Automatic contact tracing is currently used in several countries in order to limit the spread of SARS-CoV-2. Many governments decided to develop smartphone apps based on the "Exposure Notifications" designed by Apple and Google according to a decentralized approach previously proposed by the DP-3T team. Decentralization was pushed as a key feature to protect privacy in contrast to centralized approaches that could leverage automatic contact tracing to realize mass-surveillance programs.In this work, taking into account the privacy and integrity vulnerabilities of DP-3T systems, we show the design of a decentralized contact tracing system named Pronto-C2 that has better resilience against various attacks. We also discuss the significant overhead of Pronto-C2 when used in real-world scenarios.

A major problem in blockchain-based supply chain management is the unclear validity of digital twins when dealing with digital representations of physical goods. Indeed, the use of blockchain technology to trace goods is ineffective if there is no strong correspondence between what is physically exchanged and the digital information that appears in blockchain transactions.In this work we propose a model for strengthening the supply chain management of physical goods leveraging blockchain technology through a digital-twin verification feature. Our model can be instantiated in various scenarios and we have in particular considered the popular case of food traceability. In contrast to weaker models known in the literature that propose their own ad-hoc properties to assess the robustness of their supply chain management systems, in this work we use the formalism of secure computation, where processes are described through generic and natural ideal functionalities.

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In this work, we consider executions of smart contracts for implementing secure multiparty computation (MPC) protocols on forking blockchains (e.g., Ethereum), and we study security and delay issues due to forks. In this setting, the classical double-spending problem tells us that messages of the MPC protocol should be confirmed on-chain before playing the next ones, thus slowing down the entire execution.Our contributions are twofold: For the concrete case of fairly tossing multiple coins with penalties, we notice that the lottery protocol of Andrychowicz et al. (S&P '14) becomes insecure if players do not wait for the confirmations of several transactions. In addition, we present a smart contract that instead retains security even when all honest players immediately answer to transactions appearing on-chain. We analyze the performance using Ethereum as testbed.We design a compiler that takes any "digital and universally composable" MPC protocol (with or without honest majority), and transforms it into another one (for the same task and same setup) which maintains security even if all messages are played on-chain without delays. The special requirements on the starting protocol mean that messages consist only of bits (e.g., no hardware token is sent) and security holds also in the presence of other protocols. We further show that our compiler satisfies fairness with penalties as long as honest players only wait for confirmations once. By reducing the number of confirmations, our protocols can be significantly faster than natural constructions.

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