Paper 2026/2288

Kettle: Short Post-Quantum Threshold Signatures from the HAWK Signature Scheme

Abstract

HAWK is a lattice-based hash-and-sign signature scheme that was a third round candidate in the NIST additional call for post-quantum signatures. It has short signatures and keys, as well as fast and portable signing and verification. However, in response to a highly-publicized AI-driven cryptanalytic result that reduced its security (roughly doubling the required dimension to achieve a given security level), the authors de- cided to withdraw from the competition. The need to increase parameters was seen as limiting the size advantage of HAWK compared to the already selected lattice schemes ML-DSA and Falcon, which was one of its primary selling points along- side speed and the lack of floating-point arithmetic. In this paper, we identify another attractive aspect of the HAWK design that is not affected by the AI attack and has been overlooked so far: its surprising friendliness to multi- party computation (MPC). Indeed, HAWK signing can be achieved in a generic way from a small collection of basic composable MPC functionalities (like integer multiplication, uniform random generation, etc.) in such a way that instanti- ating those functionalities using MPC protocols with given security properties (semi-honest vs. malicious, honest vs. dis- honest majority, etc.) yields a secure protocol for MPC signing with those same properties. As a result, we obtain Kettle, a UC-secure threshold signa- ture protocol that outputs specification-compliant HAWK sig- natures and scales to an arbitrary number of parties. It offers essentially the shortest signature size so far for a lattice-based threshold signature, while achieving significantly lower round complexity and online communication cost than other thresh- old protocols built from pre-existing, non-threshold lattice- based signatures (like ML-DSA and Falcon): as low as 2 online rounds (a dozen rounds total) and 2 kB online com- munication per party, depending on the specific setting and optimization choices. We also provide a proof-of-concept implementation of our approach in the MP-SPDZ framework, which can be instanti- ated with any number of parties, any threshold, and a variety of security properties.