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For k = ω(log n), we prove a Ω(k²n / log(kn)) lower bound on private simultaneous messages (PSM) with k parties who receive n-bit inputs. This extends the Ω(n) lower bound due to Appelbaum, Holenstein, Mishra and Shayevitz [Journal of Cryptology, 2019] to the many-party (k = ω(log n)) setting. It is the first PSM lower bound that increases quadratically with the number of parties, and moreover the first unconditional, explicit bound that grows with both k and n. This note extends the work of Ball, Holmgren, Ishai, Liu, and Malkin [ITCS 2020], who prove communication complexity lower bounds on decomposable randomized encodings (DREs), which correspond to the special case of k-party PSMs with n = 1. To give a concise and readable introduction to the method, we focus our presentation on perfect PSM schemes.
@InProceedings{ball_et_al:LIPIcs.ITC.2022.7,
author = {Ball, Marshall and Randolph, Tim},
title = {{A Note on the Complexity of Private Simultaneous Messages with Many Parties}},
booktitle = {3rd Conference on Information-Theoretic Cryptography (ITC 2022)},
pages = {7:1--7:12},
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.2022.7},
URN = {urn:nbn:de:0030-drops-164855},
doi = {10.4230/LIPIcs.ITC.2022.7},
annote = {Keywords: Secure computation, Private Simultaneous Messages}
}