Having just added account activity monitoring to Gmail and released a Web app security assessment tool called ratproxy, Google is adding yet another security offering: phishing protection.

Gmail already tries to flag phishing messages. But now the free e-mail service has begun using DomainKeys and DomainKeys Identified Mail (DKIM), related e-mail authentication protocols, to detect phishing attempts in e-mail messages that purport to come from eBay and PayPal, two of the most heavily phished domains.

"Now any e-mail that claims to come from 'paypal.com' or 'ebay.com' (and their international versions) is authenticated by Gmail and -- here comes the important part -- rejected if it fails to verify as actually coming from PayPal or eBay," explained Google engineer Brad Taylor in a blog post. "That's right: You won't even see the phishing message in your spam folder. Gmail just won't accept it at all. Conversely, if you get a message in Gmail where the 'From' says '@paypal.com' or '@ebay.com,' then you'll know it actually came from PayPal or eBay. It's e-mail the way it should be."

DomainKeys was developed by Yahoo and has since been adopted by other Internet companies. It also spawned DKIM, a related standard. It appends a cryptographic signature to e-mail messages that the receiving server can use to authenticate the purported domain of origin.

As part of its efforts to protect Internet users, Google also publishes a blacklist of known phishing sites. This blacklist informs the anti-phishing features in Firefox and Google Desktop. Google makes this list available for other applications through its Safe Browsing API.

Gmail's use of DomainKeys will be able to block messages that pretend to come from eBay or PayPal. It won't help much against phishers who are communicating through hijacked eBay accounts. In such cases, the recipient has to hope that any malicious URLs in the message are detected using Google's blacklist.