Lightweight Image Encryption Algorithm Based on Round-Reduced SM4

Images, as an important information medium, are widely used in various industries due to their strong intuitive visual effects, large information carrying capacity, and high correlation between pixels. However, images often face the risk of malicious theft and cyber-attacks. Consequently, devising efficacious methodologies for image encryption and decryption, which concurrently preserve the fundamental characteristics of the visual data, has escalated into an exigent research challenge demanding immediate attention. This paper focuses on cryptographic methods of image encryption and proposes a lightweight image encryption scheme based on the round-reduced SM4 algorithm. The experimental results show that: 1) As the number of encryption rounds increases, the average correlation coefficient of ciphertext images gradually approaches zero, which significantly destroys the correlation between pixels inside the image and effectively achieving the encryption effect of the image; 2) The number of pixels change rate (NPCR) and unified average change intensity (UACI) are close to the ideal values of 99.60% and 33.34%, respectively; 3) The information entropy of encrypted images gradually approaches the ideal value of 8 as the number of encryption rounds increases. Moderately reducing the number of encryption rounds can effectively improve encryption and decryption efficiency without significantly reducing image security, achieving a good balance between efficiency and security.