Ask HN: How do iOS apps know you're logged into other apps?

3 points·slimebot80··5 comments

Can someone please explain it to me like I'm a two year old:

How is it, when I sign into Facebook app on iOS, that when I later install Messenger it knows who I am?

Similarly, install Gmail, and later GMaps knows it's me.

Is this "cookies" at an app level? Cross-app cookies?

How does it even work?

What else can they see between apps?

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I'm pretty sure Google uses shared keychain access.

Since iPhone OS 3.0 it has been possible to share data between a family of applications. This can provide a better user experience if you follow the common path of free/premium applications or if you have a set of related applications that need to share some common account settings.

https://developer.apple.com/documentation/security/keychain_...

Apps from the same developer can share information such as login credentials between them. In your example of Gmail and Google Maps, they’re both Google-developed apps and such can share account information.

So is there a secure/hard restriction where only apps by the same author can share data?

Yes, at least on the phone itself. App developers are generally free to share freely with others from their own server platforms, as long as they stay within bounds of applicable privacy laws, etc.

I guess these apps report back to Facebook your device ID? So no direct comms between apps?

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