Duniter is a software which allowing the creation of a free money as described by the RTM (Relative Theory of Money). This theory implies that monetary units are co-produced by each of the members of a given community. It is therefore essential that the members of the community are identified and recognized. In a word without dishonest people, a simple declaration of identity could be enough to become a member (and thus co-produce monetary units). But in our world, when it is about money, cheating cases can potentially come. Actually, it is tempting to register with many identities and thus produce a surplus of monetary units for its own advantage. This is even more true when it is about digital identities and when it is easy to create as much as we wish. We call such a thing a sybil attack.