Instant Hotspot security
Instant Hotspot connects other Apple devices to a personal iPhone and iPad hotspot. iPhone and iPad devices that support Instant Hotspot use Bluetooth Low Energy (BLE) to discover and communicate to all devices that have signed in to the same individual iCloud account or accounts used with Family Sharing (in iOS 13 and iPadOS). Compatible Mac computers with OS X 10.10 or later use the same technology to discover and communicate with Instant Hotspot iPhone and iPad devices.
Initially, when a user enters Wi-Fi settings on a device, it emits a BLE advertisement containing an identifier that all devices signed in to the same iCloud account agree upon. The identifier is generated from a DSID (Destination Signaling Identifier) that’s tied to the iCloud account and rotated periodically. When other devices signed in to the same iCloud account are in close proximity and support Personal Hotspot, they detect the signal and respond, indicating the availability to use Instant Hotspot.
When a user who isn’t part of Family Sharing chooses an iPhone or iPad for Personal Hotspot, a request to turn on Personal Hotspot is sent to that device. The request is sent across a link that is encrypted using BLE encryption, and the request is encrypted in a fashion similar to iMessage encryption. The device then responds across the same BLE link using the same per-message encryption with Personal Hotspot connection information.
For users that are part of Family Sharing, Personal Hotspot connection information is securely shared using a mechanism similar to that used by HomeKit devices to sync information. Specifically, the connection that shares hotspot information between users is secured with an ECDH (Curve25519) ephemeral key that is authenticated with the users’ respective device-specific Ed25519 public keys. The public keys used are those that had previously synced between the members of Family Sharing using IDS when the Family Share was established.