One of the best features of iOS 13 is that Find My Phone can run even if your phone has been stolen and removed SIM (no Wi-Fi, no mobile network). It works by temporarily borrowing Apple devices around the thief, such as the iPhone or MacBook of passers-by, people sitting in cafes, in pawn shops … to locate lost machines and send the location back to you. In addition to iOS 13, the new macOS 10.15 also supports this feature.
Find My will use Bluetooth to signal
Explaining more about this function, Apple said that the phone will use Bluetooth signals to communicate with other Apple items. If you don't know, wireless devices using Wi-Fi or Bluetooth have a mode called "advertising", which will "advertise" the surrounding device to know about its presence as well as to detect Therefore, it is possible for other devices to detect that the lost iPhone is feasible. Apple does this because of the large number of iPhone users, so it is easy to catch advertising signals, but if the number of users is too small it will not work.
And by using Bluetooth to detect, if the iPhone is turned off, (probably) the Find My feature cannot work. Haven't seen Apple talking about this case.
Why is Find My safe way?
Going back to the way Find My works, the data from your device will be forwarded to another intermediary machine, is it safe?
Apple said that the data sent was completely anonymous and encrypted from beginning to end. It is interesting that you need to own 2 or more Apple devices to use the new Find My mode because your other device will be the place to store the key (private key) to decode the packets (in addition there is another key called public key also created). Your private key will be shared between devices that you own safely to miss this device and the other device will remain.
Assuming someone uses their iPhone to read the packets to detect your location, they can't know who you are, which one of your millions of iPhones is operating globally. because they do not have private key. Advertising companies also cannot know the location you have visited because of this encryption method.
Specifically, the operation process of Find My feature on iOS 13 will be as follows
The public key is also constantly changing and it will vary from device to device to limit stolen code, Apple does not say how often it changes and how they save the key to ensure safety. Even Apple does not read the geographical location of the lost device because they do not have the private key to decrypt, only you can see this information. The Apple only uses your public key to determine which packets belong to you and then forward them to the other device for you to view.
In short: only your device decodes data from a lost device, so you need to have 2 or more Apple devices.
Apple claims that these packets are so small that it doesn't affect the mobile data or battery life of those around you, or the device you are losing.
You are interested in the encryption mechanism by public – private key can read more at this explanation.