How to generate unique identifier which should work in all iOS versions?

I was updating my application that was working based only on Unique Identifier which supported iOS 4.3 and above. So,

1) I was unable to use [UIDevice currentDevice].uniqueIdentifier; as it was no longer available

2) I could not use [UIDevice currentDevice].identifierForVendor.UUIDString because it was Available in iOS 6.0 and later only and was unable to use for lower iOS versions.

3) The mac address was not an option as it wasn’t allowed in iOS-7

4) OpenUDID was deprecated some time ago and also had issues with iOS-6.

5) Advertisement identifiers were also not available for iOS-5 and below

Finally this was what i did

a) Added SFHFKeychainUtils to the project

b) Generated CFUUID key String

 CFUUIDRef cfuuid = CFUUIDCreate(kCFAllocatorDefault);
    udidString = (NSString*)CFBridgingRelease(CFUUIDCreateString(kCFAllocatorDefault, cfuuid));

c) Saved it to Key Chain Utils or else it will generate a new Unique Each Time

Final Code

+ (NSString *)GetDeviceID {
    NSString *udidString;
   udidString = [self objectForKey:@"deviceID"];
    if(!udidString)
    {
    CFUUIDRef cfuuid = CFUUIDCreate(kCFAllocatorDefault);
    udidString = (NSString*)CFBridgingRelease(CFUUIDCreateString(kCFAllocatorDefault, cfuuid));
    CFRelease(cfuuid);
        [self setObject:udidString forKey:@"deviceID"];
    }
    return udidString;
}

+(void) setObject:(NSString*) object forKey:(NSString*) key
{
    NSString *objectString = object;
    NSError *error = nil;
    [SFHFKeychainUtils storeUsername:key
                         andPassword:objectString
                      forServiceName:@"LIB"
                      updateExisting:YES
                               error:&error];

    if(error)
        NSLog(@"%@", [error localizedDescription]);
}

+(NSString*) objectForKey:(NSString*) key
{
    NSError *error = nil;
    NSString *object = [SFHFKeychainUtils getPasswordForUsername:key
                                                  andServiceName:@"LIB"
                                                           error:&error];
    if(error)
        NSLog(@"%@", [error localizedDescription]);

    return object;
}

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