seminar

Tuesday, Nov 3, 2026 - 09:30 - 24-25/509

Lennart Braun

Succinct Two-Round Two-Party Signing from PCFs

(ALMASTY Seminar)

Modern secure computation protocols rely on trusted sources of correlated randomness, typically in the form of random oblivious transfer, multiplication triples, or vector oblivious linear evaluation (VOLE: Alice gets vectors u, v; Bob gets scalar \Delta and u*\Delta+v). Pseudorandom correlations functions (PCFs) allow to compress large amounts of such correlated (pseudo)randomness into two short correlated keys. Given to two parties, these keys can then be used to silently and on-the-fly generate any target amount of required correlated randomness.

We recap the PCF for VOLE based on the Sparse LPN assumption from Asiacrypt 2025 and show how to modify the construction to achieve an efficient distributed setup protocol along with new definitions that allow for limited adversarial influence on the key distribution. Based on our PCF we introduce new two-party threshold signature schemes with strong efficiency and security: Applying our methodology to the two most popular signatures, Schnorr and ECDSA, yields two-round, two-party, stateless and deterministic signing with 4-12 ms of computation on one core of a standard laptop, extremely low communication (96 B for Schnorr, and 128 B for ECDSA) and full concurrent simulatable security. With our PCF setup protocol we obtain an efficient end-to-end maliciously-secure and highly parallelizable distributed key generation (DKG) protocol, resulting in an estimated runtime of 44 s for the (one-time) distributed setup of Schnorr and ECDSA on one core of a standard laptop.

This is joint work with Geoffroy Couteau, Kelsey Melissaris, Mahshid Riahinia, and Elahe Sadeghi, and based on: [0]: Fast Pseudorandom Correlation Functions From Sparse LPN (Asiacrypt 2025) [1]: Succinct Two-Round Two-Party Signing from PCFs (Crypto 2026)

Tuesday, Nov 10, 2026 - 09:30 - 24-25/405

Mahshid Riahinia

(Post-Quantum) Pseudorandom Correlation Functions for Oblivious Transfer: A Constrained-PRF Approach

(ALMASTY Seminar)

Pseudorandom Correlation Functions (PCFs) allow two parties to locally generate a large amount of correlated randomness, such as Oblivious Transfer (OT) correlations, using short correlated keys. These correlations can then be used to significantly speed up secure computation protocols. In this talk, we build on the recent approach of constructing PCFs for OT from Constrained Pseudorandom Functions (CPRFs). CPRFs are pseudorandom functions where a constrained key allows the evaluation of the function only on a specific subset of inputs. We show some instantiations of this framework in the post-quantum regime, from lattices and isogenies. Our lattice-based PCF, which further allows a public-key setup, offers relatively high throughput, i.e., several hundred OTs per second. However, it requires megabyte-sized public keys and gigabyte-sized PCF keys. In contrast, our isogeny-based PCF is highly compact and has keys in the 100 kB range, while generating around 7 OTs per second.

Tuesday, Jan 5, 2027 - 09:30 - 24-25/405

Arnaud Spiasseuth

Graph-Walking and Cayley Hashes: Shorter Group Action Signatures

(ALMASTY Seminar)

We present a graph-walk then branch-swap technique for reducing signature sizes in group-action signature schemes, the setting of past NIST submissions such as LESS, MEDS, and ALTEQ. First, we show this technique can effectively compress signatures in the balanced mode: for the minimal public-key size its worst-case signature size is cut by almost half, yielding parameter trade-offs that approach those of the unbalanced mode. This gain, however, fades as the public-key size grows and does not improve on the unbalanced mode itself. Second, we show applying the technique to unbalanced mode successfully extends the range of possible parameter sets for any given security level, but the new possibilities are not necessarily better than the original sets. Third, we conclude with a theoretical generalisation of our technique to potentially compress the signature sizes even further, for which we found no instantiation but leave as an open problem with Cayley hashes being a promising direction.

Tuesday, Sep 29, 2026 - 10:00 - 25-26/105

Mickaël Hamdad

Algorithms for the cryptanalysis of code-based cryptographic schemes

(Soutenance Thèse)

Friday, Sep 25, 2026 - 10:30 - 24-25/405

Elena Kirshanova

Hybrid attacks on sparse LWE

(ALMASTY Seminar)

In this talk, I will present hybrid primal attack on Learning With Errors with sparse secret. For this, I introduce Batch Closest Vector Problem and how to solve it with state-of-the-art methods. These include efficient guessing algorithm, and Randomized Slicer Algorithm for Batch CVP. I will also speak about practical aspects of the hybrid attack. The talk is based in the joing work with Alexander Karenin, Alexander May, Julian Nowakowski “ Fast Slicer for Batch-CVP: Making Lattice Hybrid Attacks Practical” (AsiaCrypt 2025).