A cooperative model for resource sharing on grid

fined by the administrators of the physical organizations and of the Grid middleware. This approach does not require support for access control in the middleware, but since changes in the access control policy of the Virtual Organization imply the involvement of one or more administrators, it lacks the flexibility needed in several application scenarios. In this paper we pro- pose a group-based access control model for Grid environments that increases the flexibility of the access control model offered by state-of-the-art Grid platforms without requiring changes in the middleware. The approach is based on collaboration among Grid users and allows them to exchange access permissions to Virtual Resources without the intervention of administrators. We show that our solution can be defined on top of the ac- cess control mechanisms offered by state-of-the-art Grid middleware and illustrate how the proposed model can be implemented as a service in a service-oriented Grid environment.