The emergence of online games has fundamentally changed security requirements for computer games, which previously were largely concerned with copy protection. In this paper, we examine how new security requirements impact the design of online games by using online Bridge, a simple client-server game, as our case study. We argue that security is emerging as an inherent design issue for online games, after graphics and artificial intelligence, which have become important issues of the design of most games for decades. The most important new security concern in on-line game design is fairness enforcement, and most security mechanisms all contribute to a single objective, namely, making the play fair for each user.