The appsstore rejecting rejecting the apps for iphone if we use the iphone/pad/ipod unique device id(udid). so inorder to overcome this problem we have to use another source id.
and also [UIDevice CurrentDevice]uniqueIdentifier] is deprecated from ios 5.
The best one among those are open udid
the link for open idid is https://github.com/ylechelle/OpenUDID
just a single line of code specifies a 40 digit unique id so that we can use it instead of UDID for iphone.