Simultaneous broadcast (SBC) protocols [Chor et al., FOCS 1985] constitute a special class of broadcast channels which have proved extremely useful in the design of various distributed computing constructions (e.g., multiparty computation, coin flipping, e-voting, fair bidding). As with any communication channel, it is crucial that SBC security is composable, i.e., it is preserved under concurrent protocol executions. The work of [Hevia, SCN 2006] proposes a formal treatment of SBC in the Universal Composability (UC) framework [Canetti, FOCS 2001] and a construction secure assuming an honest majority. In this work, we provide a comprehensive revision of SBC in the UC setting and improve the results of [Hevia, SCN 2006]. In particular, we present a new SBC functionality that captures both simultaneity and liveness by considering a broadcast period such that (i) within this period all messages are broadcast independently and (ii) after the period ends, the session is terminated without requiring participation of all parties. Next, we employ time-lock encryption (TLE) over a standard broadcast channel to devise an SBC protocol that realizes our functionality against any adaptive adversary corrupting up to all-but-one parties. In our study, we capture synchronicity via a global clock [Katz et al., TCC 2013], thus lifting the restrictions of the original synchronous communication setting used in [Hevia, SCN 2006]. As a building block of independent interest, we prove the first TLE protocol that is adaptively secure in the UC setting, strengthening the main result of [Arapinis et al., ASIACRYPT 2021]. Finally, we formally exhibit the power of our SBC construction in the design of UC-secure applications by presenting two interesting use cases: (i) distributed generation of uniform random strings, and (ii) decentralized electronic voting systems, without the presence of a special trusted party.

翻译：同步广播（SBC）协议 [Chor 等人，FOCS 1985] 是一类特殊的广播信道，已被证明在多种分布式计算构造（如多方计算、抛硬币、电子投票、公平竞标）中极为有用。与任何通信信道一样，SBC的安全性必须具有可组合性，即在并发协议执行中保持安全。[Hevia, SCN 2006] 的工作提出了通用可组合（UC）框架 [Canetti, FOCS 2001] 下 SBC的形式化处理方法，并给出了一种假设诚实多数的安全构造。本文在UC设置下对SBC进行了全面修订，改进了 [Hevia, SCN 2006] 的结果。具体而言，我们提出了一个新的SBC功能，通过考虑广播时段同时捕获同步性和活性，使得：(i) 在该时段内所有消息独立广播；(ii) 时段结束后，会话终止且无需所有参与方参与。接着，我们利用标准广播信道上的时间锁加密（TLE）设计了一个SBC协议，该协议在任何自适应对手（可腐化除一方外的所有方）面前实现了我们的功能。在研究中，我们通过全局时钟 [Katz 等人，TCC 2013] 捕获同步性，从而摆脱了 [Hevia, SCN 2006] 中原始同步通信设置的限制。作为独立有价值的构建模块，我们证明了首个在UC设置中具有自适应安全性的TLE协议，强化了 [Arapinis 等人，ASIACRYPT 2021] 的主要结果。最后，我们通过两个有趣的应用案例正式展示了SBC构造在UC安全应用设计中的强大能力：(i) 均匀随机字符串的分布式生成；(ii) 无需特殊可信方的去中心化电子投票系统。