This talk is on authentication and authorization in a distributed environment where different trust domains are involved. Requirements towards such a system, coming from aeronautical air-to-ground communications, are formulated. The main issue is how to deal with properties that are assigned from different property authorities/trust domains. Maurer's Calculus is used for a formal description of existing PKI models in order to show how these can not meet the specified requirements. Finally, a new method for constructing digital certificates is proposed that allows building a PKI-system that can meet the requirements.