A nearby stranger's iPhone, with no interaction from its owner, will pick up the signal, check its own location, and encrypt that location data using the public key it picked up from the laptop. Even if the thief carries it around closed and disconnected from the internet, your laptop will emit its rotating public key via Bluetooth. But every time it does, the change makes it that much harder for anyone to use your Bluetooth beacons to track your movements. Apple refused to say just how often the key rotates. Thanks to some mathematical magic, that new number doesn't correlate with previous versions of the public key, but it still retains its ability to encrypt data such that only your devices can decrypt it.
That public key frequently changes, "rotating" periodically to a new number. This is the "beacon" that your devices will broadcast out via Bluetooth to nearby devices. As in other public key encryption setups, this public key can be used to encrypt data such that no one can decrypt it without the corresponding private key, in this case the one stored on all your Apple devices. Each device also generates a public key. When you first set up Find My on your Apple devices-and Apple confirmed you do need at least two devices for this feature to work-it generates an unguessable private key that's shared on all those devices via end-to-end encrypted communication, so that only those machines possess the key. "It uses just tiny bits of data that piggyback on existing network traffic so there’s no need to worry about your battery life, your data usage, or your privacy." "Now what’s amazing is that this whole interaction is end-to-end encrypted and anonymous," Federighi said at the WWDC keynote. And it turns out that Apple's elaborate encryption scheme is also designed not only to prevent interlopers from identifying or tracking an iDevice from its Bluetooth signal, but also to keep Apple itself from learning device locations, even as it allows you to pinpoint yours. That should help you locate your stolen laptop even when it's sleeping in a thief's bag. In upcoming versions of iOS and macOS, the new Find My feature will broadcast Bluetooth signals from Apple devices even when they're offline, allowing nearby Apple devices to relay their location to the cloud. 
But while security experts immediately wondered whether Find My would also offer a new opportunity to track unwitting users, Apple says it built the feature on a unique encryption system carefully designed to prevent exactly that sort of tracking-even by Apple itself. When Apple executive Craig Federighi described a new location-tracking feature for Apple devices at the company's Worldwide Developer Conference keynote on Monday, it sounded-to the sufficiently paranoid, at least-like both a physical security innovation and a potential privacy disaster.
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