The selfish mining (SM) attack of Eyal and Sirer allows a rational mining pool with a hash power (α) much less than 50% of the whole Bitcoin network to steal from the fair shares of honest miners. This attack has been studied extensively in various settings in order for its optimization and mitigation. In this context, Heilman proposes a defense “Freshness Preferred”, based on timestamps, which are issued routinely by a timestamp authority. In contrast, we consider the case where timestamps are generated by no authority; instead every miner includes the current time into a block freely. However, due to two attacks that we discover, this turns out to be a non-trivial task. These attacks are Oracle mining , which works by cleverly setting the timestamp to future, and Bold mining , which works by generating an alternative chain starting from a previous block. Unfortunately, these attacks are hard to analyze and optimize, and to our knowledge, the available tools fail to help us for this task. To ease this, we come up with generalized formulas for revenue and profitability of SM attacks. Our analyses show that the use of timestamps could be promising for selfish mining mitigation. Nevertheless, Freshness Preferred in its current form is quite vulnerable, as any rational miner with α > 0 can directly benefit from our attacks. To cope with this problem, we propose a novel SM mitigation algorithm Fortis without an authority, which protects the honest miners’ shares against any attacker with α < 27.0 against all the known SM-type attacks. By building upon the blockchain simulator BlockSim, we simulate our Oracle and Bold mining attacks against Freshness Preferred and Fortis . Simulation results also demonstrate the effectiveness of these attacks against the former and their ineffectiveness against the latter.

Private function evaluation aims to securely compute a function$f(x_1, \ldots, x_n)$without leaking any information other than what is revealed by the output, where$f$is a private input of one of the parties (say$\mathsf {Party}_1$) and$x_i$is a private input of the$i$th party$\mathsf {Party}_i$. In this article, we propose a novel and securetwo-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.

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 applications where an algorithm or a function itself needs to remain secret for reasons such as protecting intellectual property or security classification level. In this paper, we focus on improving 2-party PFE based on symmetric cryptographic primitives. In this respect, we look back at the seminal PFE framework presented by Mohassel and Sadeghian at Eurocrypt’13. We show how to adapt and utilize the well-known half gates garbling technique (Zahur et al., Eurocrypt’15) to their constant-round 2-party PFE scheme. Compared to their scheme, our resulting optimization significantly improves the efficiency of both the underlying Oblivious Evaluation of Extended Permutation (OEP) and secure 2-party computation (2PC) protocols, 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).