Round-Preserving Parallel Composition of Probabilistic-Termination Protocols

Ran Cohen, Sandro Coretti, Juan Garay, Vasileios Zikas

Research output: Chapter in Book/Report/Conference proceedingConference contribution


An important benchmark for secure multi-party computation (MPC) protocols is their round complexity. For several important MPC tasks, (tight) lower bounds on the round complexity are known. However, for some of these tasks, such as broadcast, the lower bounds can be circumvented when the termination round of every party is not a priori known, and simultaneous termination is not guaranteed. Protocols with this property are called probabilistic-termination (PT) protocols.
Running PT protocols in parallel affects the round complexity of the resulting protocol in somewhat unexpected ways. For instance, an execution of m protocols with constant expected
round complexity might take O(log m) rounds to complete. In a seminal work, Ben-Or and El-Yaniv (Distributed Computing ‘03) developed a technique for parallel execution of arbitrarily
many broadcast protocols, while preserving expected round complexity. More recently, Cohen et al. (CRYPTO ‘16) devised a framework for universal composition of PT protocols, and provided the first composable parallel-broadcast protocol with a simulation-based proof. These constructions crucially rely on the fact that broadcast is “privacy free,” and do not generalize to arbitrary protocols in a straightforward way. This raises the question of whether it is possible to execute arbitrary PT protocols in parallel, without increasing the round complexity.
In this paper we tackle this question and provide both feasibility and infeasibility results. We construct a round-preserving protocol compiler, secure against a minority of actively corrupted
parties, that compiles arbitrary protocols into a protocol realizing their parallel composition, while having a black-box access to the underlying protocols. Furthermore, we prove that the same
cannot be achieved, using known techniques, given only black-box access to the functionalities realized by the protocols, unless merely security against semi-honest corruptions is required, for
which case we provide a protocol.
Original languageEnglish
Title of host publication44th International Colloquium on Automata, Languages, and Programming (ICALP 2017)
PublisherSchloss Dagstuhl - Leibniz-Zentrum für Informatik
Number of pages15
ISBN (Electronic)978-3-95977-041-5
Publication statusPublished - 14 Jul 2017
EventICALP 2017: 44th International Colloquium on Automata, Languages, and Programming - Warsaw, Poland
Duration: 10 Jul 201714 Jul 2017

Publication series

NameLeibniz International Proceedings in Informatics (LIPIcs)
PublisherSchloss Dagstuhl--Leibniz-Zentrum fuer Informatik
ISSN (Electronic)1868-8969


ConferenceICALP 2017
Abbreviated titleICALP 2017
Internet address


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