Faster Multiplication in Z2m [x] on Cortex-M4 to Speed up NIST PQC Candidates

In this paper we optimize multiplication of polynomials in Z(2)m[x] on the ARM Cortex-M4 microprocessor. We use these optimized multiplication routines to speed up the NIST post-quantum candidates RLizard, NTRU-HRSS, NTRUEncrypt, Saber, and Kindi. For most of those schemes the only previous implementation that executes on the CortexM4 is the reference implementation submitted to NIST; for some of those schemes our optimized software is more than factor of 20 faster. One of the schemes, namely Saber, has been optimized on the Cortex-M4 in a CHES 2018 paper; the multiplication routine for Saber we present here outperforms the multiplication from that paper by 42%, yielding speedups of 22% for key generation, 20% for encapsulation and 22% for decapsulation. Out of the five schemes optimized in this paper, the best performance for encapsulation and decapsulation is achieved by NTRU-HRSS. Specifically, encapsulation takes just over 400 000 cycles, which is more than twice as fast as for any other NIST candidate that has previously been optimized on the ARM Cortex-M4.