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  1. Free, publicly-accessible full text available May 9, 2024
  2. Private Set Union (PSU) allows two players, the sender and the receiver, to compute the union of their input datasets with- out revealing any more information than the result. While it has found numerous applications in practice, not much re- search has been carried out so far, especially for large datasets. In this work, we take shuffling technique as a key to de- sign PSU protocols for the first time. By shuffling receiver’s set, we put forward the first protocol, denoted as ΠRPSU, that eliminates the expensive operations in previous works, such as additive homomorphic encryption and repeated operations on the receiver’s set. It outperforms the state-of-the-art design by Kolesnikov et al. (ASIACRYPT 2019) in both efficiency and security; the unnecessary leakage in Kolesnikov et al.’s design, can be avoided in our design. We further extend our investigation to the application sce- narios in which both players may hold unbalanced input datasets. We propose our second protocol ΠSPSU, by shuffling the sender’s dataset. This design can be viewed as a dual ver- sion of our first protocol, and it is suitable in the cases where the sender’s input size is much smaller than the receiver’s. Finally, we implement our protocols ΠRPSU and ΠSPSU in C++ on big datasets, and perform a comprehensive evaluation in terms of both scalability and parallelizability. The results demonstrate that our design can obtain a 4-5× improvement over the state-of-the-art by Kolesnikov et al. with a single thread in WAN/LAN settings. 
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  3. Private Set Union (PSU) allows two players, the sender and the receiver, to compute the union of their input datasets with- out revealing any more information than the result. While it has found numerous applications in practice, not much research has been carried out so far, especially for large datasets. In this work, we take shuffling technique as a key to design PSU protocols for the first time. By shuffling receiver’s set, we put forward the first protocol, denoted as $\Pi^R_{PSU}$, that eliminates the expensive operations in previous works, such as additive homomorphic encryption and repeated operations on the receiver’s set. It outperforms the state-of-the-art design by Kolesnikov et al. (ASIACRYPT 2019) in both efficiency and security; the unnecessary leakage in Kolesnikov et al.’s design, can be avoided in our design. We further extend our investigation to the application scenarios in which both players may hold unbalanced input datasets. We propose our second protocol $\Pi^S_{PSU}$, by shuffling the sender’s dataset. This design can be viewed as a dual version of our first protocol, and it is suitable in the cases where the sender’s input size is much smaller than the receiver’s. Finally, we implement our protocols $\Pi^R_{PSU}$ and $\Pi^S_{PSU}$ in C++ on big datasets, and perform a comprehensive evaluation in terms of both scalability and parallelizability. The results demonstrate that our design can obtain a 4-5X improvement over the state-of-the-art by Kolesnikov et al. with a single thread in WAN/LAN settings. 
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  4. The conventional (election) voting systems, e.g., representative democracy, have many limitations and often fail to serve the best interest of the people in a collective decision-making process. To address this issue, the concept of liquid democracy has been emerging as an alternative decision-making model to make better use of “the wisdom of crowds”. However, there is no known cryptographically secure e-voting implementation that supports liquid democracy. In this work, we propose a new voting concept called statement voting, which can be viewed as a natural extension of the conventional voting approaches. In the statement voting, instead of defining a concrete elec- tion candidate, each voter can define a statement in his/her ballot but leave the vote “undefined” during the voting phase. During the tally phase, the (conditional) actions expressed in the statement will be carried out to determine the final vote. We initiate the study of statement voting under the Universal Composability (UC) framework, and propose several construction frameworks together with their instantiations. As an application, we show how statement voting can be used to realize a UC-secure liquid democracy voting system. We remark that our statement voting can be extended to enable more complex voting and generic ledger-based non-interactive multi-party computation. We believe that the statement voting concept opens a door for constructing a new class of e-voting schemes. 
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  5. We study how to construct secure digital signature schemes in the presence of kleptographic attacks. Our work utilizes an offline watchdog to clip the power of subversions via only one-time black-box testing of the implementation. Previous results essentially rely on an online watchdog which requires the collection of all communicating transcripts (or active re-randomization of messages). We first give a simple but generic construction, without random oracles, in the partial-subversion model in which key generation and signing algorithms can be subverted. Then, we give the first digital signature scheme in the complete-subversion model in which all cryptographic algorithms can be subverted. This construction is based on the full-domain hash. Along the way, we enhance the recent result of Russell et al. (CRYPTO 2018) about correcting a subverted random oracle. 
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