Apple and mobile advertisers are locked in a cat-and-mouse chase over the next step in mobile advertising. And users are caught in the middle.

Last summer Apple moved discourage developers from using UDID, the 40-character code that acts as an identifier for its devices. But advertisers had been relying on UDID to track and better target users. Since Apple’s decision to get away from UDID, advertisers have been scrambling for an alternative that they hope will bypass Apple’s limitations, The Wall Street Journal reports.

Two high-profile methods now being used by advertisers are the open-source OpenUDID, which uses the phone’s copy-and-paste function to track, and Open Device Identification Number (ODIN), which piggybacks off a device’s Media Access Control address (MAC) to track users.

Advertisers maintain that both identifiers are essentially anonymous, and don’t use any kind of personalized information. Without this information, advertisers claim, most free apps wouldn’t be around and advertisers would lose their effectiveness, and, in turn, lose their cash.

But privacy advocates vehemently disagree. The problem, they say, is that with enough anonymous data, even the most de-personalized ID number can glean a pretty clear idea of a user’s identity. This has become increasingly the case as user activity has been tracked across websites, apps, and, more recently, devices.

Cross-device user tracking has become a bit of a Holy Grail for the advertising industry, which is on the quest for a way to track users no matter what device they are using. Enter Drawbridge, which last month raised $6.5 million to fund its efforts to do just that. Using “probabilistic and statistical inference models,” Drawbridge says it’s able to get a clear picture of a user and figure out where that user goes across smartphones, tablets, and their computers.

All of which should probably scare the pants off the average web user.

Photo via Flickr user Vorstius

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