Scalable Secure Biometric Authentication without Auxiliary Identifiers

The prevalence of biometric authentication has been on the rise due to its ease of use and elimination of weak passwords. To date, most biometric authentication systems have been designed for on-device authentication of the device owner (e.g., smartphones and laptops). Recently, biometric authentication systems have started to emerge that are designed to authenticate users against cloud databases storing representations of biometrics for large numbers of users (potentially millions), such as those facilitating biometric payments. However, the use of a large cloud database introduces a significant attack vector, as a breach of the database could lead to the compromise of all enrolled users' sensitive biometric data. Indeed, all such existing systems either do not adequately protect against such a breach, or are impractical to deploy and use due to their high computational overhead. In this work, we present a new biometric authentication system that provides provable security guarantees against data breaches, while remaining scalable and performant. To do so, we marry artificial intelligence with advanced cryptographic techniques in a novel fashion, providing several optimizations along the way. Our work is the first to show that real-world scalable privacy-preserving biometric authentication without auxiliary identifiers is feasible, and we believe that it will spur widespread industrial adoption and further research in this area.

Covert Attacks on Machine Learning Training in Passively Secure MPC

Secure multiparty computation (MPC) allows data owners to train machine learning models on combined data while keeping the underlying training data private. The MPC threat model either considers an adversary who passively corrupts some parties without affecting their overall behavior, or an adversary who actively modifies the behavior of corrupt parties. It has been argued that in some settings, active security is not a major concern, partly because of the potential risk of reputation loss if a party is detected cheating. In this work we show explicit, simple, and effective attacks that an active adversary can run on existing passively secure MPC training protocols, while keeping essentially zero risk of the attack being detected. The attacks we show can compromise both the integrity and privacy of the model, including attacks reconstructing exact training data. Our results challenge the belief that a threat model that does not include malicious behavior by the involved parties may be reasonable in the context of PPML, motivating the use of actively secure protocols for training.

Secure Evaluation of Quantized Neural Networks

Image classification using Deep Neural Networks that preserve the privacy of both the input image and the model being used, has received considerable attention in the last couple of years. Recent work in this area have shown that it is possible to perform image classification with realistically sized networks using e.g., Garbled Circuits as in XONN (USENIX '19) or MPC (CrypTFlow, Eprint '19). These, and other prior work, require models to be either trained in a specific way or postprocessed in order to be evaluated securely. We contribute to this line of research by showing that this postprocessing can be handled by standard Machine Learning frameworks. More precisely, we show that quantization as present in Tensorflow suffices to obtain models that can be evaluated directly and as-is in standard off-the-shelve MPC. We implement secure inference of these quantized models in MP-SPDZ, and the generality of our technique means we can demonstrate benchmarks for a wide variety of threat models, something that has not been done before. In particular, we provide a comprehensive comparison between running secure inference of large ImageNet models with active and passive security, as well as honest and dishonest majority. The most efficient inference can be performed using a passive honest majority protocol which takes between 0.9 and 25.8 seconds, depending on the size of the model; for active security and an honest majority, inference is possible between 9.5 and 147.8 seconds.