A method, apparatus, system, and computer program product are presented in which federated domains interact within a federated environment. Domains within a federation are able to initiate federated single-sign-on operations for a user at other federated domains. A point-of-contact server within a domain relies upon a trust proxy within the domain to manage trust relationships between the domain and the federation. Trust proxies interpret assertions from other federated domains as necessary. Trust proxies may have a trust relationship with one or more trust brokers, and a trust proxy may rely upon a trust broker for assistance in interpreting assertions. To enhance security, domains may also require users to re-prove their identity through proof-of-possession challenges that are executed after a user has initiated a single-sign-on operation.