Vehicular ad hoc networks (VANETs) have the potential to increase road safety and comfort. Especially because of the road safety functions, there is a strong demand for security in VANETs. Simply using digital signatures and a public key infrastructure (PKI) to protect message integrity is insufficient taking into account multilateral security. Our main goal is to develop a security architecture for VANETs that balances security requirements of all participants. We also try to identify and - if necessary - develop feasible mechanisms that fit in this architecture. Finally, we evaluate the architecture and mechanisms in simulations.