The last years have seen a major interest in designing and deploying trust management and public key infrastructures. Yet, it is still far from clear how one can pass from the organization and system requirements to the actual credentials and attribution of permissions in the PKI infrastructure. This paper presents a semi-formal and formal framework for filling this gap. We devise a methodology for modeling and analyzing security and trust requirements, that extends the Tropos methodology for Early Requirements modeling. The key intuition that underlies the framework is the identification of distinct roles for actors that manipulate resources, accomplish goals or execute tasks (aka functional requirements), and actors that own or permit usage of resources or goals (aka non functional requirements).