Incoercible multiparty computation

Current secure multiparty protocols have the following deficiency. The public transcript of the communication can be used as an involuntary commitment of the parties to their inputs and outputs. Thus parties can be later coerced by some authority to reveal their private data. Previous work that has pointed this interesting problem out contained only partial treatment. The authors present the first general treatment of the coercion problem in secure computation. They first present a general definition of protocols that provide resilience to coercion. Their definition constitutes a natural extension of the general paradigm used for defining secure multiparty protocols. They next show that if trapdoor permutations exist then any function can be incoercibly computed (i.e., computed by a protocol that provides resilience to coercion) in the presence of computationally bounded adversaries and only public communication channels. This holds as long as less than half the parties are coerced (or corrupted). In particular, theirs are the first incoercible protocols without physical security assumptions. Also, the protocols constitute an alternative solution to the recently solved adaptive security problem. Their techniques are quite surprising and include non-standard use of deniable encryptions.