,
Mary Wootters
Creative Commons Attribution 4.0 International license
A Homomorphic Secret Sharing (HSS) scheme is a secret-sharing scheme that shares a secret x among s servers, and additionally allows an output client to reconstruct some function f(x) using information that can be locally computed by each server. A key parameter in HSS schemes is download rate, which quantifies how much information the output client needs to download from the servers. Often, download rate is improved by amortizing over 𝓁 instances of the problem, making 𝓁 also a key parameter of interest. Recent work [Fosli et al., 2022] established a limit on the download rate of linear HSS schemes for computing low-degree polynomials and constructed schemes that achieve this optimal download rate; their schemes required amortization over 𝓁 = Ω(s log(s)) instances of the problem. Subsequent work [Blackwell and Wootters, 2023] completely characterized linear HSS schemes that achieve optimal download rate in terms of a coding-theoretic notion termed optimal labelweight codes. A consequence of this characterization was that 𝓁 = Ω(s log(s)) is in fact necessary to achieve optimal download rate. In this paper, we characterize all linear HSS schemes, showing that schemes of any download rate are equivalent to a generalization of optimal labelweight codes. This equivalence is constructive and provides a way to obtain an explicit linear HSS scheme from any linear code. Using this characterization, we present explicit linear HSS schemes with slightly sub-optimal rate but with much improved amortization 𝓁 = O(s). Our constructions are based on algebraic geometry codes (specifically Hermitian codes and Goppa codes).
@InProceedings{blackwell_et_al:LIPIcs.ITC.2024.7,
author = {Blackwell, Keller and Wootters, Mary},
title = {{Improved Trade-Offs Between Amortization and Download Bandwidth for Linear HSS}},
booktitle = {5th Conference on Information-Theoretic Cryptography (ITC 2024)},
pages = {7:1--7:21},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-333-1},
ISSN = {1868-8969},
year = {2024},
volume = {304},
editor = {Aggarwal, Divesh},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2024.7},
URN = {urn:nbn:de:0030-drops-205156},
doi = {10.4230/LIPIcs.ITC.2024.7},
annote = {Keywords: Error Correcting Codes, Homomorphic Secret Sharing}
}