SMAUG: Pushing Lattice-Based Key Encapsulation Mechanisms to the Limits

Recently, NIST has announced Kyber, a lattice-based key encapsulation mechanism (KEM), as a post-quantum standard. However, it is not the most efficient scheme among the NIST’s KEM finalists. Saber enjoys more compact sizes and faster performance, and Mera et al. (TCHES ’21) further pushed its efficiency, proposing a shorter KEM, Sable. As KEM are frequently used on the Internet, such as in TLS protocols, it is essential to achieve high efficiency while maintaining sufficient security. In this paper, we further push the efficiency limit of lattice-based KEMs by proposing SMAUG, a new post-quantum KEM scheme whose IND-CCA2 security is based on the combination of MLWE and MLWR problems. We adopt several recent developments in lattice-based cryptography, targeting the smallest and the fastest KEM while maintaining high enough security against various attacks, with a full-fledged use of sparse secrets. Our design choices allow SMAUG to balance the decryption failure probability and ciphertext sizes without utilizing error correction codes, whose side-channel resistance remains open. With a constant-time C reference implementation, SMAUG achieves ciphertext sizes up to 12% and 9% smaller than Kyber and Saber, with much faster running time, up to 103% and 58%, respectively. Compared to Sable, SMAUG has the same ciphertext sizes but a larger public key, which gives a trade-off between the public key size versus performance; SMAUG has 39%–55% faster encapsulation and decapsulation speed in the parameter sets having comparable security.