In this paper we address the new problem of protecting volunteer computing systems from malicious volunteers who submit erroneous results by presenting sabotage-tolerance mechanisms that work without depending on checksums or cryptographic techniques. We first analyze the traditional technique of voting, and show how it reduces error rates exponentially with redundancy, but requires all work to be done at least twice, and does not work well when there are many saboteurs. We then present a new technique called spot-checking which reduces the error rate linearly (i.e., inversely) with the amount of work to be done, while only costing an extra fraction of the original time. We then integrate these mechanisms by presenting the new idea of credibility-based fault-tolerance, which uses probability estimates to efficiently limit and direct the use of redundancy. By using voting and spot-checking together credibility-based fault-tolerance effectively allows us to exponentially shrink an already linearly-reduced error rate, and thus achieve error rates that are orders-of-magnitude smaller than those offered by voting or spot-checking alone. We validate this new idea with Monte Carlo simulations, and discuss how credibility-based fault tolerance can be used with other mechanisms and in other applications.
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