Protected Secret Sharing and its Application to Threshold Cryptography

Abstract

In the secret reconstruction of Shamir’s (t,n) secret sharing scheme (SS), shares released by shareholders need to be protected otherwise, non-shareholders can also obtain the secret. Key establishment protocol can establish pairwise keys for any pair of shareholders. Then, shareholders can use these pairwise keys to protect shares in the secret reconstruction process. However, adding a key establishment in the secret reconstruction slows down the process significantly. Shamir’s SS is based on a univariate polynomial. Shares generated by a bivariate polynomial enable pairwise keys to be shared between any pair of shareholders. But we proposed a new type of SS, called protected secret sharing scheme (PSS), in which shares of shareholders can not only be used to reconstruct the secret but also be used to protect the secrecy of shares in the secret reconstruction process. Thus, the recovered secret is only available to shareholders but not to non-shareholders. A basic (t,n) PSS based on a bivariate polynomial is proposed. Furthermore, we introduce to use this basic PSS in the applications of threshold cryptography. The PSS is unique since it protects the secrecy of the recovered secret in a very efficient way.