We'll be in room A316, on the third floor of the Arison-Lauder Building.
Join us at RUNI for the first GTACS of the post-election season.
Program
9:30-10:00 Early Morning Tea & Gathering
10:00-10:45 Matan Hamilis, Dishonest-Majority Secure Computation via PIR-Authenticated Multiplication Triples
10:45-11:15 Late Morning Tea
11:15-12:00 Eyal Ronen, Microarchitectural Weird Machines
12:00-13:30 Lunch
13:30-14:15 Alon Rosen, Adaptive Robustness of Hypergrid Johnson-Lindenstrauss.
14:15-14:45 Early Afternoon Tea
14:45-15:30 Eylon Yogev, Towards a White-Box Secure Fiat-Shamir Transformation.
15:30-16:00 Four O'Clock Tea
Abstracts
Dishonest-Majority Secure Computation via PIR-Authenticated Multiplication Triples
Speaker: Matan Hamilis
We revisit the question of minimizing the overhead of security against malicious parties in dishonest-majority secure computation.
A leading approach, pioneered by the SPDZ line of protocols, uses homomorphic MACs to authenticate computation: Parties effectively compute a MAC on the computation output using authenticated multiplication triples (AMT). However, securely generating these AMTs presently sits as the cost bottleneck.
In this talk, we introduce a new technique for enabling SPDZ-style verification via homomorphic MACs, while bypassing the need for AMT. We instead rely on the specific structure of state-of-the-art fast pseudorandom correlation generators (PCG) for generating standard (unauthenticated) multiplication triples (MT).
Parties authenticate the computation result via an authenticated variant of private information retrieval (PIR), relying on the sparse representation of MT produced by these PCGs.
This opens the door to a wide range of PIR optimizations and tradeoffs from the literature, resulting in asymptotic and concrete improvements over the traditional AMT-based approach. For example, in the Boolean 2-party case with sigma=40 (statistical security parameter), we get a 3x to 8x computation improvement (and comparable communication) over best approaches using PCG to generate AMT, where the latter exploits variants of the Stationary Syndrome Decoding assumption of Kolesnikov et al. (Crypto 2025). With m>=3 parties we obtain even larger improvements while reducing the asymptotic dependence on m from cubic to quadratic.
Adaptive Robustness of Hypergrid Johnson-Lindenstrauss
Speaker: Alon Rosen
In 1984 Johnson and Lindenstrauss showed that a random projection of an arbitrary point set S into low-dimensional space is approximately distance-preserving, as long as S is of size at most exponential in the target dimension. The resulting embedding has found many uses in data science.
If S is larger than exponential, however, its points can contract arbitrarily under the projection, opening the door to adversarial attacks.
We give evidence that when S is the n-dimensional hypergrid of integral points with bounded infinity-norm, the task of finding a contracting pair of points exhibits a computational-statistical gap. In a certain parameter range, contracting pairs are abundant, but finding such a pair is computationally infeasible.
As a cryptographic application, we show that the rounded Johnson-Lindenstrauss embedding is a robust property-preserving hash function. Such hash functions h compress data while preserving distances between inputs up to some distortion factor, with the guarantee that even knowing h, no computationally bounded adversary can find a pair of points that violates the distortion bound.
Based on joint work with Andrej Bogdanov, Neekon Vafa, and Vinod Vaikutanathan.
Microarchitectural Weird Machines
Speaker: Eyal Ronen
Over the last few decades, computers have become incredibly complex. Modern computers are an intricate combination of hundreds of microarchitectural components, such as caches, execution engines, and various predictors. These are all orchestrated to achieve the ever-increasing performance we expect as the field advances. As in many complex systems, the complexity of modern computers' microarchitectural components and their interaction gives rise to emergent behavior that was not anticipated from the design. Microarchitectural side-channel and speculative execution attacks (such as Spectre and Meltdown) exploit this behavior to compromise cryptographic keys.
In this talk, we demonstrate that the microarchitectural state is complex enough to support arbitrary computations hidden from the user at the architectural state. The main mechanism we use is weird gates, which are software constructs whose execution performs logical functions on cache state. We then show how these gates can be exploited to improve the state-of-the-art in microarchitectural attacks, and conclude by discussing how more advanced constructions can be realized
Towards a White-Box Secure Fiat-Shamir Transformation.
Speaker: Eylon Yogev
The Fiat–Shamir transformation is a fundamental cryptographic technique widely used to convert public-coin interactive protocols into non-interactive ones. This transformation is crucial in both theoretical and practical applications, particularly in the construction of succinct non-interactive arguments (SNARKs). While its security is well-established in the random oracle model, practical implementations replace the random oracle with a concrete hash function, where security is merely assumed to carry over.
A growing body of work has given theoretical examples of protocols that remain secure under the Fiat–Shamir transformation in the random oracle model but become insecure when instantiated with any white-box implementation of the hash function. Recent research has shown how these attacks can be applied to natural cryptographic schemes, including real-world systems. These attacks rely on a general diagonalization technique, where the protocol exploits its access to the white-box implementation of the hash function. These attacks cast serious doubt on the security of cryptographic systems deployed in practice today, leaving their soundness uncertain.
We propose a new Fiat–Shamir transformation (XFS) that aims to defend against a broad family of attacks. Our approach is designed to be practical, with minimal impact on the efficiency of the prover and verifier and on the proof length. At a high level, our transformationcombines the standard Fiat–Shamir technique with a new type of proof-of-work that we construct.
We hope that our transformation will help preserve the security of systems relying on the Fiat–Shamir transformation.
Joint work with Gal Arnon.