Paper 2026/231 Original: eprint.iacr.org/2026/231 ↗

RAGtime-PIANO: Efficient Secure Remote RAG

Abstract

Retrieval Augmented Generation (RAG) can enhance the performance of Large Language Models (LLMs) when used in conjunction with a comprehensive knowledge database. However, the space required to store the necessary information can be taxing when RAG is used locally. As such, the concept of RAG-as-a-Service (RaaS) has emerged, in which third-party servers can be used to process client queries via an external database. Unfortunately, using such a service would expose the client's query to a third party, making the product unsuitable for processing sensitive queries. Our scheme ensures that throughout the entire RAG processing, neither the query, any distances, nor retrieval information is known to the database hosting server. Using a two-pronged approach, we employ Fully Homomorphic Encryption (FHE) and Private Information Retrieval (PIR) to ensure complete security during RAG processing. FHE is used to maintain privacy during initial query processing, during which the query embedding is encrypted and sent to the server for k-means centroid scoring to obtain a similarity ranking. Then, a series of PIR queries is used to privately retrieve the centroid-associated embeddings and the top-ranked documents. A first-of-its-kind, lightweight, fully secure RAG protocol, RAGtime-PIANO, enables efficient secure RAG.