Most existing work on secure multi-party computation (MPC) ignores a key idiosyncrasy of modern communication networks, that there are a limited number of communication paths between any two nodes, many of which might even be corrupted. The problem becomes particularly acute in the information-theoretic setting, where the lack of trusted setups (and the cryptographic primitives they enable) makes communication over sparse networks more challenging. The work by Garay and Ostrovsky [EUROCRYPT'08] on almost-everywhere MPC (AE-MPC), introduced "best-possible security" properties for MPC over such incomplete networks, where necessarily some of the honest parties may be excluded from the computation. In this work, we provide a universally composable definition of almost-everywhere security, which allows us to automatically and accurately capture the guarantees of AE-MPC (as well as AE-communication, the analogous "best-possible security" version of secure communication) in the Universal Composability (UC) framework of Canetti. Our results offer the first simulation-based treatment of this important but under-investigated problem, along with the first simulation-based proof of AE-MPC. To achieve that goal, we state and prove a general composition theorem, which makes precise the level or "quality" of AE-security that is obtained when a protocol’s hybrids are replaced with almost-everywhere components.
@InProceedings{chandran_et_al:LIPIcs.ITC.2022.14, author = {Chandran, Nishanth and Forghani, Pouyan and Garay, Juan and Ostrovsky, Rafail and Patel, Rutvik and Zikas, Vassilis}, title = {{Universally Composable Almost-Everywhere Secure Computation}}, booktitle = {3rd Conference on Information-Theoretic Cryptography (ITC 2022)}, pages = {14:1--14:25}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-238-9}, ISSN = {1868-8969}, year = {2022}, volume = {230}, editor = {Dachman-Soled, Dana}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2022.14}, URN = {urn:nbn:de:0030-drops-164929}, doi = {10.4230/LIPIcs.ITC.2022.14}, annote = {Keywords: Secure multi-party computation, universal composability, almost-everywhere secure computation, sparse graphs, secure message transmission} }