Most Apple iOS users have come across the term UDID number at some stage or other. Not everyone knows what it is or what it is for though. UDID stands for Unique Device Identifier and is a string of letters and numbers that identify a specific iOS device. The normal length of a UDID is a 40 character hex value that is 20 bytes.
The UDID number is an alphanumeric code made up of alpha characters a-f and numeric characters 0-9. Developers that are registered with the Apple iOS developers program are allowed to add up to 100 devices for the purpose of testing their apps and, for this, the UDID of each device is required.
How is a UDID Calculated?
A UDID is made up of a number of different hardware identifiers from the chips that are in each iOS device, hence the reason it is unique to each one. A typical UDID would be calculated by creating a 59 or 60-character test string using:
- The 11 or 12-character serial number (from the Settings app)
- 15-character IMEI number OR
- 12-character ECID number with no leading zeroes
- 17-character Wi-Fi Mac address
- 17-character Bluetooth Mac address
Calculate the SHA1 hash, and the result is the UDID.
Changing the UDID :
The UDID can be changed by running this command – nvram wifiaddr=ma:ca:dd:re:as followed by a DFU restore. If the restore is not done, a lot of apps may not work properly afterwards, including iTunes. You must also make a note of the original Mac address so things can be changed back if necessary. Do not attempt to do this unless you are highly experienced at programming and are willing to take the chance on your device being bricked.