Paper 2025/2257 Original: eprint.iacr.org/2025/2257 ↗

\textsc{Npir}: High-Rate PIR for Databases with Moderate-Size Records

Abstract

Private information retrieval (PIR) is a widely used technique in privacy-preserving applications that enables users to retrieve records from a database without revealing any information about their queries. This study focuses on a type of PIR that has a high ratio between the size of the record retrieved by the client and the server's response. Although significant progress has been made in high-rate PIR in recent years, the computational overhead on the server side remains rather high. This results in low server throughput, particularly for applications involving databases with moderate-size records (i.e. tens of kilobytes), such as private advertising system. In this paper, we present \textsc{Npir}, a high-rate single-server PIR that is based on NTRU encoding and outperforms the state-of-the-art Spiral (Menon \& Wu, S\&P 2022) and NTRUPIR (Xia \& Wang, EuroS\&P 2024) in terms of server throughput for databases with moderate-size records. In specific, for databases ranging from 1 GB to 32 GB with 32 KB records, the server throughput of \textsc{Npir} is 1.50 to 2.84 times greater than that of Spiral and 1.77 to 2.55 times greater than that of NTRUPIR. To improve server throughput without compromising the high-rate feature, we propose a novel tool called NTRU packing, which compresses the constant terms of underlying polynomials of multiple NTRU encodings into a single NTRU encoding, thereby reducing the size of the server's response. Furthermore, \textsc{Npir} naturally supports batch processing for moderate-size records, and can easily handle retrieving for records of varying sizes.tions, we advance secure communication protocols under challenging conditions.