# Technical Report MSC-2013-08

 TR#: MSC-2013-08 Class: MSC Title: Lossy Chains and Fractional Secret Sharing Authors: Omer Strulovich Supervisors: Yuval Ishai, Eyal Kushilevitz PDF MSC-2013-08.pdf Abstract: Motivated by the goal of controlling the amount of work required to access a shared resource or to solve a cryptographic puzzle, we introduce and study the related notions of lossy chains and fractional secret sharing. Fractional secret sharing generalizes traditional secret sharing by allowing a fine-grained control over the amount of uncertainty about the secret. More concretely, a fractional secret sharing scheme realizes a fractional access structure f:2^{[n]}\to [m] by guaranteeing that from the point of view of each set T\subseteq [n] of parties, the secret is uniformly distributed over a set of f(T) potential secrets. We show that every (monotone) fractional access structure can be realized. For symmetric structures, in which f(T) depends only on the size of $T$, we give an efficient construction with share size poly(n,log m). Our construction of fractional secret sharing schemes is based on the new notion of lossy chains which may be of independent interest. A lossy chain is a Markov chain (X_0,...,X_n) which starts with a random secret $X_0$ and gradually loses information about it at a rate which is specified by a loss function g. Concretely, in every step t, the distribution of X_0 conditioned on the value of X_t should always be uniformly distributed over a set of size g(t). We show how to construct such lossy chains efficiently for any possible loss function g, and prove that our construction achieves an optimal asymptotic information rate. Copyright The above paper is copyright by the Technion, Author(s), or others. Please contact the author(s) for more information

Remark: Any link to this technical report should be to this page (http://www.cs.technion.ac.il/users/wwwb/cgi-bin/tr-info.cgi/2013/MSC/MSC-2013-08), rather than to the URL of the PDF files directly. The latter URLs may change without notice.