Homomorphic Encryption: An Analysis of its Applications in Searchable Encryption

TL;DR

This paper surveys the intersection of homomorphic encryption (HE) and searchable encryption (SE), addressing the challenge of performing privacy-preserving searches over encrypted cloud data. It systematically compiles 23 HE-enabled SE schemes, classifying them by search structure, functionalities, and user multiplicity, and analyzing the HE types used (PHE, SWHE, and FHE). Key findings show a strong shift toward index-based SE with prevalent ranked search, largely driven by PHE (notably Paillier) but with meaningful use of FHE in several schemes; dynamic updates and verifiability are active areas, while fuzzy search and delegation remain underexplored. The work highlights practical pathways for future research, including leveraging recent HE advances (e.g., TFHE), combining HE with other primitives, reducing reliance on PIR, and expanding functionalities like fuzzy matching and robust verifiability, ultimately guiding both researchers and practitioners in privacy-preserving search design.

Abstract

The widespread adoption of cloud infrastructures has revolutionised data storage and access. However, it has also raised concerns regarding the privacy of sensitive data stored in the cloud. To address these concerns, encryption techniques have been widely used. However, traditional encryption schemes limit the efficient search and retrieval of encrypted data. To tackle this challenge, innovative approaches have emerged, such as the utilisation of Homomorphic Encryption (HE) in Searchable Encryption (SE) schemes. This paper provides a comprehensive analysis of the advancements in HE-based privacy-preserving techniques, focusing on their application in SE. The main contributions of this work include the identification and classification of existing SE schemes that utilize HE, a comprehensive analysis of the types of HE used in SE, an examination of how HE shapes the search process structure and enables additional functionalities, and the identification of promising directions for future research in HE-based SE. The findings reveal the increasing usage of HE in SE schemes, particularly Partially Homomorphic Encryption. The analysis also highlights the prevalence of index-based SE schemes using HE, the support for ranked search and multi-keyword queries, and the need for further exploration in functionalities such as verifiability and the ability to authorise and revoke users. Future research directions include exploring the usage of other encryption schemes alongside HE, addressing omissions in functionalities like fuzzy keyword search, and leveraging recent advancements in Fully Homomorphic Encryption schemes.

Figures (6)

• Figure 1: Conceptual overview of a cloud-based SE system.
• Figure 2: Key characteristics of a cloud-based SE scheme.
• Figure 3: Types of HE schemes used in SE
• Figure 4: Use of HE on the Search structure
• Figure 5: Uses of HE on Search functionalities