• CFUUIDCreate 사용 + Keychain
• MacAddress -> SHA1
• Appsfire - OpenUDID
• iOS6의 API 사용

• 예제로 사용한 소스 링크
  

- (NSString*) udidUsingCFUUID
{
    // initialize keychaing item for saving UUID.
    KeychainItemWrapper *wrapper =
        [[KeychainItemWrapper alloc] initWithIdentifier:@"UUID"
            accessGroup:nil];

    NSString *uuid = [wrapper objectForKey:kSecAttrAccount];
    if( uuid == nil || uuid.length == 0)
    {

        // if there is not UUID in keychain, make UUID and save it.
        CFUUIDRef uuidRef = CFUUIDCreate(NULL);
        CFStringRef uuidStringRef = CFUUIDCreateString(NULL, uuidRef);
        CFRelease(uuidRef);
        uuid = [NSString stringWithString:(NSString *) uuidStringRef];
        CFRelease(uuidStringRef);

        // save UUID in keychain
        [wrapper setObject:uuid forKey:kSecAttrAccount];
    }

    return uuid;
}

- (NSString*) udidUsingMacAddress
{
    // can use up to  iOS 6.0 and before
    // mac address from system in iOS 7 and later will be
    // '02:00:00:00:00:00
    NSString *macaddr = [self getMacAddress];

    // hash |macaddr| using SHA1 to make udid which is 40 long.
    const char *ptr = [macaddr UTF8String];
    unsigned char buf[CC_SHA1_DIGEST_LENGTH];
    CC_SHA1(ptr, strlen(ptr), buf);

    NSMutableString *output =
        [NSMutableString stringWithCapacity:CC_SHA1_DIGEST_LENGTH * 2];

    for(int i = 0; i < CC_SHA1_DIGEST_LENGTH; i++)
        [output appendFormat:@"%02x",buf[i]];
    NSString *uuid = [NSString stringWithString:output];

    return uuid;
}

#import "OpenUDID.h" // OpenUDID
- (NSString*) udidUsingAppsfire
{
    NSString* openUDID = [OpenUDID value];
    return openUDID;
}

- (NSString*) udidUsingiOS6UIDevice
{
    // can use iOS 6.0 and later
    return [[[UIDevice currentDevice] identifierForVendor] UUIDString];
}

- (NSString*) udidUsingiOS6ASIdentifier
{
    // can use iOS 6.0 and later
    ASIdentifierManager *manager = [ASIdentifierManager sharedManager];
    return [[manager advertisingIdentifier] UUIDString];
}