We propose in this paper a framework in which the security policies of services in a distributed environment can be expressed. Services interact by exchanging credentials. Each service is made up of an access control policy protecting the access to the service, and of a trust negotiation policy controlling the accessibility of the credentials for other services. We add a workflow layer for each service to model its dynamic evolution with respect to the performed accesses. Unlike most of the access control policies which are uniquely based on roles, we choose an attribute based framework leading to more flexibility in the characterization of users. The strengths of this framework are its ability to control and check the access control aspect of the services and its dynamic evolution based on an exchange of credentials. We provide a unified framework for reasoning on access control policies, trust negotiation policies and workflows.