A chaos-based word-wise stream cipher using keyed strong S-Box

Stream ciphers are crucial in the realm of information security. However, many existing stream ciphers suffer from several vulnerabilities, including insufficient security levels and the use of Sboxes with low non-linearity, fixed points, reverse fixed points and short cycles. To address these issues, a robust word-wise stream cipher was proposed that incorporates a keyed strong S-box and a two-dimensional (2D) chaotic map, significantly enhancing overall security. A 2D nondegenerate quadratic map (2D-NDQM) was constructed and evaluated using bifurcation and phase diagrams, Lyapunov exponent, Kolmogorov entropy, sample entropy, correlation dimension, and TestU01. The 2D-NDQM is then utilized to design a keyed strong S-box construction algorithm. Based on this, a word-wise stream cipher leveraging both the 2D-NDQM and the keyed strong S-box was developed. A series of tests were conducted to evaluate the proposed stream cipher's performance. The results were promising, showing a key space of 2179, an average number of bits change rate of 50.0229%, a unified average changing intensity of 33.4402 %, and a number of pixels change rate of 99.6671 %. Additional metrics such as Hamming distance, linear complexity, and Shannon entropy also closely aligned with ideal values, confirming the security and feasibility of the proposed algorithm.