4-Round Concurrent Non-Malleable Commitments

The round complexity of non-malleable commitments and non-malleable zero knowledge arguments has been an open question for long time. Very recent results of Pass [TCC 2013] and of Goyal et al. [FOCS 2014, STOC 2016], gave almost definitive answers. In this work we show how to construct round-efficient non-malleable protocols via compilers. Starting from protocols enjoying limited non-malleability features, our compilers obtain fullfledged non-malleability without penalizing the round complexity. By instantiating our compilers with known candidate constructions, the resulting schemes improve the current state of the art. Indeed we show the first 4-round non-malleable zero knowledge argument system and the first 4-round concurrent non-malleable commitment scheme under standard polynomial-time hardness assumptions. We also discuss concrete man-in-the-middle adversaries that invalidate security proofs of previously known constructions for 4-round concurrent non-malleable commitments and 4-round non-malleable zero knowledge under polynomial-time hardness assumptions.