  This paper studies pipelined algorithms for protecting distributed grid
computations from cheating participants, who wish to be rewarded for tasks they
receive but don't perform. We present improved cheater detection algorithms
that utilize natural delays that exist in long-term grid computations. In
particular, we partition the sequence of grid tasks into two interleaved
sequences of task rounds, and we show how to use those rounds to devise the
first general-purpose scheme that can catch all cheaters, even when cheaters
collude. The main idea of this algorithm might at first seem
counter-intuitive--we have the participants check each other's work. A naive
implementation of this approach would, of course, be susceptible to collusion
attacks, but we show that by, adapting efficient solutions to the parallel
processor diagnosis problem, we can tolerate collusions of lazy cheaters, even
if the number of such cheaters is a fraction of the total number of
participants. We also include a simple economic analysis of cheaters in grid
computations and a parameterization of the main deterrent that can be used
against them--the probability of being caught.
