## Cryptology ePrint Archive: Report 2019/1383

Communication-Efficient Proactive Secret Sharing for Dynamic Groups with Dishonest Majorities

Karim Eldefrawy and Tancrède Lepoint and Antonin Leroux

Abstract: In standard Secret Sharing (SS), a dealer shares a secret $s$ among $n$ parties such that an adversary corrupting no more than $t$ parties does not learn $s$, while any $t+1$ parties can efficiently recover $s$. Proactive Secret Sharing (PSS) retains confidentiality of $s$ even when a mobile adversary corrupts all parties over the lifetime of the secret, but no more than a threshold $t$ in each epoch (called a refresh period). Withstanding such adversaries has become of increasing importance with the emergence of settings where private keys are secret shared and used to sign cryptocurrency transactions, among other applications. Feasibility of single-secret PSS for static groups with dishonest majorities was demonstrated but with a protocol that requires inefficient communication of $O(n^4)$.

In this work, we improve over prior work in three directions: batching without incurring a linear loss in corruption threshold, communication efficiency, and handling dynamic groups. While each of properties we improve upon appeared independently in the context of PSS and in other previous work, handling them simultaneously (and efficiently) in a single scheme faces non-trivial challenges. Some PSS protocols can handle batching of $\ell \sim n$ secrets, but all of them are for the honest majority setting. Techniques typically used to accomplish such batching decrease the tolerated corruption threshold bound by a linear factor in $\ell$, effectively limiting the number of elements that can be batched with dishonest majority. We solve this problem by reducing the threshold decrease to $\sqrt{\ell}$ instead, allowing us to deal with the dishonest majority setting when $\ell \sim n$. This is accomplished based on new bivariate-polynomials-based techniques for sharing, and refreshing and recovering of shares, that allow batching of up to $n-2$ secrets in our PSS. To tackle the efficiency bottleneck the constructed PSS protocol requires only $O(n^3/\ell)$ communication for $\ell$ secrets, i.e., an amortized communication complexity of $O(n^2)$ when the maximum batch size is used. To handle dynamic groups we develop three new sub-protocols to deal with parties joining and leaving the group.

Category / Keywords: cryptographic protocols / proactive secret sharing, dishonest majority, dynamic groups

Original Publication (with major differences): 18th International Conference on Applied Cryptography and Network Security (ACNS 2020)