This category provides UUIDOfUser static method which saves and fetches an user identifier at user defaults or at key chain. The code is based on @ux_design_tokyo's post: http://qiita.com/ux_design_tokyo/items/ace0caf4b909c785836f

NSUUID+UUIDOfUser.h

//
//  NSUUID+UUIDOfUser.h
//
//  Created by Yusuke Kawasaki on 2013/11/27.
//  Copyright (c) 2013 Kawanet. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface NSUUID (UUIDOfUser)

+ (NSUUID*)UUIDOfUser;

@end

NSUUID+UUIDOfUser.m

//
//  NSUUID+UUIDOfUser.m
//
//  Created by Yusuke Kawasaki on 2013/11/27.
//  Based on @ux_design_tokyo's post:
//  http://qiita.com/ux_design_tokyo/items/ace0caf4b909c785836f
//

#import "NSUUID+UUIDOfUser.h"

@implementation NSUUID (UUIDOfUser)

static NSString * const NSUUID_UUIDOfUser = @"NSUUID_UUIDOfUser";

+ (NSUUID*)UUIDOfUser {
    NSString *uuidString = nil;
    
    // Check UserDefault at first
    NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
    uuidString = [userDefaults stringForKey:NSUUID_UUIDOfUser];
    BOOL notFoundAtSaveDefaults = !uuidString.length;

    // Check keychain otherwise
    if (!uuidString.length) {
        NSDictionary *query = [NSDictionary dictionaryWithObjectsAndKeys:
                               (__bridge id)kSecClassGenericPassword,   (__bridge id)kSecClass,
                               NSUUID_UUIDOfUser,                           (__bridge id)kSecAttrGeneric,
                               NSUUID_UUIDOfUser,                           (__bridge id)kSecAttrAccount,
                               [[NSBundle mainBundle] bundleIdentifier],(__bridge id)kSecAttrService,
                               (__bridge id)kSecMatchLimitOne,          (__bridge id)kSecMatchLimit,
                               (__bridge id)kCFBooleanTrue,             (__bridge id)kSecReturnAttributes,
                               nil];
        CFTypeRef attributesRef = NULL;
        OSStatus result = SecItemCopyMatching((__bridge CFDictionaryRef)query, &attributesRef);
        if (result == noErr) {
            NSDictionary *attributes = (__bridge_transfer NSDictionary *)attributesRef;
            NSMutableDictionary *valueQuery = [NSMutableDictionary dictionaryWithDictionary:attributes];
            
            [valueQuery setObject:(__bridge id)kSecClassGenericPassword  forKey:(__bridge id)kSecClass];
            [valueQuery setObject:(__bridge id)kCFBooleanTrue            forKey:(__bridge id)kSecReturnData];
            
            CFTypeRef passwordDataRef = NULL;
            OSStatus result = SecItemCopyMatching((__bridge CFDictionaryRef)valueQuery, &passwordDataRef);
            if (result == noErr) {
                NSData *passwordData = (__bridge_transfer NSData *)passwordDataRef;
                // Assume the stored data is a UTF-8 string.
                uuidString = [[NSString alloc] initWithBytes:[passwordData bytes]
                                                      length:[passwordData length]
                                                    encoding:NSUTF8StringEncoding];
            }
        }
    }

    if (!uuidString.length) {
        // Generate the new UIID
        CFUUIDRef uuidRef = CFUUIDCreate(kCFAllocatorDefault);
        uuidString = (__bridge_transfer NSString *)CFUUIDCreateString(kCFAllocatorDefault, uuidRef);
        CFRelease(uuidRef);
        
        // iOS 6 has an invalid UUID bug: 00000000-0000-0000-0000-000000000000
        // id identifier = [[UIDevice currentDevice] performSelector:@selector(identifierForVendor)];
        // uuidString = [identifier performSelector:@selector(UUIDString)];
        
        // UIID must be persistent even if the application is removed from devices
        // Use keychain as a storage
        NSMutableDictionary *query = [NSMutableDictionary dictionaryWithObjectsAndKeys:
                                      (__bridge id)kSecClassGenericPassword,    (__bridge id)kSecClass,
                                      NSUUID_UUIDOfUser,                            (__bridge id)kSecAttrGeneric,
                                      NSUUID_UUIDOfUser,                            (__bridge id)kSecAttrAccount,
                                      [[NSBundle mainBundle] bundleIdentifier], (__bridge id)kSecAttrService,
                                      @"",                                      (__bridge id)kSecAttrLabel,
                                      @"",                                      (__bridge id)kSecAttrDescription,
                                      nil];
        
        if ([[[UIDevice currentDevice] systemVersion] floatValue] < 4) {
            // kSecAttrAccessible is iOS 4 or later only
            // Current device is running on iOS 3.X, do nothing here
        } else {
            // Set kSecAttrAccessibleAfterFirstUnlock so that background applications are able to access this key.
            // Keys defined as kSecAttrAccessibleAfterFirstUnlock will be migrated to the new devices/installations via encrypted backups.
            // If you want different UIID per device, use kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly instead.
            // Keep in mind that keys defined as kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly will be removed after restoring from a backup.
            [query setObject:(__bridge id)kSecAttrAccessibleAfterFirstUnlock forKey:(__bridge id)kSecAttrAccessible];
        }
        [query setObject:[uuidString dataUsingEncoding:NSUTF8StringEncoding] forKey:(__bridge id)kSecValueData];
        
        OSStatus result = SecItemAdd((__bridge CFDictionaryRef)query, NULL);
        if (result != noErr) {
            NSLog(@"[ERROR] Couldn't add the Keychain Item. result = %ld query = %@", result, query);
            return nil;
        }
    }
    
    // save UUID at UserDefault
    if (notFoundAtSaveDefaults) {
        [userDefaults setObject:uuidString forKey:NSUUID_UUIDOfUser];
    }
    
    NSUUID *uuid = [[NSUUID alloc] initWithUUIDString:uuidString];
    return uuid;
}

@end