Efficient Single-Round Obfuscation of Search and Result Patterns in Searchable Encryption

Searchable Symmetric Encryption (SSE) enables data owners to securely store encrypted data on untrusted cloud servers while retaining the ability to perform secure searches and retrieve relevant documents. However, standard SSE schemes expose search patterns (whether two queries are identical), and result patterns (which documents are returned), making them susceptible to leakage-abuse attacks that can infer sensitive information such as the queried keywords and/or document contents. While Oblivious RAM (ORAM) and Private Information Retrieval (PIR) can hide these patterns, their high computation and communication overhead often render them impractical for real-world search workloads. A more efficient alternative is to obfuscate search and result patterns using Differential Privacy (DP). Unfortunately, existing DP-based SSE schemes either provide insufficient query privacy protection, or still incur substantial performance overhead. In this paper, we propose FROST, a novel differentially private SSE scheme that efficiently obfuscates both search and result patterns, while providing strong resilience against all known statistical leakage-abuse attacks. The core component of FROST is our new rerandomized PIR (RePIR) scheme designed for private databases, which allows server-side rerandomization of encrypted PIR query responses. In FROST, we also introduce a novel method for applying DP noises to SSE for search result obfuscation using only simple arithmetic operations. An important property of FROST is that it requires only a single round of communication, with small user-side storage as an additional benefit. We fully implemented FROST and conducted extensive experiments over realworld datasets to rigorously assess its practical performance and resilience. Our experiments showed that FROST not only effectively mitigates pattern-leakage attacks while maintaining reasonable utility, but also achieves up to four orders of magnitude faster keyword search and three orders of magnitude lower bandwidth overhead than prior DP-based SSE schemes. CCS Concepts • Security and privacy → Database and storage security; Privacypreserving protocols; Cryptography.