Exposing Computationally-Challenged Byzantine Impostors

Internet protocols permit a single machine to masquerade as many, allowing an adversary to appear to control more nodes than it actually does. The possibility of suchSybil attackshas been taken to mean that distributed algorithms that tolerate only a fixed fract ion of faulty nodes are not useful in peer-to-peer systems unless identities can be verified externally. The pr esent work argues against this assumption, by presenting practical algorithms for the distributed com puting problem ofByzantine agreement that defend against Sybil attacks by using moderately hard puzzl es as a pricing scheme for identities. Though our algorithms do not prevent Sybil attacks entirely, they s olve Byzantine agreement (and some useful variants) when the limited fraction of nodes that can fail is replaced by a limited fraction of the total computational power. These results suggest that Byzantine agr ement and similar tools from the distributed computing literature are likely to help solve the p roblem of adversarial behavior by components of peer-to-peer systems.

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