Ask HN: How do Google's iOS apps track your identity even after reinstalling?

In the old way, an implementation detail of the Keychain was that data saved in the Keychain would not be purged on app uninstall. Basically, the Keychain can be thought of as a fancy key-value store where your keyspace is further scoped to your app's bundle ID. It's intended to provide a secure place to generate and store keys, etc., but you can also just stick a bit of data in there.

So, user logs in, save their username to keychain. User deletes app. User reinstalls app. On app start, try to read the key you saved the username under. Is it there? Ok, this is a reinstall. No? Ok, this is a new installation or new phone.

There is no Apple documentation that says that this is the way it works, it's merely a quirk of how it's implemented. Obviously, it's a privacy concern. Apple closed this hole in one of the betas of iOS 10.3, but then restored the old behavior. I don't know why, but it's possible that many user workflows were relying on this behavior, or that it subtly broke something else (e.g. app groups).

The second way is using App Groups: https://developer.apple.com/library/archive/documentation/Mi...

In more recent versions of iOS, Apple introduced support for "app groups", which basically means a bunch of rights and privs that were scoped to the bundle id in the past are now optionally scoped to the bundle prefix (team id) or to a whitelisted set of apps that share a bundle prefix.

This allows for Facebook to share login session with Facebook Messenger, for instance. If you're Google, and you have multiple Google-owned products on the user's device, you can add them all to an app group. If up to all of the apps but one are deleted, and the last maintains user login data in the keychain, reinstallation of the other apps will allow them to recover data shared by other apps in their app group.

I don't know if there's a way a user on a non-jailbroken device can wipe the keychain without resetting the device. (You should also look into iCloud Keychain.)

Your Google credentials are likely being stored in a shared container.

I'm not sure when or even whether shared containers are garbage collected at all, it's not documented either way.

You can easily track the device and user with keychain.