Access controlin modern computing environments is different from access controlin the traditional setting of operating systems. For distributed computing systems, specification and enforcement of permissions can be based on a public key infrastructure which deals with public keys for asymmetric cryptography. Previous approaches and their implementations for applying a public key infrastructure are classified as based either on trusted authorities with licencing or on owners with delegations. We present the architecture and main features of a trust management infrastructure based on a hybrid model which unifies and extends the previous public key infrastructure approaches. The trust management infrastructure constitutes a flexible framework for experimenting with the applications of different trust models.