Enhancing Text Encryption and Secret Document Watermarking through Hyperladder Graph-Based Keystream Construction on Asymmetric Cryptography Technology

Message security remains a vital concern in cryptography. This paper introduces a novel enhancement to the classical Caesar cipher by generating a keystream from a Hyper-Ladder Graph, which combines hypergraph and ladder graph properties to produce complex and unpredictable patterns. The proposed method is evaluated against AES, DES, ChaCha20, and XChaCha20, showing superior performance in encryption time and memory efficiency, especially in constrained environments. To demonstrate broader applicability, we implemented the keystream in grayscale image watermarking. The binary keystream was first encrypted using RSA public key encryption, then embedded using the least significant bit (LSB) method. The results showed high imperceptibility with a PSNR of 57.05 dB and an SSIM of 0.9989. This integration of graph-based keystream and asymmetric cryptography offers robust security and flexibility, making it suitable for various domains such as secure text encryption, digital watermarking, and document authentication.