Paper 2025/1340

Zelda: Efficient Multi-server Preprocessing PIR with Unconditional Security

Abstract

Private Information Retreival (PIR) schemes without preprocessing are known to incur linear server computation per client query. Several recent works have shown that by relying on a one-time preprocessing phase, we can get around this barrier, and achieve sublinear computation per query without relying on any cryptographic assumptions. Beimel et al. (CRYPTO'00) first showed a family of schemes whose bandwidth and computation per query scale as fast as $n^{O(1/S)}$ where $S$ denotes the number of servers and $n$ denotes the database size. Unfortunately, their schemes are not practical partly because the servers must each store an encoded version of the database, and the encoding length grows sharply as we increase $S$. The recent work of Singh et al. (TCC'24) showed how to achieve similar bandwidth scaling but without the server space blowup. To get this, they rely on a different type of preprocessing called client-specific preprocessing, where the stateful client stores some hints and the servers store only the original database. Unfortunately, Singh et al.'s result is completely impractical due to the reliance on Dvir and Gopi's PIR as a building block. We propose Zelda (short for ZEro-Leakage Data Access), the first concretely efficient, information-theoretic multi-server PIR scheme with sublinear computation. Our work makes both theoretical and practical contributions. On the theoretical front, we devise a unified framework for constructing multi-server PIR with client-specific preprocessing. This gives us a parametrizable family of schemes that asymptotically outperform all prior constructions in the same setting, including Singh et al. (TCC'24) and Ishai et al. (CRYPTO'24). On the practical front, Zelda is conceptually simple, self-contained, and does not rely on any underlying PIR as a building block. We implemented Zelda and open sourced our code. We compared the concrete performance of Zelda with a state-of-the-art PIR scheme called QuarterPIR (Eurocrypt'24), which relies on pseudorandom functions for security. Experimental results show that Zelda outperforms QuarterPIR in terms of online response time and client space (assuming typical fiber optical links), at the price of increased costs for offline maintenance operations.