How to store local data after user delete my iOS app?

Question

I thought it's impossible, because all the data store in a sandbox, when user delete the app, all the data should be removed from the device.

But somehow, this happened:

I downloaded a wallpaper app from the appstore.

It's a free app, if you don't pay, you can download limited wallpapers, up to 105.

• I download a wallpaper and save to my album. it's now 1/105.
• I delete the app, and delete the wallpaper in my album.

• I turn off the iCloud backup function. OK, It's totally removed from my device, right?

• I download it again.

• I enter flight mode, so there is no internet connection.
• I open the app, no wallpaper preview can be download from internet.

But the save count shows 1/105!

I'm not trying to steal their wallpapers, I did not change my wallpaper for 4 months. I just wanna know, how can this app know I saved 1 wallpaper already?

• I delete the app. so it's not store in the app documents or tmp folder.
• I delete the wallpaper in my album, SO it can't get it from an MD5 of my album.
• I am in flight mode, so it can't get it from their server or anywhere on the internet.

Anyone knows where this app stored the save/download count?

Answer 1

This is done using the keychain on iOS most likely.

Read up on it here: https://developer.apple.com/library/mac/documentation/security/Reference/keychainservices/Reference/reference.html