  With the growth of connectivity to smart grids, new applications, and the
changing interaction between customer and energy clouds, clouds are more
vulnerable to denial-of-service attacks. Efficient detection methods are
required to authenticate, detect and control attackers. Completely Automated
Public Turing test to tell Computers and Humans Apart, CAPTCHA, is one
efficient tool to thwart denial of service attacks. The server presents the
user with a client puzzle to solve in order to gain access to the service or
website. The puzzle should be hard enough for computers, but easy for humans to
solve. Several methods have been suggested including the popular image-based,
as well as video-based, and text-based CAPTCHAs. In this paper, we present a
new Flash-based gaming CAPTCHA to differentiate bots from humans. We propose a
drag and drop client puzzle where the user will play a simple game to answer a
visual question. Our method turns out to be convenient, easy for users and
challenging for bots. Additionally, it has gaming aspect, which makes it
interesting to users of all age groups.
