International Association
for Cryptologic Research

Streamlined NTRU Prime is a lattice-based Key Encapsulation Mechanism (KEM) that is, together with X25519, currently the default algorithm in OpenSSH 9. Being based on lattice assumptions, it is assumed to be secure also against attackers with access to large-scale quantum computers. While Post-Quantum Cryptography (PQC) schemes have been subject to extensive research in the recent years, challenges remain with respect to protection mechanisms against attackers that have additional side-channel information such as the power consumption of a device processing secret data. As a countermeasure to such attacks, masking has been shown to be a promising and effective approach. For public-key schemes, including any recent PQC schemes, usually a mixture of Boolean and arithmetic approaches are applied on an algorithmic level. Our generic hardware implementation of Streamlined NTRU Prime decapsulation, however, follows an idea that until now was assumed to be only applicable to symmetric cryptography: gate-level masking. There, a hardware design that consists of logic gates is transformed into a secure implementation by replacing each gate with a composably secure gadget that operates on uniform random shares of secret values. In our work, we show the feasibility of applying this approach also to PQC schemes and present the first Public-Key Cryptography (PKC) – pre- and post-quantum – implementation masked at gate level considering several trade-offs and design choices. We synthesize our implementation both for Artix-7 Field-Programmable Gate Arrays (FPGAs) and 45 nm Application-Specific Integrated Circuits (ASICs), yielding practically feasible results regarding area, randomness demand and latency. Finally, we also analyze the applicability of our concept to Kyber which will be standardized by the National Institute of Standards and Technology (NIST).