A remark on hierarchical threshold secret sharing

The main results of this paper are theorems which provide a solution to the open problem posed by Tassa [1]. He considers a specific family Γv of hierarchical threshold access structures and shows that two extreme members Γ∧ and Γ∨ of Γv are realized by secret sharing schemes which are ideal and perfect. The question posed by Tassa is whether the other members of Γv can be realized by ideal and perfect schemes as well. We show that the answer in general is negative. A precise definition of secret sharing scheme introduced by Brickell and Davenport in [2] combined with a connection between schemes and matroids are crucial tools used in this paper. Brickell and Davenport describe secret sharing scheme as a matrix M with n+1 columns, where n denotes the number of participants, and define ideality and perfectness as properties of the matrix M. The auxiliary theorems presented in this paper are interesting not only because of providing the solution of the problem. For example, they provide an upper bound on the number of rows of M if the scheme is perfect and ideal

Алгоритм вычисления элемента Штикельбергера для мнимых мультиквадратичных полей

Представлен алгоритм вычисления идеала Штикельбергера для мультиквадра- тичного поля K = Q^/d1^/d2,..., л/ЗП), где di = 1 (mod 4), i = 1,..., n, и di попарно взаимно просты. Мы алгоритмизируем идеи, описанные в работе Р. Кучеры 1996 г., доказываем корректность полученных алгоритмов и анализируем их сложность. Для 2n = [K : Q] алгоритм работает за время O(2n). Полученный результат полезен для решения криптоаналитических задач поиска короткого вектора в идеалах мультиквадратичных полей

Об однородных матроидах, соответствующих блок-схемам

Исследуются взаимосвязи однородных матроидов и блок-схем. Эта задача связана с изучением структур доступа идеальных совершенных схем разделения секрета. Под однородностью матроида понимается одинаковая мощность его циклов, при этом, возможно, не все подмножества этой мощности являются циклами. Для мощности циклов пять доказано, что однородный связный разделяющий матроид является равномерным. При этом если матроид связный и разделяющий, то двойственный ему матроид будет простым. Доказано, что если каждый цикл однородного разделяющего связного матроида является его гиперплоскостью, то ему соответствует блок-схема

On Representable Matroids and Ideal Secret Sharing

In secret sharing, the exact characterization of ideal access structures is a longstanding open problem. Brickell and Davenport (J. of Cryptology, 1991) proved that ideal access structures are induced by matroids. Subsequently, ideal access structures and access structures induced by matroids have attracted a lot of attention. Due to the difficulty of finding general results, the characterization of ideal access structures has been studied for several particular families of access structures. In all these families, all the matroids that are related to access structures in the family are representable and, then, the matroid-related access structures coincide with the ideal ones. In this paper, we study the characterization of representable matroids. By using the well known connection between ideal secret sharing and matroids and, in particular, the recent results on ideal multipartite access structures and the connection between multipartite matroids and discrete polymatroids, we obtain a characterization of a family of representable multipartite matroids, which implies a sufficient condition for an access structure to be ideal. By using this result and further introducing the reduced discrete polymatroids, we provide a complete characterization of quadripartite representable matroids, which was until now an open problem, and hence, all access structures related to quadripartite representable matroids are the ideal ones. By the way, using our results, we give a new and simple proof that all access structures related to unipartite, bipartite and tripartite matroids coincide with the ideal ones

Secret sharing schemes on sparse homogeneous access structures with rank three

One of the main open problems in secret sharing is the characterization of the ideal access structures. This problem has been studied for several families of access structures with similar results. Namely, in all these families, the ideal access structures coincide with the vector space ones and, besides, the optimal information rate of a non-ideal access structure is at most 2/3. An access structure is said to be r-homogeneous if there are exactly r participants in every minimal qualified subset. A first approach to the characterization of the ideal 3-homogeneous access structures is made in this paper. We show that the results in the previously studied families can not be directly generalized to this one. Nevertheless, we prove that the equivalences above apply to the family of the sparse 3-homogeneous access structures, that is, those in which any subset of four participants contains at most two minimal qualified subsets. Besides, we give a complete description of the ideal sparse 3-homogeneous access structures