Content-based publish/subscribe (CBPS) system is a wide-area communication infrastructure. It facilitates the dissemination of notifications from publishers to interested consumers through an event notification service. More specifically, the event notification service can be asked to apply a filter to the contents of event notifications. The issue of confidentiality is that the plain texts of events should be kept private from the infrastructure to counter against eavesdropping and that the infrastructure must remain able to make correct routing decisions efficiently. This paper presents a solution that allows confidentiality in a CBPS infrastructure without decreasing its performances such as efficiency, expressivity and scalability. This solution is based on smart cards as trusted devices. In this research work, we implemented it on one of the most popular CBPS systems: SIENA.