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Opportunistic Networks are envisioned to complement infrastruc-ture-based communication in overloaded cellular settings, in remote areas, during and immediately after large scale disasters. On account of their highly distributed and dynamic nature, as well as of their dependence on the honest cooperation of nodes, Opportunistic Networks are particularly vulnerable to sybil attacks. In a sybil attack, a node assumes multiple identities and attempts to form many links to the rest of the network, with the aim of gaining access to resources, influencing the network, circumventing detection of misbehavior ("spread the blame"), etc. Sybil attacks have been studied extensively in the context of distributed systems and online social networks. However, the Opportunistic Networking setting brings new challenges, specific to the network conditions: forming links may require significant resources from the attacker (e.g. time, speed …