iOS7以降でuniqueIdentifierを取得（生成）する。

UIDevice+GINUniqueIdentifier.h

//
// Created by gin0606 on 2013/10/03.
//


#import <Foundation/Foundation.h>

@interface UIDevice (GINUniqueIdentifier)
- (NSString *)gin_uniqueIdentifier;
@end

UIDevice+GINUniqueIdentifier.m

//
// Created by gin0606 on 2013/10/03.
//


#import "UIDevice+GINUniqueIdentifier.h"


// 参考 http://qiita.com/items/ace0caf4b909c785836f
@implementation UIDevice (GINUniqueIdentifier)
- (NSString *)gin_bundleIdentifier {
    return [[[NSBundle mainBundle] infoDictionary] objectForKey:(NSString *) kCFBundleIdentifierKey];
}

- (NSString *)gin_uuidKey {
    return [NSString stringWithFormat:@"%@.keychain.uniqueIdentifier.key", [self gin_bundleIdentifier]];
}

- (NSString *)gin_userDefaultsKey{
    return [NSString stringWithFormat:@"%@.uniqueIdentifier.key", [self gin_bundleIdentifier]];
}

- (NSString *)gin_uniqueIdentifier {
    NSString *uuidString = [self gin_loadUUIDFromUserDefaults];

    // UserDefaultsに無かったらキーチェーンから探す
    if (!uuidString) {
        uuidString = [self gin_findUUIDFromKeyChain];
    }

    // キーチェーンにも無かったら生成する
    if (!uuidString) {
        uuidString = [self gin_generateNewUUID];
    }

    //UUIDをUserDefaultに保存する
    [[NSUserDefaults standardUserDefaults] setObject:uuidString forKey:[self gin_userDefaultsKey]];
    [[NSUserDefaults standardUserDefaults] synchronize];

    return uuidString;
}

- (NSString *)gin_loadUUIDFromUserDefaults {
    return [[NSUserDefaults standardUserDefaults] stringForKey:[self gin_userDefaultsKey]];
}

- (NSString *)gin_findUUIDFromKeyChain {
    NSString *uuidString;
    NSDictionary *query = @{
            (__bridge id) kSecClass : (__bridge id) kSecClassGenericPassword,
            (__bridge id) kSecAttrGeneric : [self gin_uuidKey],
            (__bridge id) kSecAttrAccount : [self gin_uuidKey],
            (__bridge id) kSecAttrService : [[NSBundle mainBundle] bundleIdentifier],
            (__bridge id) kSecMatchLimit : (__bridge id) kSecMatchLimitOne,
            (__bridge id) kSecReturnAttributes : (__bridge id) kCFBooleanTrue,
    };

    CFTypeRef attributesRef;
    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;
        OSStatus res = SecItemCopyMatching((__bridge CFDictionaryRef) valueQuery, &passwordDataRef);
        if (res == 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];
        }
    }
    return uuidString;
}

- (NSString *)gin_generateNewUUID {
    CFUUIDRef uuidRef = CFUUIDCreate(kCFAllocatorDefault);
    NSString *uuidString = (__bridge_transfer NSString *) CFUUIDCreateString(kCFAllocatorDefault, uuidRef);
    CFRelease(uuidRef);

    //以下はuuidが00000000-0000-0000-0000-000000000000になってしまう可能性があるので使わない
    //id identifier = [[UIDevice currentDevice] performSelector:@selector(identifierForVendor)];
    //uuidString = [identifier performSelector:@selector(UUIDString)];

    NSMutableDictionary *query = [@{
            (__bridge id) kSecClass : (__bridge id) kSecClassGenericPassword,
            (__bridge id) kSecAttrGeneric : [self gin_uuidKey],
            (__bridge id) kSecAttrAccount : [self gin_uuidKey],
            (__bridge id) kSecAttrService : [[NSBundle mainBundle] bundleIdentifier],
            (__bridge id) kSecAttrLabel : @"",
            (__bridge id) kSecAttrDescription : @"",
    } mutableCopy];

    [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;
    }
    return uuidString;
}

@end