Robust Secret Sharing Schemes Against Local Adversaries

We study robust secret sharing schemes in which between one third and one half of the players are corrupted. In this scenario, robust secret sharing is possible only with a share size larger than the secrets, and allowing a positive probability of reconstructing the wrong secret. We focus on the most challenging case where the number corruptions is just one less than the number of honest players. In the standard model, it is known that at least m + k bits per share are needed to robustly share a secret of bit-length m with an error probability of 2-k; however, to the best of our knowledge, no efficient scheme matches this lower bound: the one that gets closest has share size m+ O (n+ k), where n is the number of players in the scheme. We show that it is possible to obtain schemes with close to minimal share size in a model of local adversaries, i. e. in which corrupt players cannot communicate between receiving their respective honest shares and submitting corrupted shares to the reconstruction procedure, but may coordinate before the execution of the protocol and can also gather information afterwards. In this limited adversarial model, we prove a lower bound of roughly m + k bits on the minimal share size, which is (somewhat surprisingly) similar to the lower bound in the standard model, where much stronger adversaries are allowed. We then present efficient scheme that essentially meets our lower bound, and has shorter share size than any known efficient construction in the standard model for the same set of parameters. For our construction, we introduce a novel procedure that compiles an error correcting code into a new randomized one, with the following two properties: a single local portion of a codeword leaks no information on the encoded message itself, and any set of portions of a codeword reconstructs the message with error probability exponentially low in the set size.