SybilRes: A Sybil-resilient Flow-Based Decentralized Reputation Mechanism

Due to the possibility of cheap identity creation, decentralized online reputation mechanisms are susceptible to sybil attacks. Barter Cast is a reputation mechanism used in the Internet-deployed Tribler file-sharing client. In this paper we study the opportunities for sybil attacks in Barter Cast and we devise a method for making Barter Cast sybil resilient, which is incorporated in a protocol called Sybil Res. Like in Barter Cast, in Sybil Res each peer maintains a local subjective weighted directed graph reflecting data transfer actions in Tribler, from which it computes the reputations of other peers using a flow based algorithm taking the edge weights as flows. In Sybil Res, after an upload action, the uploading peer discounts the weights of the edges on the paths from the down loader to itself. As a consequence, due to the way reputations are computed, the reputation of a peer performing a sybil attack decreases fast. To mitigate the negative impact of edge weight discounting on the reputations of honest peers, after a download action, the downloading peer increases the weights of the edges on the paths from the up loader to itself. We demonstrate that Sybil Res is effective in practice by means of trace-driven simulations using data collected from the Tribler network. The results show that Sybil Res effectively marginalizes attackers while having a minimal effect on the reputations of honest peers.