ALGARADI: a Hybrid SPN–Feistel Block Cipher with Dynamic Round Keys

With the exponential growth of digital data transmission and the increasing sophistication of cyber threats, the demand for secure and efficient encryption algorithms has become more critical than ever. This paper presents a novel lightweight block cipher, the ALGARADI cipher, which combines principles from the Substitution-Permutation Network (SPN) and the Feistel structure to provide a secure and resource-efficient cryptographic solution. The proposed design employs compact 4×4 S-boxes constructed over GF(2⁴) to ensure strong nonlinearity, along with an MDS-based diffusion matrix and a customized 16-bit derangement P-box to maximize both confusion and diffusion. A key innovation of the cipher is its dynamic SHA-256-driven key schedule, which generates four unique subkeys per round and applies them through interleaved key mixing significantly enhancing sensitivity to key variations. Security evaluations demonstrate strong resistance against linear and differential cryptanalysis, with worst-case differential probabilities below 2⁻³⁶ and linear correlation biases under 2⁻¹²⁰. Moreover, the cipher exhibits an ideal avalanche effect and high ciphertext entropy, indicating a high degree of randomness and cryptographic strength. Designed with performance efficiency in mind, the cipher is well-suited for both high-throughput systems and resource-constrained devices. This work presents a promising candidate for future encryption standards where both security and efficiency are of paramount importance.