# WAHC ’18- Proceedings of the 6th Workshop on Encrypted Computing & Applied Homomorphic Cryptography

## SESSION: Session 1

• Kurt Rohloff

### Doing Real Work with FHE: The Case of Logistic Regression

• Jack L. H. Crawford
• Craig Gentry
• Shai Halevi
• Daniel Platt
• Victor Shoup

We describe our recent experience, building a system that uses fully-homomorphic encryption (FHE) to approximate the coefficients of a logistic-regression model, built from genomic data. The aim of this project was to examine the feasibility of a solution that operates “deep within the bootstrapping regime,” solving a problem that appears too hard to be addressed just with somewhat-homomorphic encryption. As part of this project, we implemented optimized versions of many bread and butter FHE tools. These tools include binary arithmetic, comparisons, partial sorting, and low-precision approximation of arbitrary functions (used for reciprocals, logarithms, etc.). Our solution can handle thousands of records and hundreds of fields, and it takes a few hours to run. To achieve this performance we had to be extremely frugal with expensive bootstrapping and data-movement operations. We believe that our experience in this project could serve as a guide for what is or is not currently feasible to do with fully-homomorphic encryption.

### High-Throughput Secure AES Computation

• Koji Chida
• Dai Ikarashi
• Ryo Kikuchi
• Benny Pinkas

This work describes a three-times ($3\times$) improvement to the performance of secure computation of AES over a network of three parties with an honest majority. The throughput that is achieved is even better than that of computing AES in some scenarios of local (non-private) computation. The performance improvement is achieved through an optimization of the generic secure protocol, and, more importantly, through an optimization of the description of the AES function to support more efficient secure computation, and an optimization of the protocol to the underlying architecture. This demonstrates that the development process of efficient secure computation must include adapting the description of the computed function to be tailored to the protocol, and adapting the implementation of the protocol to the architecture. This work focuses on the secure computation of AES since it has been widely investigated as a de-facto standard performance benchmark for secure computation, and is also important by itself for many applications. Furthermore, parts of the improvements are general and not specific to AES, and can be applied to secure computation of arbitrary functions.

### More Practical Privacy-Preserving Machine Learning as A Service via Efficient Secure Matrix Multiplication

• Wen-jie Lu
• Jun Sakuma

An efficient secure two-party computation protocol of matrix multiplication allows privacy-preserving cloud-aid machine learning services such as face recognition and traffic-aware navigation. We use homomorphic encryption to construct a secure matrix multiplication protocol with a small communication overhead and computation overhead on the client’s side, which works particularly well when a large number of clients access to the server simultaneously. The fastest secure matrix multiplication protocols have been constructed using tools such as oblivious transfer, but a potential limitation of these methods is the needs of using a wide network bandwidth between the client and the server, e.g., 10~Gbps. This is of particular concern when thousands of clients interact with the server concurrently. Under this setting, the performance oblivious transfer-based methods will decrease significantly, since the server can only allocate a small ratio of its outgoing bandwidth for each client. With three proposed optimizations, our matrix multiplication protocol can run very fast even under the high concurrent setting. Our benchmarks show that it takes an Amazon instance (i.e., 72 CPUs and 25 Gbps outgoing bandwidth) less than 50 seconds to complete 1000 concurrent secure matrix multiplications with $128\times 128$ entries. In addition, our method reduces more than $74% – 97%$ of the precomputation time of two privacy-preserving machine learning frameworks, SecureML (S&P’17) and MiniONN (CCS’17).

## SESSION: Session 2

### Session details: Session 2

• Michael Brenner

### Bit Decomposition Protocols in Secure Multiparty Computation

• Peeter Laud
• Alisa Pankova

We present improved protocols for the conversion of secret-shared bit-vectors into secret-shared integers and vice versa, for the use as subroutines in secure multiparty computation (SMC) protocols and for protocols verifying the adherence of parties to prescribed SMC protocols. The protocols are primarily designed for three-party computation with honest majority. We evaluate our protocols as part of the Sharemind three-party protocol set and see a general reduction of verification overheads, thereby increasing the practicality of covertly or actively secure Sharemind protocols.

### Marble: Making Fully Homomorphic Encryption Accessible to All

• Alexander Viand
• Hossein Shafagh

With the recent explosion of data breaches and data misuse cases, there is more demand than ever for secure system designs that fundamentally tackle today’s data trust models. One promising alternative to today’s trust model is true end-to-end encryption without however compromising user experience nor data utility. Fully homomorphic encryption (FHE) provides a powerful tool in empowering users with more control over their data, while still benefiting from computing services of remote services, though without trusting them with plaintext data. However, due to the complexity of fully homomorphic encryption, it has remained reserved exclusively for a small group of domain experts. With our system Marble, we make FHE accessible to the broader community of researchers and developers. Marble takes away the complexity of setup and configuration associated with FHE schemes. It provides a familiar programming environment. Marble allows rapid feasibility assessment and development of FHE-based applications. More importantly, Marble benchmarks the overall performance of an FHE-based application, as part of the feasibility assessment. With real-world application case-studies, we show the practicality of Marble.

### Implementation and Evaluation of Improved Gaussian Sampling for Lattice Trapdoors

• Kamil Doruk Gür
• Yuriy Polyakov
• Kurt Rohloff
• Gerard W. Ryan
• Erkay Savas

We report on our implementation of a new Gaussian sampling algorithm for lattice trapdoors. Lattice trapdoors are used in a wide array of lattice-based cryptographic schemes including digital signatures, attributed-based encryption, program obfuscation and others. Our implementation provides Gaussian sampling for trapdoor lattices with prime moduli, and supports both single- and multi-threaded execution. We experimentally evaluate our implementation through its use in the GPV hash-and-sign digital signature scheme as a benchmark. We compare our design and implementation with prior work reported in the literature. The evaluation shows that our implementation 1) has smaller space requirements and faster runtime, 2) does not require multi-precision floating-point arithmetic, and 3) can be used for a broader range of cryptographic primitives than previous implementations.