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UDID is an acronym for Unique Device ID. The UDID is a feature of Apple iDevices. It is a unique id for a single device that is being fetched by Apple servers every time a user tries to activate the device using iCloud or 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 cannot be changed by the user nor by Apple as it is assigned to the motherboard of the device.
This Unique ID is created from 40 letters and numbers, 25 for an iPhone XR or XS. It is not written on the iDevice cover as the IMEI is, but it can be retrieved by iTunes in normal mode.
The UDID is calculated as follows:
For the Verizon iPhone 4 and newer:
UDID = SHA1(serial + ECID + wifiMac + bluetoothMac)
All other devices use:
UDID = SHA1(serial + IMEI + wifiMac + bluetoothMac)