Cooperative environments where multiple organizations interact for providing e-services to their customers are widely diffused and often referred as virtual enterprises. IT systems supporting these inter-organizational models must be designed by taking into account both functional and non-functional issues. Among the non-functional issues, information security solutions play an important role as mechanisms for reinforcing trust among members of a virtual enterprise and their supplier/customers. In this paper, we outline a set of non-functional requirements for IT systems supporting virtual enterprises, and describe the federated identity management system which has been implemented in the context of an EU project (LD-CAST) as an example of a trust-reinforcing mechanism.