Novel multiple color images encryption and decryption scheme based on a bit-level extension algorithm

Digital images play a crucial role in modern communication, but the accompanying privacy concerns have become increasingly prominent. In this context, image encryption technology has emerged as a vital means to safeguard the security of digital image information. Over the past few years, a number of effective image encryption schemes have been developed. However, the amalgamation of bit-level encryption techniques with compression algorithms remains underexplored. To leverage the advantages of these two technologies in terms of security and practicality, a novel multiple color images encryption and decryption scheme is devised based on a bit-level extension algorithm. Firstly, four color images are compressed by the discrete wavelet transform (DWT) to reduce transmission and storage consumption. Secondly, to carry out the bit-level scrambling and diffusion operations on pixels with fractional parts in the DWT domain and maintain the decryption image quality as much as possible, a bit-level extension algorithm is developed. Finally, three rounds of scrambling and diffusion operations based on the bit-level extension algorithm are conducted to enhance the randomness of the ciphertext image and conceal the plaintext information. To reinforce the resistance of the presented image encryption and decryption scheme towards the chosen-plaintext and the differential attacks, secret keys are produced with the hash function SHA-512. The presented scheme undergoes a comprehensive analysis including decryption quality, key security, and resistance to the statistical and the differential attacks. The results are: average PSNR = 31.2631 dB, key space reaches 2472, correlation coefficients are almost 0, average entropy = 7.9985 bits, average NPCR = 99.6089%, and average UACI = 33.4658%. Moreover, experimental simulations demonstrate that our solution exhibits a degree of resilience to noise interference and data loss. In summary, our presented multiple color images encryption and decryption scheme based on a bit-level extension algorithm is effective and reliable.