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UDID is an acronym for Unique Device Identifier. The UDID is a feature of Apple's iOS devices. It is a unique identifier for a single device that is fetched from Apple servers when a user tries to activate the device using iCloud or the Setup app. This ID is also used by iTunes to detect the phone or to communicate with it while restoring the iPSW firmware.
This unique ID has been formatted in two ways: for devices introduced between 2007 and 2018 it was represented as a 40-digit lowercase hex code, and for device models introduced after 2018, as a 25-digit uppercase hex code. It is not written on the device cover as the IMEI is, but it can be retrieved by iTunes in normal mode.
Starting from iOS 11, Apple's verification server will check the device's UDID before it could be set up. If the device's UDID is malformed or not present in Apple's database, the device cannot be activated and will be denied access to the verification server. If said device is connected to iTunes, an error message will appear stating that the iPhone could not be activated because "the activation information could not be obtained from the device."
Calculation of 2007 format
UDID = SHA1(serial + ECID + wifiMac + bluetoothMac)
All other devices use:
UDID = SHA1(serial + IMEI + wifiMac + bluetoothMac)
- "What Is A UDID And Why Is Apple Killing Apps That Track Them?". Retrieved 2019-08-13.
- "Apple Unique Device Identifiers (UDIDs)". Retrieved 2019-08-13.
- "The iPhone Wiki". Retrieved 2019-08-13.