ROYALE: A Framework for Universally Composable CardGames with Financial Rewards and Penalties Enforcement

While many tailor made card game protocols are known, the vast majority of those suffer from three main issues: lack of mechanisms for distributing financial rewards and punishing cheaters, lack of composability guarantees and little flexibility, focusing on the specific game of poker. Even though folklore holds that poker protocols can be used to play any card game, this conjecture remains unproven and, in fact, does not hold for a number of protocols (including recent results). We both tackle the problem of constructing protocols for general card games and initiate a treatment of such protocols in the Universal Composability (UC) framework, introducing an ideal functionality that captures general card games constructed from a set of core card operations. Based on this formalism, we introduce Royale, the first universally composable protocol for securely playing general card games with financial rewards/penalties enforcement. We remark that Royale also yields the first universally composable poker protocol. Interestingly, such an instantiation of a poker game based on Royale performs better than most previous works without composability guarantees, which we highlight through a detailed concrete complexity analysis and benchmarks obtained from a prototype implementation.