MPX: A Unified Systolic Array for Matrix and Polynomial Multiplication

Abstract: Polynomial multiplication is a fundamental kernel in Fully Homomorphic Encryption (FHE) and post-quantum cryptography (PQC) and is commonly accelerated through Number Theoretic Transforms (NTTs). To avoid the cost of designing dedicated cryptographic accelerators, recent efforts have mapped NTT computations onto existing systolic matrix engines, enabling the reuse of AI hardware for cryptographic workloads. In this work, we take the opposite approach. We observe that the wavefront dataflow of systolic arrays naturally aligns with the accumulation pattern of polynomial multiplication and leverage this correspondence to design MPX, a dual-mode systolic array that supports both matrix multiplication and direct polynomial multiplication within the same hardware fabric. Experimental results show that extending a conventional systolic array with this dual-mode capability requires only 20% additional area and introduces negligible power overhead during matrix-multiplication execution. In polynomial-multiplication mode, MPX achieves more than 1.2x lower latency compared to NTT-based polynomial multiplication on systolic matrix engines.