Peer-to-peer Grids that seek to harvest idle cycles available throughout the Internet are vulnerable to hosts that fraudulently accept computational tasks and then maliciously return arbitrary results. Current strategies employed by popular cooperative computing Grids, such as SETI@Home, rely heavily on task replication to check results. However, result verification through replication suffers from two potential shortcomings: (1) susceptibility to collusion in which a group of malicious hosts conspire to return the same bad results and (2) high fixed overhead incurred by running redundant copies of the task.

In this paper, we first propose a scheme called Quiz to combat collusion. The basic idea of Quiz is to insert indistinguishable quiz tasks with verifiable results known to the client within a package containing several normal tasks. The client can then accept or reject the normal task results based on the correctness of quiz results. Our second contribution is the promotion of trust-based task scheduling in peer-to-peer Grids. By coupling a reputation system with the basic verification schemes - Replication and Quiz - a client can potentially avoid malicious hosts and also reduce the overhead of verification for trusted hosts.