Multi-linear Secret-Sharing Schemes

Multi-linear secret-sharing schemes are the most common secret-sharing schemes. In these schemes the secret is composed of some field elements and the sharing is done by applying some fixed linear mapping on the field elements of the secret and some randomly chosen field elements. If the secret contains one field element, then the scheme is called linear. The importance of multi-linear schemes is that they provide a simple non-interactive mechanism for computing shares of linear combinations of previously shared secrets. Thus, they can be easily used in cryptographic protocols. In this work we study the power of multi-linear secret-sharing schemes. On one hand, we prove that multi-linear secret-sharing schemes in which the secret is composed of p field elements are more powerful than schemes in which the secret is composed of less than p field elements (for every prime p). On the other hand, we prove super-polynomial lower bounds on the share size in multi-linear secretsharing schemes. Previously, such lower bounds were known only for linear schemes. ∗Partially supported by ISF grant 938/09 and by the Frankel Center for Computer Science.