Approximate thumbnail encryption utilizing a chaotic map with enhanced parameter space

Abstract Cloud-based image storage has emerged as a pivotal approach for managing image data. However, storing plaintext images directly in the cloud poses significant risks of data leakage, while traditional encryption methods render ciphertext images as noise-like entities devoid of visual usability. To address the dual demands of usability and security in cloud storage, this paper introduces an innovative Thumbnail-Preserving Encryption (TPE) scheme. The key expansion mechanism of this scheme leverages a novel chaotic map that integrates the Sine map, cosine map, and Logistic map, characterized by a broad parameter range. Experimental analyses reveal that this chaotic map excels in dynamic properties, including aperiodicity, sample entropy, and Lyapunov exponent. The TPE encryption process comprises two fundamental stages: intra-block scrambling and bit-plane group-based encryption. Intra-block scrambling reorganizes pixel positions within image sub-blocks to achieve confusion while maintaining thumbnail visibility. Bit-plane group-based encryption applies confusion to the highest 2-bit planes and diffusion to the lowest several bit planes, ensuring the encryption system’s ability to conceal statistical patterns and induce the avalanche effect. Comprehensive experimental evaluations confirm the scheme’s robustness in terms of encrypted image visual quality, encryption efficiency, histogram stability, image expansion rate, and resilience against diverse security threats such as differential attacks, face attacks, information loss, noise attacks, and plaintext attacks. These findings underscore the scheme’s capability to fulfill both security and usability requirements for cloud-based image storage.