In a secret sharing scheme, a secret s is divided into multiple shares and distributed among n participants. As more shares are collected, the mutual information between the secret and the shares increases monotonically. In the perfect case, the best known explicit lower bound on the share size is given by $\frac{{{2^k} - 1}}{k} \cdot H(s)$ , where H(s) is the entropy of the secret and k is the largest integer such that n ≥ 2k+k−2. This paper considers the non-perfect case, in which each share reveals at most a $\frac{1}{L}$ fraction of the secret's entropy, for an integer L > 1. A prior result in this setting provides a lower bound of $\frac{{{2^k} - 1}}{{kL}} \cdot H(s)$ , under the condition n ≥ 2k+k+L−3. We provide a new lower bound that approaches the perfect-case bound, namely $\frac{{{2^k} - 2 + 1/L}}{k} \cdot H(s)$ , which holds under the condition n ≥ 2k+k+2L−4. The proof generalizes perfect-case techniques by increasing the number of participants by 2(L−1). These results refine our understanding of the trade-off between share size and information leakage in non-perfect secret sharing. In particular, we construct non-perfect access structures that nearly attain the perfect lower bound with fewer participants than previous non-perfect bounds, thus complementing and extending prior work.In a secret sharing scheme, a secret s is divided into multiple shares and distributed among n participants. As more shares are collected, the mutual information between the secret and the shares increases monotonically. In the perfect case, the best known explicit lower bound on the share size is given by $\frac{{{2^k} - 1}}{k} \cdot H(s)$, where H(s) is the entropy of the secret and k is the largest integer such that n ≥ 2k+k−2. This paper considers the non-perfect case, in which each share reveals at most a $\frac{1}{L}$ fraction of the secret's entropy, for an integer L > 1. A prior result in this setting provides a lower bound of $\frac{{{2^k} - 1}}{{kL}} \cdot H(s)$, under the condition n ≥ 2k+k+L−3. We provide a new lower bound that approaches the perfect-case bound, namely $\frac{{{2^k} - 2 + 1/L}}{k} \cdot H(s)$, which holds under the condition n ≥ 2k+k+2L−4. The proof generalizes perfect-case techniques by increasing the number of participants by 2(L−1). These results refine our understanding of the trade-off between share size and information leakage in non-perfect secret sharing. In particular, we construct non-perfect access structures that nearly attain the perfect lower bound with fewer participants than previous non-perfect bounds, thus complementing and extending prior work.

IEEE Information Theory Workshop (ITW)Sep 29, 20250 citations

On the Impossibility of d-Multiplicative Non-perfect Secret Sharing

Maki Yoshida, Toru Fujiwara

A secret sharing scheme is said to be d-multiplicative if the scheme allows the players to multiply shared d secrets by locally converting their shares into an additive sharing of the product. In the previous work, the following negative result for perfect secret sharing has been shown: The d-multiplicative secret sharing for d players is impossible. This paper extends the impossibility result to non-perfect secret sharing. Our main result is a proof that d-multiplicative secret sharing for d players is impossible even if every player has partial information on the secret (e.g., all but one bit). This result means that there is no need to relax the privacy requirement with leakage of partial information only for the purpose of d-multiplication.A secret sharing scheme is said to be d-multiplicative if the scheme allows the players to multiply shared d secrets by locally converting their shares into an additive sharing of the product. In the previous work, the following negative result for perfect secret sharing has been shown: The d-multiplicative secret sharing for d players is impossible. This paper extends the impossibility result to non-perfect secret sharing. Our main result is a proof that d-multiplicative secret sharing for d players is impossible even if every player has partial information on the secret (e.g., all but one bit). This result means that there is no need to relax the privacy requirement with leakage of partial information only for the purpose of d-multiplication.

IEICE Transactions on Fundamentals of Electronics Communications and Computer SciencesJan 1, 20153 citations

\frac{{{2^k} - 1}}{k} \cdot H(s)

, where H(s) is the entropy of the secret and k is the largest integer such that n ≥ 2k+k−2. This paper considers the non-perfect case, in which each share reveals at most a

\frac{1}{L}

fraction of the secret's entropy, for an integer L > 1. A prior result in this setting provides a lower bound of

\frac{{{2^k} - 1}}{{kL}} \cdot H(s)

, under the condition n ≥ 2k+k+L−3. We provide a new lower bound that approaches the perfect-case bound, namely

\frac{{{2^k} - 2 + 1/L}}{k} \cdot H(s)

, which holds under the condition n ≥ 2k+k+2L−4. The proof generalizes perfect-case techniques by increasing the number of participants by 2(L−1). These results refine our understanding of the trade-off between share size and information leakage in non-perfect secret sharing. In particular, we construct non-perfect access structures that nearly attain the perfect lower bound with fewer participants than previous non-perfect bounds, thus complementing and extending prior work.