This paper discusses the security in the fieldbuses, and presents a security architecture for the centralized fieldbuses, such as WorldFIP. The discussion takes into account, on one hand, the security services defined by the OSI Security Architecture, and on the other hand, the security mechanisms defined by WorldFIP. The discussion shows that there are two critical points for attacking this network: the Bus Arbitrator and the bus. This paper presents then the specification, using the Specification and Description Language (SDL), of a security architecture for WorldFIP based on the RC4 algorithm, which is a lightweight cryptographic stream cipher.