ShiftPIR: An Efficient PIR System with Gravity Shifting from Client to Server

We present ShiftPIR, a single-server Private Information Retrieval (PIR) protocol that gravity shifts both computation and communication overhead from the client to the server, thereby significantly improving overall efficiency. This shift is driven by the growing asymmetry between resource-constrained clients and compute-intensive servers, where server-side tasks can be effectively parallelized and scaled. To achieve this, ShiftPIR introduces a novel request generation method in which the client transmits only a compact plaintext offset derived from pre-uploaded seed ciphertexts. The server then reconstructs the full query ciphertexts using homomorphic rotations, eliminating the need for costly ciphertext generation and transmission on the client side. We further design a highly parallelizable query expansion mechanism that removes data dependencies between ciphertext rotations, enabling efficient GPU-based execution. Our experiments demonstrate that ShiftPIR reduces client-side latency to microseconds while maintaining a communication cost within 4X of the non-private baseline—far outperforming prior protocols with 104 -105 × overhead. Compared to the state-of-the-art protocol YPIR, ShiftPIR achieves up to 26X lower end-to-end latency.