HWAX: Metric-Hardness-Based Lightweight Block Cipher for Postquantum IoT Security on 32-bit Microcontrollers

Abstract

Every lightweight cipher standardized to date uses a 128-bit key, which Grover’s algorithm reduces to 64-bit postquantum security—below the NIST postquantum threshold. We present <bold>HWAX</bold>, a 128-bit block cipher with a 256-bit key designed for the ARM Cortex-M4 pipeline. HWAX combines three table-free layers: 1) <italic>Hamming-weight confusion</italic> (HWC), the first symmetric primitive to import metric-hardness from code-based postquantum cryptography; 2) an ARX F-function operating entirely in registers; and 3) a Hadamard diffusion step using only <sc>xor</sc> operations. A Hamming-distance key schedule provably resists Simon’s algorithm. All Hamming-weight computations use a constant-time bit-counting sequence, eliminating timing side channels. Software evaluation confirms 50.00% avalanche effect (<inline-formula> <tex-math notation="LaTeX">$\sigma ~=~ 5.66$ </tex-math></inline-formula> bits, 50 000 samples) and all nine implemented NIST SP 800–22 tests pass at <inline-formula> <tex-math notation="LaTeX">$p ~\geq ~0.15$ </tex-math></inline-formula>. Hardware profiling on STM32F407VG is underway.