AEX-128: A Key-Dependent Finite-State SPN–ARX Hybrid Block Cipher

Abstract

The design of modern block ciphers faces a fundamental trade-off between the strong security properties of Substitution–Permutation Network (SPN) structures and the high efficiency of Addition–Rotation–XOR (ARX) constructions. Existing ciphers typically rely on fixed and predictable round structures, which may introduce exploitable patterns and limit adaptability across diverse computational environments. Despite recent hybrid approaches, most designs still employ static combinations of SPN and ARX operations, leaving a gap in developing dynamic, key-dependent architectures that enhance structural unpredictability. To address this limitation, this paper proposes AEX-128, a 128-bit hybrid block cipher that integrates SPN and ARX transformations within a finite state machine (FSM) framework. The proposed methodology introduces key-dependent control over round operations, where the sequence of transformations, rotation parameters, and permutations are dynamically selected based on entropy derived from the secret key. Unlike conventional designs, the key schedule not only generates subkeys but also governs the FSM transitions, creating multiple possible execution paths for encryption. Experimental evaluation demonstrates that AEX-128 achieves strong diffusion properties, with an avalanche effect close to the ideal 50%, indicating high sensitivity to input changes. Performance analysis indicates that the proposed cipher achieves competitive throughput while demonstrating strong preliminary diffusion properties under avalanche metrics. These results suggest that the hybrid FSM-based design offers an effective balance between structural innovation, diffusion efficiency, and computational performance.