Sybil defenses via social networks

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Abstract
The sybil attack in distributed systems refers to individual malicious users joining the system multiple times under multiple fake identities. Sybil attacks can easily invalidate the overarching prerequisite of many fault-tolerant designs which assume that the fraction of malicious nodes is not too large. This article presents a tutorial and survey on effective sybil defenses leveraging social networks. Since this approach of sybil defenses via social networks was introduced 5 years ago, it has attracted much more attention from the research community than many other alternatives. We will first explain the intuitions and insights behind this approach, and then survey a number of specific sybil defense mechanisms based on this approach, including SybilGuard, SybilLimit, SybilInfer, Gatekeeper, SumUp, Whanau, and Ostra. We will also discuss some practical implications and deployment considerations of this approach.