Apple wants to support the ad economy, but lately the primary focus has been on user privacy and security. In Safari, cross-site tracking, which allows content providers to track you across websites and apps to show you more targeted ads, is turned off by default. However, content providers can get around that by using less privacy-invasive ad measures, but you can stop that in iOS 14.5 too.
The feature in question is called Private Click Measurement, or PCM, which is something Apple has been working on for years and is hoping to become a web standard that all web browsers will implement, not just Safari. Mozilla has actively discussed the project with Apple and it could one day be included in Firefox. And Apple talks to Brave, Chrome and Edge about it too.
PCM is good for advertisers and users
Overall, PCM is actually a good thing. It̵
In general, PCM allows ad networks to measure the effectiveness of ad clicks on websites and from iOS (or iPadOS) apps to websites, while maintaining anonymity for users. It doesn’t help to kill the ad industry – it gives them another way to do what they need to do without compromising the safety of the people opening the ads.
Without PCM, cross-site tracking disabled, or content blocking, advertisers would know you tapped an ad from an iOS app or website to go to the ad’s website in Safari. But advertisers don’t need to know you tapped the link. That’s why Apple has already baked in the “Prevent Cross-Site Tracking” option for Safari – to periodically delete this kind of data.
How PCM works in Safari
What PCM does is anonymize those taps, or ‘clicks,’ so advertisers see just that someone tapped on the ad, not that you did. This helps support the app developers and the ad system by giving them more knowledge without putting you at any risk. According to Apple:
[N]neither advertisers nor Apple can see which ads are being clicked or which purchases are being made. This solution avoids trusting any of the parties involved – the ad network, merchant or other intermediaries – so that none of them can track users while clicking ads and making purchases in Safari.
But how do advertisers get the information they need? Also from Apple:
[A]ttribution reports [to advertisers] with limited data in a special mode for private browsing without cookies, randomly delaying reports between 24 and 48 hours to unlink events in time and processing data on the device.
PCM uses an 8-bit identifier on the source side of the click, while it is only a 4-bit identifier on the conversion side. A previous implementation of PCM set it to 6 bits for each side. Eight bits means “256 parallel ad campaigns can be measured per website or app,” and 4 bits means “16 different conversion events can be discerned,” WebKit said. These numbers make it difficult for advertisers to create a unique ID that can be used to track users across websites and apps.
You can find more detailed information about Apple’s new Private Click Measurement standard and better explanations of how it works on the Safari whitepaper and WebKit site.
If you were curious about the types of tracking apps and websites have can running with iOS 14.5’s newly required tracking requests that app developers must include using the AppTrackingTransparency framework, this would be an example since “PCM app-to-web [does not ] require the app to be allowed to track according to AppTracking Transparency. “
Delete saved clicks
The new feature certainly helps support advertisers and app and website owners by not blocking ads, but it also protects users’ privacy. But if you want to clear the saved clicks on ad measurements in your Safari browser, you can.
Saved clicks can be removed when you delete website data, which can be done by going to Settings -> Safari -> Advanced -> Website Data. Here you can tap “Delete all website data” to start clean. You can also delete them from Settings -> Safari -> Clear History and Website Data, but that will also clear your browsing history.
When Safari doesn’t save clicks
PCM does not record data when using private browsing mode, and content blockers can add parameters to detect and block the known path, blocking Private Click Measurement. Also, WebViews within apps cannot use it, but apps using SFSafariViewController may use PCM in the future. More importantly, you can opt out entirely if you are still not comfortable with content providers and advertisers learning from your ad interactions.
How to completely disable PCM in Safari
To opt out of PCM, go to Settings -> Safari and turn off the “Maintain privacy of ad measurement” switch. When it is disabled, “no click metadata will be saved and attribution reports will not be sent.” It’s that easy.
PCM is still a work in progress, and fraud prevention via non-linkable tokens is coming soon – something Mozilla and other browser makers have agreed is important.
Note 1: PCM for app-to-website ad metrics is available for testing in iOS 14.5, which is currently in beta, but it may not be officially implemented in iOS 14.5 when the stable release comes out. Apple says it will be released “in the early spring of 2021,” so it could be iOS 14.5 or iOS 14.6.
Note 2: iOS 14.5 also includes “SKAdNetwork 2.2,” which Apple claims “supports view-through attribution for ad formats such as video, audio, and interactive ads. [a developer] view [its] choice of ad sizes and measure which ads are most effective, while preserving user privacy. “
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