Pointing the Way, Hiding the Destination: Practical Private Dense Retrieval at Scale

2026-08-26

AI Summary

本文提出使用深度哈希作为隐私过滤器，通过随机二进制码生成候选列表，并结合加密重排和无意识密钥传输，实现高效且高质量的大规模私密密集检索。

Abstract

Hosted retrieval-augmented generation (RAG) and semantic search allow users to query valuable provider-held corpora, raising two competing demands: to hide each query and chosen result, yet reveal only the documents that the user is authorized to receive. Existing cryptographic approaches either make this costly by processing the entire corpus for every query, or sacrifice quality for efficiency by scanning a few clusters. We repurpose learned deep hashing as a private filter: a randomized binary code points the provider to a short candidate list, while encrypted reranking and oblivious key transfer protect the precise query and final selection. This shortlist short-circuits full-corpus cryptographic search without sacrificing retrieval quality: with 200-500 candidates, it closely matches full-corpus retrieval across five zero-shot corpora spanning 25K to 5.4M documents. On the full 2.68M-passage NQ corpus over a 10-Gbps link, our protocol only adds 0.73 seconds, or 10 percent, to a 128-token Qwen3-32B RAG pipeline. The released code satisfies directional metric differential privacy (DP) and substantially reduces embedding-inversion and property-inference leakage, demonstrating that a carefully learned shortlist can make private dense retrieval both accurate and practical.

Problem

• private dense retrieval
• retrieval-augmented generation
• semantic search
• cryptographic approaches

Innovation

• deep hashing
• private filter
• encrypted reranking
• oblivious key transfer
• differential privacy