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Complete Characterization of Adversaries Tolerable in Secure Multi-Party Computation

Martin Hirt and Ueli Maurer

The classical results in unconditional multi-party computation among a set of $n$ players state that less than $n/2$ passive or less than $n/3$ active adversaries can be tolerated; assuming a broadcast channel the threshold for active adversaries is $n/2$. Strictly generalizing these results we specify the set of potentially misbehaving players as an arbitrary set of subsets of the player set. We prove the necessary and sufficient conditions for the existence of secure multi-party protocols in terms of the potentially misbehaving player sets.

For every function there exists a protocol secure against a set of potential passive collusions if and only if no two of these collusions add up to the full player set. The same condition applies for active adversaries when assuming a broadcast channel. Without broadcast channels, for every function there exists a protocol secure against a set of potential active adverse player sets if and only if no three of these sets add up to the full player set.

The complexities of the protocols not using a broadcast channel are polynomial, that of the protocol with broadcast is only slightly higher.