Private function evaluation (PFE) is a special case of secure multi-party computation (MPC), where the function to be computed is known by only one party. PFE is useful in several real-life settings where an algorithm or a function itself needs to remain secret due to its confidential classification or intellectual property. In this work, we look back at the seminal PFE framework presented by Mohassel and Sadeghian at Eurocrypt'13. We show how to adapt and utilize the wellknown half gates garbling technique (Zahur et al., Eurocrypt'15) to their constant round 2-party PFE scheme. Compared to their scheme, our resulting optimization considerably improves the efficiency of both the underlying Oblivious Evaluation of Extended Permutation (OEP) and secure 2-party computation (2PC) protocol, and yields a more than 40% reduction in overall communication cost (the computation time is also slightly decreased, and the number of rounds remains unchanged).

Use of game theory and mechanism design in cloud security is a well-studied topic. When applicable, it has the advantages of being efficient and simple compared to cryptography alone. Most analyses consider two-party settings, or multi-party settings where coalitions are not allowed. However, many cloud security problems that we face are in the multiparty setting and the involved parties can almost freely collaborate with each other. To formalize the study of disincentivizing coalitions from deviating strategies, a well-known definition named k-resiliency has been proposed by Abraham et al. (ACM PODC '06). Since its proposal, k-resiliency and related definitions are used extensively for mechanism design. However, in this work we observe the shortcoming of k-resiliency. That is, although this definition is secure, it is too strict to use for many cases and rule out secure mechanisms as insecure. To overcome this issue, we propose a new definition named -repellence against the presence of a single coalition to replace k-resiliency. Our definition incorporates transferable utility in game theory as it is realistic in many distributed and multi-party computing settings. We also propose m-stability definition against the presence of multiple coalitions, which is inspired by threshold security in cryptography. We then show the advantages of our novel definitions on three mechanisms, none of which were previously analyzed against coalitions: incentivized cloud computation, forwarding data packages in ad hoc networks, and connectivity in ad hoc networks. Regarding the former, our concepts improve the proposal by Küpçü (IEEE TDSC '17), by ensuring a coalition-proof mechanism.

Private function evaluation aims to securely compute a function f (x1,. .. , xn) without leaking any information other than what is revealed by the output, where f is a private input of one of the parties (say Party 1) and xi is a private input of the i-th party Party i. In this work, we propose a novel and secure two-party private function evaluation (2PFE) scheme based on the DDH assumption. Our scheme introduces a reusability feature that significantly improves the state-of-the-art. Accordingly, our scheme has two variants, one is utilized in the initial execution of the function f , and the other is utilized in its subsequent evaluations. To the best of our knowledge, this is the first and most efficient 2PFE scheme that enjoys a reusablity feature. Our protocols achieve linear communication and computation complexities and a constant number of rounds which is at most three.

E-cash and cryptocurrency schemes have been a focus of applied cryptography for a long time. However, we acknowledge the continuing need for a cryptographic protocol that provides global scale, decentralized, secure, and fair delivery of donations. Such a protocol would replace central trusted entities (e.g., charity organizations) and guarantee the privacy of the involved parties (i.e., donors and recipients of the donations). In this work, we target this online donation problem and propose a practical solution for it. First, we propose a novel decentralized e-donation framework, along with its operational components and security definitions. Our framework relies on a public ledger that can be realized via a distributed blockchain. Second, we instantiate our e-donation framework with a practical scheme employing privacy-preserving cryptocurrencies and attributebased signatures. Third, we provide implementation results showing that our operations have feasible computation and communication costs. Finally, we prove the security of our e-donation scheme via formal reductions to the security of the underlying primitives.

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