structure of UDID of iPhone

can anyone tell me the structure of开发者_Go百科 UDID of a iphone . I want to use the UDID to identify each user saperately in my app. The UDID seems to be a 40 digit string . In my app i dont want to use such a big ID. Can any one tell me the structure of the UDID so that i can cut short and make out a small unique string from UDID....

From the documentation:

A unique device identifier is a hash value composed from various hardware identifiers such as the device’s serial number.

I wouldn't recommend changing it, as then you aren't guaranteed any uniqueness. There isn't a structure to it such that you can ignore characters; it's a hash. Do you have a good reason for preferring fewer than 40 characters?