Visual Cryptography using Sigmoid and Transposition Cipher Technique

In a world where secure image transmission is crucial—especially in areas like telemedicine, surveillance, and digital documentation—this paper presents a novel visual cryptography approach that protects image data while also enhancing its clarity. Traditional cryptographic techniques often burden systems with complex computations or degrade visual quality. To address this, the proposed work consist of a dual- layer method: first, an image is encrypted using a transposition cipher that scrambles pixel positions based on a secure, hashed key; second, a sigmoid function is applied to enhance contrast, ensuring the final image is both secure and visually sharp. This method ensures that even if unauthorized users access partial data, the image remains unreadable and visually distorted. The process involves converting images into matrix form, applying encrypted sorting operations on rows and columns, and then enhancing contrast using a sigmoid transformation that improves brightness differentiation without introducing noise. The encryption and decryption steps are carefully designed to reverse each other, maintaining the integrity of the original image. The proposed technique was implemented in MATLAB and validated through histogram analysis, which showed that the encrypted images had high randomness, while decrypted images preserved structural and contrast details. Overall, the method achieves a strong balance between image security and visual quality, making it highly suitable for real-time, resource- constrained applications.