  The Internet has become a critical communication infrastructure for citizens
to organize protests and express dissatisfaction with their governments. This
fact has not gone unnoticed, with governments clamping down on this medium via
censorship, and circumvention researchers working to stay one step ahead.
  In this paper, we explore a promising new avenue for covert channels:
real-time strategy-video games. Video games have two key features that make
them attractive cover protocols for censorship circumvention. First, due to the
popularity of gaming platforms such as Steam, there are a lot of different
video games, each with their own protocols and server infrastructure. Users of
video-game-based censorship-circumvention tools can therefore diversify across
many games, making it difficult for the censor to respond by simply blocking a
single cover protocol. Second, games in the same genre have many common
features and concepts. As a result, the same covert channel framework can be
easily adapted to work with many different games. This means that circumvention
tool developers can stay ahead of the censor by creating a diverse set of tools
and by quickly adapting to blockades created by the censor.
  We demonstrate the feasibility of this approach by implementing our coding
scheme over two real-time strategy-games (including a very popular
closed-source game). We evaluate the security of our system prototype -- Castle
-- by quantifying its resilience to a censor-adversary, its similarity to real
game traffic, and its ability to avoid common pitfalls in covert channel
design. We use our prototype to demonstrate that our approach can provide
throughput which is amenable to transfer of textual data, such at e-mail, SMS
messages, and tweets, which are commonly used to organize political actions.
