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UDID is an acronym for Unique Device ID[1]. 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[2]. It is not written on the device cover as the IMEI is, but it can be retrieved by iTunes in normal mode.


Apple usually uses this ID to assign Apple ID and iCloud ID to the device. This also holds the Find My iPhone activation status.

Calculation of 2007 format[edit]

The UDID is calculated as follows for the Verizon iPhone 4[3]:

For the:

UDID = SHA1(serial + ECID + wifiMac + bluetoothMac)

All other devices use:

UDID = SHA1(serial + IMEI + wifiMac + bluetoothMac)

See also[edit]


  1. ^ "What Is A UDID And Why Is Apple Killing Apps That Track Them?". Retrieved 2019-08-13.
  2. ^ "Apple Unique Device Identifiers (UDIDs)". Retrieved 2019-08-13.
  3. ^ "The iPhone Wiki". Retrieved 2019-08-13.