Digital privacy: How Apple aims to stop web ads from tracking you all across the internet

Apple unveils Privacy Preserving Ad Click Attribution For the Web.

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Apple has proposed a privacy feature for Safari and its WebKit engine that would stop many adverts from identifying users across the web, but still giving advertisers enough information about who is clicking on ads and buying products.

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The feature, called Privacy Preserving Ad Click Attribution For the Web, aims to "allow attribution of ad clicks on the web while preserving user privacy," Apple web engineer John Wilander explains. 

The technique would limit the amount of identifiable information about a user while also allowing marketers to attribute a purchase via an ad click. 

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Apple designed the technology with the view that users shouldn't be tracked across websites just so marketers, such as ad networks and retailers, can attribute an ad click. The tech would also limit the ability to measure clicks and conversions to sites that a user actually visits and clicks ads on, cutting out third-party trackers that users may not be aware of. 

Importantly, the browser or 'user agent' "should act on behalf of the user and do its best to preserve privacy while reporting on ad click attribution," notes Wilander. 

This last goal is achieved by disallowing all cookies for reporting purposes, delaying click and conversion reports to merchants by between one and two days, and removing support for Privacy Preserving Ad Click Attribution when the browser is set to private mode. 

Wilander explains the problem with the conventional approach to click attribution is using unseen tracking pixels that are emitted from online shops whether or not a user clicks an ad. Essentially it allows some parties, in his example a firm called "search", to build a detailed profile of web users. 

"In browsers without appropriate privacy protections, search.example will identify John through his cookies every time shop.example fires such a tracking pixel to search.example. This pervasive technology allows search.example to learn everything John does on shop.example and all other websites that fire similar tracking pixels. Even worse, all these pixels fire regardless of whether John has clicked an ad or not.

"Needless to say, tracking pixels that carry cookies enable sites such as search.example to build up a huge profile of people's interests, purchasing power, habits, age, et cetera. We refer to this as cross-site tracking and Safari prevents it from happening through the WebKit feature Intelligent Tracking Prevention."

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In Apple's alternative model, the browser stores ad clicks, which happens on a page hosting the ad that was clicked. Then the site the ad navigated to after it was clicked would match conversions against stored ad clicks. Then the browser would send out ad click attribution data.   

"Critically, our solution avoids placing trust in any of the parties involved — the ad network, the merchant, or any other intermediaries — and dramatically limits the entropy of data passed between them to prevent communication of a tracking identifier," explains Wilander. 

Apple will enable Privacy Preserving Ad Click Attribution as an experimental feature in Safari Technology Preview 82+.