Conjunctive Searchable Symmetric Encryption from Hard Lattices

Searchable Symmetric Encryption (SSE) supports efficient keyword searches over encrypted outsourced document collections while minimizing information leakage. All practically efficient SSE schemes supporting conjunctive queries rely crucially on quantum-broken cryptographic assumptions (such as discrete-log hard groups) to achieve compact storage and fast query processing. On the other hand, quantum-safe SSE schemes based on purely symmetric-key crypto-primitives either do not support conjunctive searches, or are practically inefficient. In particular, there exists no quantum-safe yet practically efficient conjunctive SSE scheme from lattice-based hardness assumptions. We solve this open question by proposing Oblivious Post-Quantum Secure Cross Tags (OQXT) – the first lattice-based practically efficient and highly scalable conjunctive SSE scheme. The technical centerpiece of OQXT is a novel oblivious cross-tag generation protocol with provable security guarantees derived from lattice-based hardness assumptions. We prove the post-quantum simulation security of OQXT with respect to a rigorously defined and thoroughly analyzed leakage profile. We then present a prototype implementation of OQXT and experimentally validate its practical efficiency and scalability over extremely large real-world databases. Our experiments show that OQXT has competitive end-to-end search latency when compared with the best (quantum-broken) conjunctive SSE schemes

SEC: Fast Private Boolean Circuit Evaluation from Encrypted Look-ups

Encrypted computation has over the past thirty years, turned into one of the holy grails of modern cryptography especially with the advent of cloud computing. Modern cryptographic techniques like Fully Homomorphic Encryption (FHE) allow arbitrary Boolean circuit evaluation with encrypted inputs. However, the prohibitively high computation and storage overhead coupled with high communication bandwidth of FHE severely limit its scalability in practical applications like real-time analytics or machine learning inference. In summary, the current cryptographic literature lacks robust and scalable methods for efficient encrypted computation in practical outsourced applications. In this work, we introduce a new approach for encrypted computation called SEC (Symmetric Encryption-based Computation) which offers fast Boolean circuit evaluation with optimal storage and communication overhead while scaling smoothly to real applications. SEC relies on an efficient Searchable Symmetric Encryption (SSE) construction to leverage the power of encrypted lookups in Boolean circuit evaluation. SEC is specifically suited for client-server systems, and the server, honest-but-curious receives the client’s encrypted inputs and outputs the encrypted evaluation result while leaking only benign information to the server. SEC essentially extends the capabilities of SSE schemes from searching over encrypted databases to arbitrary function evaluation over encrypted inputs. SEC supports Boolean function composition, allowing it to evaluate complex functions efficiently without blowing up storage overhead. SEC outperforms the state-of-the-art FHE, namely, Torus FHE (TFHE) scheme with an average 103× speed-up in basic Boolean gate evaluations. We present a prototype implementation of SEC and experimentally validate its practical efficiency. Our experiments show that SEC executes arbitrary depth Boolean circuit in a single round of communication between client and server with a significant improvement in performance than the fastest TFHE backends. We exemplify the applicability of our scheme by implementing one byte AES SBox using SEC and comparing the results with TFHE

TWo-IN-one-SSE: Fast, Scalable and Storage-Efficient Searchable Symmetric Encryption for Conjunctive and Disjunctive Boolean Queries

Searchable Symmetric Encryption (SSE) supports efficient yet se- cure query processing over outsourced symmetrically encrypted databases without the need for decryption. A longstanding open question has been the following: can we design a fast, scalable, linear storage and low-leakage SSE scheme that efficiently sup- ports arbitrary Boolean queries over encrypted databases? In this paper, we present the design, analysis and prototype implementa- tion of the first SSE scheme that efficiently supports conjunctive, disjunctive and more general Boolean queries (in both the con- junctive and disjunctive normal forms) while scaling smoothly to extremely large encrypted databases, and while incurring linear storage overheads and supporting extremely fast query processing in practice. We quantify the leakage of our proposal via a rigorous cryptographic analysis and argue that it achieves security against a well-known class of leakage-abuse and volume analysis attacks. Finally, we demonstrate the storage-efficiency and scalability of our proposed scheme by presenting experimental results of a prototype implementation of our scheme over large real-world databases