrative assistant. This assistant passes to you, unopened, mail from friends, as well as all mail, regardless of sender, bearing a valid stamp. After reading the remainder, the assistant tosses the junk, delivers the good mail, and asks your opinion about the questionable mail.

Camram's hybrid sender-pays system has several advantages over other anti-spam techniques. It is completely decentralized: stamps can be generated and validated at any point in the process, and even offline. It is incrementally adoptable: it benefits the first user, and benefits accrue as the number of users grows. And the techniques can be used over a wide range of configurations, from the individual through the enterprise and ISPs.
 
The two most common objections to sender-pays systems are the impact on mailing lists and the risks from "zombie" systems generating stamps. Mailing lists present spammer-like loads to an e-mail system, and therefore Camram-like systems would indeed slow them down. The short-term solution is not to use stamps on mailing-list messages-let them traverse the content filter and, after a short time, the recipients' training of their filters will assure that such messages pass through unhindered. The longer-term solution is to employ a different kind of stamp based on cryptographic signatures. Such signature stamps present a much lower computational load than work stamps and therefore could be used by mailing lists and other bulk mailers to identify themselves to list members as "friends."

The zombie challenge comes from security flaws in Microsoft software. In the last year, as many as 1.5 million systems running Windows XP or Windows 2000 have been taken over in virus or worm attacks. By some estimates as much as half of all spam sent today is relayed through such zombies.

Even if spammers controlled all of these systems-which is almost certainly not the case-they still would lack the computational power to generate enough 23-bit stamps to deliver today's volume of spam. And if spammers do begin exploiting zombies to generate stamps, the computational cost of a stamp could easily be raised by increasing the number of bits in a valid stamp. (Individual Camram users are able to decide how many bits comprise an acceptable stamp.) Every additional bit doubles the workload for the spammer.

Hybrid sender-pays systems as exemplified by the Camram project have the potential to make e-mail friendly again. Worries about e-mail from business associates or friends and family going astray become a thing of the past. The work of slogging through a spam trap to recover miscategorized messages is significantly reduced. Good e-mail gets through and bad e-mail is filtered, and these benefits ensue with an absolute minimum of extra work on the part of the email recipient. If compatible sender-pays systems become widely deployed, spammers will have to begin to look for another line of work.