Paper 2026/1814 Original: eprint.iacr.org/2026/1814 ↗

LetoPIR: Fast Keyword Private Information Retrieval with Logarithmic Communication

Abstract

Keyword private information retrieval (PIR) allows a client to retrieve a record associated with a keyword from a database without revealing any information about the keyword. In the standard single-server setting, existing hintless keyword PIR protocols incur substantial communication and computation costs. In this paper, we propose an efficient approach to generate $k$-hot vectors (i.e., vectors with exactly $k$ non‑zero components) in homomorphic-encryption form, and present a bucket-merging technique to decrease the maximum size of buckets. Based on these techniques, we construct LetoPIR, a hintless keyword PIR protocol that outperforms previous PIR protocols in the same setting. Compared to the state-of-the-art hintless keyword PIR scheme, SparsePIR (USENIX'23), LetoPIR achieves a $12.4\times \sim 17.0\times$ improvement in communication cost for databases ranging from $256$ MB to $4$ GB with records of $256$ bytes, and more than $3.0\times$ improvement in computation cost for the $256$ MB database. Compared to the state-of-the-art keyword PIR scheme with client hint, KPIR (USENIX'25), LetoPIR reduces the communication cost by $51.4\times \sim184.8\times$, while achieving a similar (even better) computation cost.