The critical infrastructure cannot operate without SCADA systems; this has made the task of securing SCADA systems a national security priority. While progress has been made in securing control networks, security at the field device level is still lacking. Field devices present unique security challenges and these challenges are compounded by the presence of legacy devices. This paper describes a technique that uses Bloom filters to implement challenge-response authentication and role-based access control in field devices. The approach, which is implemented in an in-line security pre-processor, provides for rapid and constant access check times. Experiments involving a prototype device demonstrate that the false positive rate can be kept arbitrarily low and that the real-time performance is acceptable for many SCADA applications.