澁谷 智治

conventional secret sharing, password protected secret sharing (PPSS) was invented. Recently, Nakahara et al. improved Ogata's multiple-use PPSS (mPPSS), which protects multiple data by one password, by employing secure computation known as CSEC70 method in the user authentication. In this paper, we propose an attack on Nakahara's mPPSS that presumes on a security hall in the adoption of CSEC70 method. Then, we show that the proposed attack enables an administrator of any server of the system to disclose the password of an arbitrary user in Nakahara's mPPSS.

In this talk, a group theoretic representation suitable for the rank-modulation (RM) scheme over the multi-cell ranking developed by En Gad et al. is presented. By introducing an action of the group of all permutation matrices on the set of all permutations, the scheme is clearly reformulated. Moreover, we introduce the concept of r-dominating sets over the multi-cell ranking, which is a generalization of conventional dominating sets, in the design of rank-modulation rewriting codes. The concept together with the presented group theoretic representation helps to yield an explicit formula of an upper bound on the size of the set of messages that can be stored in the memory by using RM rewriting codes over multi-cell ranking. We also note that this bound enables us to consider the trade-off between the size of the storable message set and the rewriting cost more closely.