A mechanism for trust management is required in vehicular ad hoc networks (VANETs) to record the behavior of the vehicles. However, it is a complex matter to follow the historical behavior of the vehicles, since their identities are protected by pseudonyms, which are constantly changing to prevent tracking. In our proposal based on a centralized reputation system, being a vehicle well behaved in the opportunistic forwarding of messages comprises two components: reputation for messages creation and for messages forwarding. The vehicles can forward messages at a given time, and the types of messages that are sent in each transaction may require a vehicle reputation score. In this case, the decision of a vehicle to accept a message is based on the reputation of the sender vehicle that created or forwarded the message. We describe the reputation system through the first three levels of a functional ontology and finite state machines. We evaluate the system by means of simulations in an urban scenario to obtain the reputations of forwarding and generation of messages, varying the number of misbehaving vehicles. It was found that a centralized reputation scheme reflects the behavior of the vehicles insofar as feedback is delivered to the reputation server.