Paper 2019/597

A Candidate Access Structure for Super-polynomial Lower Bound on Information Ratio

Shahram Khazaei

Abstract

The contribution vector (convec) of a secret sharing scheme is the vector of all share sizes divided by the secret size. A measure on the convec (e.g., its maximum or average) is considered as a criterion of efficiency of secret sharing schemes, which is referred to as the information ratio. It is generally believed that there exists a family of access structures such that the information ratio of any secret sharing scheme realizing it is $2^{\mathrm{\Omega}(n)}$, where the parameter $n$ stands for the number of participants. The best known lower bound, due to Csirmaz (1994), is $\mathrm{\Omega}(n/\log n)$. Closing this gap is a long-standing open problem in cryptology. Using a technique called \emph{substitution}, we recursively construct a family of access structures by starting from that of Csirmaz, which might be a candidate for super-polynomial information ratio. We provide support for this possibility by showing that our family has information ratio ${n^{\mathrm{\Omega}(\frac{\log n}{\log \log n})}}$, assuming the truth of a well-stated information-theoretic conjecture, called the \emph{substitution conjecture}. The substitution method is a technique for composition of access structures, similar to the so called block composition of Boolean functions, and the substitution conjecture is reminiscent of the Karchmer-Raz-Wigderson conjecture on depth complexity of Boolean functions. It emerges after introducing the notion of convec set for an access structure, a subset of $n$-dimensional real space, which includes all achievable convecs. We prove some topological properties about convec sets and raise several open problems.