The shift from paper documents to their respective electronic formats is producing important advantages in the functioning of businesses and Public Administrations. However, this shift is often limited to the internal operation of each entity because of the lack of security in the electronic communication mechanisms. Traditionally, these entities have managed their Local Area Networks (LANs) or even Virtual Private Networks (VPN) as isolated islands, where local identity-based authorization schemes were appropriate. But, the trend towards paperless procedures leads to the need for these entities to interoperate. As an advance, extranets were proposed to connect entities that share common goals in a way that automates their administrative interactions using Internet technology. However, the limited authorization and access control capabilities provided by extranets is a mayor drawback for their application in open and heterogeneous scenarios. Trust appears as the main issue to address i...