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WSJ | Advertisers no longer want to just buy ads. They want to buy access to specific people. So, Mr. Norris is building a "credit bureau for devices" in which every computer or cellphone will have a "reputation" based on its user's online behavior, shopping habits and demographics. He plans to sell this information to advertisers willing to pay top dollar for granular data about people's interests and activities.

Device fingerprinting is a powerful emerging tool in this trade. It's "the next generation of online advertising," Mr. Norris says.

It might seem that one computer is pretty much like any other. Far from it: Each has a different clock setting, different fonts, different software and many other characteristics that make it unique. Every time a typical computer goes online, it broadcasts hundreds of such details as a calling card to other computers it communicates with. Tracking companies can use this data to uniquely identify computers, cellphones and other devices, and then build profiles of the people who use them.

Until recently, fingerprinting was used mainly to prevent illegal copying of computer software or to thwart credit-card fraud. BlueCava's own fingerprinting technology traces its unlikely roots to an inventor who, in the early 1990s, wanted to protect the software he used to program music keyboards for the Australian pop band INXS.

Tracking companies are now embracing fingerprinting partly because it is much tougher to block than other common tools used to monitor people online, such as browser "cookies," tiny text files on a computer that can be deleted.

As controversy grows over intrusive online tracking, regulators are looking to rein it in. This week, the Federal Trade Commission is expected to release a privacy report calling for a "do-not-track" tool for Web browsers.

Ad companies are constantly looking for new techniques to heighten their surveillance of Internet users.

Deep packet inspection, a potentially intrusive method for peering closely into the digital traffic that moves between people's computers and the broader Internet, is being tested in the U.S. and Brazil as a future means to deliver targeted advertising.

Akamai Technologies Inc., an Internet-infrastructure giant that says it delivers 15% to 30% of all Web traffic, is marketing a technique to track people's online movements in more detail than traditional tools easily can.

It's tough even for sophisticated Web surfers to tell if their gear is being fingerprinted. Even if people modify their machines—adding or deleting fonts, or updating software—fingerprinters often can still recognize them. There's not yet a way for people to delete fingerprints that have been collected. In short, fingerprinting is largely invisible, tough to fend off and semi-permanent.
How to 'Fingerprint' a Computer

A typical computer broadcasts hundreds of details about itself when a Web browser connects to the Internet. Companies tracking people online can use those details to 'fingerprint' browsers and follow their users.

Device fingerprinting is legal. U.S. Rep. Bobby Rush (D.,Ill.), proposed legislation in July that would require companies that use persistent identifiers, such as device fingerprints, to let people opt out of being tracked online.