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A fishy Challenge-Response press release

I have a Google News notification set up for mentions of “SpamAssassin”, which is how I came across this press release on PRNewsWire:

Study: Challenge-Response Surpasses Other Anti-Spam Technologies in Performance, User Satisfaction and Reliability; Worst Performing are Filter-based ISP Solutions

NORTHBOROUGH, Mass., July 17 /PRNewswire/ — Brockmann & Company, a research and consulting firm, today released findings from its independent, self-funded “Spam Index Report– Comparing Real-World Performance of Anti-Spam Technologies.”

The study evaluated eight anti-spam technologies from the three main technology classes — filters, real-time black list services and challenge- response servers. The technologies were evaluated using the Spam Index, a new method in anti-spam performance measurement that leverages users’ real-world experiences.

[…] The report finds that the best performing anti-spam technology is challenge-response, based on that technology’s lowest average Spam Index score of 160.

[…] Filter – Open Source software-(Spam Index: 388): This technology is frequently configured to work in conjunction with PC email client filters. The server adds * * SPAM * * to the subject line so that the client filter can move the message into the junk folder. This class of software includes projects such as ASSP, Mail Washer and SpamAssassin, among others.

The “Spam Index” is a proprietary measurement of spam filtering, created by Brockmann and Company. A lower “Spam Index” score is better, apparently, so C/R wins! (Funny that. The author, Peter Brockmann, seems to have some kind of relationship with C/R vendor Sendio, being quoted in Sendio press releases like this one and this one, and providing a testimonial on the front page.)

However — there’s a fundamental flaw with that “Spam Index” measurement, though; it’s designed to make C/R look good. Here’s how it’s supposed to work. Take these four measurements:

  • Average number of spam messages each day x 20 (to get approximate number per work-month)
  • Average minutes spent dealing with spam each day x 20 (to get approximate minutes per work-month)
  • Number of resend requests last month
  • Number of trapped messages last month

Then sum them, and that gives you a “Spam Index”.

First off, let’s translate that into conventional spam filter accuracy terms. The ‘minutes spent dealing with spam each day’ measures false negatives, since having to ‘deal with’ (ie delete) spam means that the spam got past the filter into the user’s inbox. The ‘number of trapped messages’ means, presumably, both true positives — spam marked correctly as spam — and false positives — nonspam marked incorrectly as spam. The ‘number of resend requests last month’ also measures false positives, although it will vastly underestimate them.

Now, here’s the first problem. The “Spam Index” therefore considers a false negative as about as important as a false positive. However, in real terms, if a user’s legit mail is lost by a spam filter, that’s a much bigger failure than letting some more spam through. When measuring filters, you have to consider false positives as much more serious! (In fact, when we test SpamAssassin, we consider FPs to be 50 times more costly than a false negative.)

Here’s the second problem. Spam is sent using forged sender info, so if a spammer’s mail is challenged by a Challenge/Response filter, the challenge will be sent to one of:

  • (a) an address that doesn’t exist, and be discarded (this is fine); or
  • (b) to an invalid address on an innocent third-party system (wasting that system’s resources); or
  • (c) to an innocent third-party user on an innocent third-party system (wasting that system’s resources and, worst of all, the user’s time).

The “Spam Index” doesn’t measure the latter two failure cases in any way, so C/R isn’t penalised for that kind of abusive traffic it generates.

Also, if a good, nonspam mail is challenged, either

  • (a) the sender will receive the challenge and take the time to jump through the necessary hoops to get their mail delivered (“visit this web page, type in this CAPTCHA, click on this button” etc.); or
  • (b) they’ll receive the challenge, and not bother jumping through hoops (maybe they don’t consider the mail that important); or
  • (c) they’ll not be able to act on the challenge at all (for example, if an automated mail is challenged).

Again, the “Spam Index” doesn’t measure the latter two failure cases.

In other words, the situations where C/R fails are ignored. Is it any wonder C/R wins when the criteria are skewed to make that happen?