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VertiSplitRAG: Privacy-Preserving Retrieval-Augmented Generation Through Vertical Attribute Partitioning

Vaishak Balachandra

Purdue, 2026

Abstract

Retrieval-Augmented Generation systems enhance Large Language Model responses by incorporating external knowledge but inherently expose sensitive information through joint attribute encoding in embeddings. Existing privacy-preserving approaches apply post-hoc protection after embeddings are formed, leaving multi-attribute leakage vulnerabilities unaddressed. This thesis introduces VertiSplitRAG, a privacy-by-design framework that restructures data through vertical attribute partitioning before embedding, ensuring that no single component observes complete records. We evaluated four architectural variants: Baseline RAG (BAS), VertiSplitRAG for single-attribute queries (VSS), VertiSplitRAG for multi-attribute with shared LLM (VSM), and VertiSplitRAG with multi-LLM architecture (VSML), on synthetic medical patient records to assess privacy protection, answer quality, and system latency. Our multi-LLM architecture achieves complete cross-attribute isolation through information-theoretic structural separation, maintaining full privacy budget across all queries while other systems progressively exhaust their isolation guarantees over time. This privacy improvement comes at an acceptable utility cost, with privacy-preserving variants maintaining competitive faithfulness scores comparable to the baseline across query sequences. Additionally, the multi-LLM architecture demonstrates lower latency than the baseline for single-attribute queries, supporting practical deployment in…

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