Secure function computation has been thoroughly studied and optimized in the past decades. We extend techniques used for secure computation to simulate arbitrary protocols involving a mediator. The key feature of our notion of simulation is that it is bidirectional: not only does the simulation produce only outputs that could happen in the original protocol, but the simulation produces all such outputs. In a synchronous system, it can be shown that this requirement can already be achieved by the standard notion of secure computation. However, in an asynchronous system, new subtleties arise because the scheduler can influence the output. We provide a construction that is secure if $n > 4t$, where $t$ is the number malicious agents, which is provably the best possible. We also show that our construction satisfies additional security properties even if $3t < n \le 4t$.