Cloud federation aims to cost-effective assets and resources optimization among heterogeneous environments where clouds can cooperate together with the goal of obtaining unbounded computation resources, hence new business opportunities. This paper describes an architecture for the federation establishment, where clouds that need external resources ask to federated clouds the renting of extra physical resources. Our architecture introduces a new module named Cross-Cloud Federation Manager including three agents (Discovery, Match-making and Authentication). In this work, we specifically focus on the authentication agent, which is responsible for a secure federation. To address such problem we propose a technical solution based on the IdP/SP model along with the SAML technology.