The correct identification of the various entities connected to a network is the first and most important step on top of which it is possible to create the chain of trust needed, for example, for authorization and encryption purposes. Unfortunately in fully decentralized systems, typical, for example, of Peer-to-peer scenarios, such identification results extremely difficult, mainly for the lack of a central authority. This paper presents the Green Card Protocol, a novel protocol that allows entities exchanging information to gain a sufficient level of trustworthiness on the other peers? real identity.