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Comment by dymk | Hacker News Reader

I don't understand Mozilla's ask for changing the cycling of these tokens to monthly.

If this was implemented, it wouldn't be very difficult to associate a cycled out profile with a new one. Only days of behavioral information. So if out of a 30 day month, 3 days are used to re-associate user profiles, they're only using a partial profile 10% of the time?

I don't even get why Moz thinks IDFAs are particularly bad for privacy anyways. They're only shared across apps that come from the same developer; your Facebook and Snapchat apps see different tokens. They don't expose any personal information, they only identify a unique piece of hardware. It wouldn't be particularly difficult for apps themselves to generate and persist their own random token; how often do people reinstall apps anyways?

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The IDFA is device universal, and is only reset when the user takes manual action.

Apple was aware of the privacy challenges of this, and laid out the rules for using IDFA in this WWDC 2014 presentation. https://developer.apple.com/videos/play/wwdc2014/715/

It's a video with no transcript, but if you click the link to the slides, you can see on slide 7 that the lifetime of the Advertising ID is "Reset Advertising ID."

Each time you submit an app binary to Apple for review, you have to click a box that solemnly swears that you're using the IDFA to attribute activity to an advertisement, proving that the advertisement did its job. "I, USER NAME, confirm…"

I don't know whether the solemn vows really do anything; I get the impression that IDFA abuse is detected via privacy researchers making noise in the tech press. But it has been enforced a few times.

> It's a video with no transcript

While Apple only started posting transcripts of WWDC presentations last year, https://asciiwwdc.com has been around for a while and is a great searchable archive of WWDC transcripts. Here's the transcript for the presentation you referenced: https://asciiwwdc.com/2014/sessions/715?q=user%20privacy%20i....

>If this was implemented, it wouldn't be very difficult to associate a cycled out profile with a new one. Only days of behavioral information. So if out of a 30 day month, 3 days are used to re-associate user profiles, they're only using a partial profile 10% of the time?

It's even worse than that. The IDFA might reset every month, but there's nothing preventing apps from storing a persistent identifier in its data. As soon as there's a new IDFA, the ad network can immediately link that back to the old IDFA. As long as you have one app that belongs to the ad network, you can be linked back. You can even combine this with keychain data to persist across reinstalls (technically you're not supposed to use the keychain for this purpose, but storing the currently logged in user's credentials is fine, so you can use that instead).

> there's nothing preventing apps from storing a persistent identifier in its data.

Can't link the PDF since it's behind a login wall, but the developer agreement already prohibits this (3.3.12):

> If a user resets the Advertising Identifier, then You agree not to combine, correlate, link or otherwise associate, either directly or indirectly, the prior Advertising Identifier and any derived information with the reset Advertising Identifier.

Uh-huh. And I'm sure advertisers do this.

I believe in technical measures over pinky-swears in cases like this.

It's a type of security in depth. The problem being solved isn't that companies can make anonymized profiles; it's that all that a company needs to do is associate one ID with a person to fully de-anonymize that person's data, whereas under the new scheme associating an ID would only de-anonymize a month's data. Not wonderful but, arguably, not nearly as bad.

That said, this is engineering-oriented security thinking in a behavioral playing field. People's habits don't change very rapidly, and I'd expect that (with the exception of life-changing events) a month's worth of data would tell you 95% of what a (device) lifetime of data would.

You may be thinking of IDFV (identifierForVendors). The IDFA usually stays the same until the user resets it.

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