Shorter Messages and Faster Post-Quantum Encryption with Round5 on Cortex M

Round5 is a Public Key Encryption and Key Encapsulation Mechanism (KEM) based on General Learning with Rounding (GLWR), a lattice problem. We argue that the ring variant of GLWR is better suited for embedded targets than the more common RLWE (Ring Learning With Errors) due to significantly shorter keys and messages. Round5 incorporates GLWR with error correction, building on design features from NIST Post-Quantum Standardization candidates Round2 and Hila5. The proposal avoids Number Theoretic Transforms (NTT), allowing more flexibility in parameter selection and making it simpler to implement. We discuss implementation techniques of Round5 ring variants and compare them to other NIST PQC candidates on lightweight Cortex M4 platform. We show that the current development version of Round5 offers not only the shortest key and ciphertext sizes among Lattice-based candidates, but also has leading performance and implementation size characteristics.