Game-Theoretic Security for Two-Party Protocols.

Shota Goto, Junji Shikata

In this paper, we consider the following question: Does composing protocols having game-theoretic security result in a secure protocol in the sense of game-theoretic security? In order to discuss the composability of game-theoretic properties, we study security of cryptographic protocols in terms of the universal composability (UC) and game theory simultaneously. The contribution of this paper is the following: (i) We propose a compiler of two-party protocols in the local universal composability (LUC) framework such that it transforms any two-party protocol secure against semi-honest adversaries into a protocol secure against malicious adversaries in the LUC framework; (ii) We consider the application of our compiler to oblivious transfer (OT) protocols, by which we obtain a construction of OT meeting both UC security and game-theoretic-security.

Gilad Asharov, Ran Canetti, Carmit Hazay

Edouard Pelchat

Two parties, wishing to establish a shared random bit at a distance, must solve a problem known as coin flipping. Classical two-party secure computations such as coin flipping are impossible in the information theoretic setting. However quantum information allows for the possibility of unconditional security. Mochon defines a model of secure two-party computations, point games, to show the existence of optimal quantum weak coin flipping protocols in [29]. His ground-breaking result is widely accepted, but its proof is very complicated, has never been validated, and remains poorly understood despite the efforts of several researchers [12,25,35]. In this thesis we use analytical and mathematical methods to study point games. We discover several new primitives, and construct new building blocks to define a new family of point games. We present a new truncation technique that is simpler, more efficient, is more intuitive, and achieves better results than the best previously known. We provide concrete examples of these point games. We analyze Mochon's optimal point games, and we identify the key elements of their construction that are not specified. We also provide a new recursion technique that achieves better results than the one proposed by Mochon. Furthermore, we construct simple and elegant point games using binomial coefficients. Our point games are simpler than Mochon's, since his point games are constructed using techniques in mathematical analysis. Studying point games with new techniques may lead to a simpler formalism of two-party secure computations, and to more efficient protocols.

Haruna Higo, Keisuke Tanaka, Kenji Yasunaga

Shota Goto, Junji Shikata

Carmit Hazay, Yehuda Lindell