TY - EJOU AU - Li, Yanjun AU - Lin, Yiping AU - Ni, Yuting AU - Zhang, Lixian AU - Huo, Shanshan TI - Security Analysis of a Modulo-2¹⁶ Implementation Variant of the MBRISI Lightweight ARX Block Cipher: Weak Keys, Deterministic Differentials, and Equivalent Keys T2 - Computers, Materials \& Continua PY - VL - IS - SN - 1546-2226 AB - This paper presents an exact security evaluation of a closed modulo-

2^{16}

implementation variant of the lightweight Add–Rotate–XOR (ARX) block cipher named MBRISI. We first resolve an arithmetic ambiguity in the original specification: modulo-65,537 addition on 16-bit words may produce the unrepresentable value 65,536, while representing its outputs by 16-bit overflow is non-injective. We construct distinct plaintext pairs that merge after the first round and consequently produce identical ciphertexts. In contrast, modulo-

2^{16}

addition is closed and bijective over the 16-bit word space; therefore, all subsequent weak-key and equivalent-key results apply exclusively to this implementation variant. We prove that addition by a fixed round key is affine over

G F (2)^{16}

if and only if the key belongs to

{0 x 0000, 0 x 4000, 0 x 8000, 0 x C 000}

, in which case every XOR difference propagates deterministically. Because this set is closed under the MBRISI round-key recurrence, weak initial subkeys make the complete ten-round encryption mapping affine over

G F (2)^{32}

. By modeling the rotation–XOR preprocessing as linear maps, we show that their images equal the even-parity subspace, their kernels have dimension one, and every image element has exactly two complementary preimages. These properties reduce exact weak-key counting and structural identification from

2^{32}

half-key-pair tests to

KW  - Lightweight block cipher; ARX cipher; IoT security; differential cryptanalysis; weak key; deterministic differential; equivalent key; key schedule

DO  - 10.32604/cmc.2026.087189