Promising antispam technique gets nod

Internet Engineering Task Force approves a technique designed to put junk e-mailers out of business forever.
Written by Declan McCullagh, Contributor
Spammers, phishers and other Internet bottom-feeders, be warned.

A key Internet standards body gave preliminary approval on Tuesday to a powerful technology designed to detect and block fake e-mail messages. It's called DomainKeys Identified Mail, and it promises to give Internet users the best chance so far of stanching the seemingly endless flow of fraudulent junk e-mail.

Yahoo, Cisco Systems, Sendmail and PGP Corporation are behind the push for DomainKeys, which the companies said in a joint statement will provide "businesses with heightened brand protection by providing message authentication, verification and traceability to help determine whether a message is legitimate."

The draft standard that the Internet Engineering Task Force adopted is more promising than most other anti-spam and antiphishing technologies because it harnesses the power of cryptographically secure digital signatures to thwart online miscreants.

The way it works is straightforward: if PayPal sends an e-mail notice to customers about their accounts, the company's outgoing mail server will quietly insert a digital signature into the legitimate message. (Because the signature is embedded in the message headers, it's generally not visible to human readers.)

Let's say the recipient has a Yahoo Mail address. Yahoo's mail servers can automatically check PayPal's Internet domain name listing to verify that the digital signature is valid and the message truly originated at Paypal.com. Signatures by authorized third parties are permitted as well, which is useful for outsourced e-mail.

If the signature doesn't check out, the message is probably spam--or a phishing attack designed to try to fool someone into divulging their details about their PayPal account. While the DomainKeys standard doesn't actually specify that messages with invalid signatures should be flagged as junk, Internet service providers are likely to do just that.

DomainKeys explained

DomainKeys works by embedding a digital signature in the headers of an outgoing e-mail message. If the cryptographically secure signature checks out, the message can be delivered as usual. Otherwise, it can be flagged as spam.

Here's an example of an embedded DomainKeys header:

DKIM-Signature a=rsa-sha1; q=dns;
s=jun2005.eng; c=relaxed/simple;
t=1117574938; x=1118006938;

All of these steps represent a belated effort to fix a fundamental problem with Internet e-mail: it was designed in a far more innocent era and came with little built-in security. (An additional benefit of fixing e-mail is that, in addition to targeting phishing attacks, DomainKeys can also help in identifying the kind of spoofed e-mail that led Engadget to falsely report last week that Apple's iPhone would be delayed.)

In the long run, DomainKeys is more promising than existing antispam and antiphishing technologies, which rely on techniques like assembling a "blacklist" of known fraudsters or detecting such messages by trying to identify common characteristics.

But spammers have invented increasingly creative counterattacks, such as inserting image advertisements in the text of messages and appending excerpts from news articles and fiction works in an attempt to defeat the popular antispam method of Bayseian filtering. That kind of counterattack is called Bayesian poisoning.

DomainKeys represents a radical shift in the arms race between phishers, in particular, and Internet users: it's effectively a tactical nuclear attack that can't be countered. The digital signatures, which use public key cryptography, are viewed as unforgeable.

But the DomainKeys approach does suffer from one serious, short-term problem: it's only effective if both the sender and recipient's mail systems are upgraded to support the standard.

Also, it does not do anything to flag junk e-mail sent by a legitimate company, or identify spam sent from a domain name with a true DomainKeys record. By restricting spammers to a limited set of domain names, however, Yahoo believes "a persistent reputation profile can be established for that sending domain" that can be updated over time and posted publicly.

Other advocates so far include antispam vendors and frequent e-mail senders: AOL, EarthLink, IBM, VeriSign, IronPort Systems, Cox Communications and Trend Micro.

MediaPost puts DomainKey adoption at 48 percent among large online retailers. But that doesn't include large ones such as Dell, Wal-Mart Stores, Target, Gap, Macy's and Circuit City, even though they would likely benefit from being able to send authenticated e-mail. Yahoo, on the other hand, has used earlier versions of DomainKeys to sign all outgoing e-mail since 2004.

The Internet Engineering Task Force's preliminary approval does make DomainKeys, or DKIM, an official proposed standard. But because it's the only technology that has achieved that status--Microsoft's competing Sender ID idea has not--it has a visible edge.

In a blog posting on Tuesday, Yahoo engineer Mark Delany said: "Everything hinges on wide-spread adoption. Now that DKIM is on Standards Track, the hurdle to global adoption has been greatly reduced, but not cleared. I joked earlier that someone might not have heard of DKIM, but the email industry is so big and diverse that evangelizing, education and encouragement are needed to ensure the success of DKIM."

While the Sender ID program is similar in principle to DomainKeys, its acceptance has been limited because Microsoft initially did not agree to license patents in ways that are compatible with GNU General Public License. For its part, Yahoo has agreed to open up a number of its pending and granted patents for use with DomainKeys.

DomainKeys Identified Mail is a reworked and enhanced version of the DomainKeys concept initially invented by Yahoo. The newer version supports features like greater security and digital signatures by authorized third parties. A list of frequently asked questions describes how to configure an e-mail server to use DomainKeys.

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