A DNA-Based Image Encryption Scheme Using Optimized Lorenz-Sprott Hyperchaotic System

Chaotic systems have been widely investigated for color image encryption because of their nonlinear dynamics, initial-condition sensitivity, and pseudorandom behavior. However, locating numerically robust parameter regions in high-dimensional hyperchaotic systems remains difficult, while many DNA-based schemes employ fixed or weakly varying rules. This study proposes a color image encryption scheme combining a 6D Lorenz-Sprott system optimized by particle swarm optimization (PSO), symbol-level feedback-dependent DNA transformation, and bidirectional cross-channel chained diffusion. The second-largest Lyapunov exponent is used as the optimization objective to locate parameter sets with at least two positive exponents. The selected system has the Lyapunov spectrum (0.8118, 0.2736, -0.0013, -4.8957, -8.5499, -12.6746) and retains two positive exponents under refined numerical settings and ±1% single-parameter perturbations. One hundred independently initialized sequences satisfy all 15 categories of the NIST SP 800-22 test suite. Tests on six images and three secret keys achieve exact reconstruction in all 18 cases. Across 180 randomly located one-bit plaintext perturbations, the mean NPCR and UACI are 99.6089% and 33.4554%, respectively. For the tested Baboon case, perturbing any initial-state component by approximately 10^-14 prevents meaningful plaintext recovery. Ablation results further demonstrate the contributions of dynamic DNA transformation and bidirectional diffusion. The proposed scheme therefore provides reproducible hyperchaotic parameter modulation and strong empirical statistical and differential performance.