International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Christian Knabenhans

Publications and invited talks

Year
Venue
Title
2025
RWC
A Privacy-Preserving Aid Distribution System with Assessment Capabilities; Or, a Case Study on Threat Modelling and System Design
Today, humanitarian distribution heavily relies on manual processes that can be slow, error-prone, and costly. Humanitarian aid organizations therefore have a strong incentive to digitalize the aid distribution process. This would allow them to scale up their operations, reduce costs, and increase the impact of their limited resources. Digitalizing the aid distribution process introduces new challenges, especially in terms of privacy and security. These challenges are particularly acute in the context of humanitarian aid, where the recipients are often vulnerable populations, and where the aid distribution process is subject to a high degree of scrutiny by the public, the media, and the donors. This is compounded by a very strong threat model, with adversaries ranging from corrupt officials to armed groups, and by the fact that the recipients themselves may not be able to protect their own privacy. This talk we propose is split into three main parts: first, we stress the need for assessments when deploying privacy-preserving applications in the real world, using concrete examples. In particular, we discuss the tension between supporting assessments and the security and privacy of the application's users. Second, we reflect on our experience in designing privacy-preserving applications for various use cases, and discuss how we go from an informal, high-level need expressed by our partners, to a formal model and a concrete protocol. Here, we stress common pitfalls, and outline a methodology that we have synthesized from our experience. Finally, we discuss how we tackled the use case of a privacy-preserving aid distribution system with statistics, in collaboration with partners from the International Committee of the Red Cross. We present a general framework to collect and evaluate statistics in a privacy-preserving way (including one-time functional evaluation, a new primitive that we introduce), and we present three concrete instantiations of this framework (based on trusted execution environments, linear secret sharing, and threshold fully homomorphic encryption, respectively).
2024
ASIACRYPT
Lova: Lattice-Based Folding Scheme from Unstructured Lattices
Folding schemes (Kothapalli et al., CRYPTO 2022) are a conceptually simple, yet powerful cryptographic primitive that can be used as a building block to realise incrementally verifiable computation (IVC) with low recursive overhead without general-purpose non-interactive succinct arguments of knowledge (SNARK). Most folding schemes known rely on the hardness of the discrete logarithm problem, and thus are both not quantum-resistant and operate over large prime fields. Existing post-quantum folding schemes (Boneh, Chen, ePrint 2024/257) based on lattice assumptions instead are secure under structured lattice assumptions, such as the Module Short Integer Solution Assumption (MSIS), which also binds them to relatively complex arithmetic. In contrast, we construct Lova, the first folding scheme whose security relies on the (unstructured) SIS assumption. We provide a Rust implementation of Lova, which makes only use of arithmetic in hardware-friendly power-of-two moduli. Crucially, this avoids the need of implementing and performing any finite field arithmetic. At the core of our results lies a new exact Euclidean norm proof which might be of independent interest
2024
RWC
Towards robust FHE for the real world
Christian Knabenhans Alexander Viand Anwar Hithnawi
In recent years, FHE has made significant gains in performance and usability. As a result, we see a first wave of real-world deployments and an increasing demand for practical applications of FHE. However, deploying FHE in the real world requires addressing challenges that have so far received less attention, as the community was primarily focused on achieving efficiency and usability. Specifically, the assumption of a semi-honest evaluating party, which is at the core of most FHE research, is incompatible with a large number of deployment scenarios. Scenarios that violate this assumption do not simply suffer from correctness issues, as one might expect, but in fact enable an adversary to completely undermine the confidentiality guarantees of FHE, up to and including very practical key-recovery attacks. As a response, a variety of works have tried to augment FHE for settings beyond the traditional semi-honest assumption. This fundamentally revolves around guaranteeing some form of integrity for FHE, while retaining sufficient malleability to allow homomorphic computations. However, it remains unclear to what extent existing approaches actually address the challenges of real-world deployment, as we identify significant gaps between the assumptions these works generally make and the way state-of-the-art FHE schemes are used in practice. In this talk, we survey and analyze existing approaches to FHE integrity in the context of real-world deployment scenarios, identify capabilities, shortcomings, and promising candidates. We also implemented and evaluated these constructions experimentally on realistic workloads, and we give some numbers. Finally, we conclude with a discussion on current capabilities, recommendations for future research directions, and an overview of the hurdles on the path to our ideal end-goal: a cryptographic equivalent of a trusted execution environment, i.e., a cryptoprocessor enabling fully private and verifiable computation.