Abstract
As the demand for secure visual data transmission grows, specifically concerning the areas of medical imaging, surveillance, and secure communications, image encryption has emerged as a popular research area. Traditional ciphers such as Rivest Cipher 4 (RC4) are fast but present challenges in the form of a biased keystream along with weak key scheduling. For this reason, the use of RC4 alone is undesirable. In this paper, a hybrid scheme is discussed for encrypting digital images that uses RC4 and combines it with chaotic transformations, along with nonlinear cryptography, to help enhance the security of normal RC4. In the first stage of the proposed scheme, a secure 256-bit key is generated using a Password-Based Key-Derivation Function (PBKDF2) and HMAC-SHA256, which then is expanded using a novel scheme. The original Keystream Generation Algorithm used to generate the keystream in RC4 undergoes modifications. The keystream is further augmented using a Multiply-With-Carry (MWC) pseudo-random sequence to provide a more secure keystream while mitigating bias. The proposed scheme employs a unique process of encryption that employs pixel-wise substitution in a preliminary step using the PRESENT S-box, followed by a stage of spatial shuffling using Arnold's Cat Map with the added step of diffusion to increase entropy and statistical randomness. All the results metrics demonstrate that the proposed scheme dilutes the known weaknesses presented in normal RC4, while simultaneously exhibiting strong secure image encryption capabilities, with desirable entropy, NPCR and UACI.