RCTPE : A Resistant JPEG Compression Thumbnail‐Preserving Encryption Scheme

With the rapid growth of image data, users increasingly store images in the cloud to ease local storage demands and improve accessibility. This convenience raises significant concerns about image security. Traditional encryption methods often distort image content, rendering it visually meaningless and unusable, thus creating a challenge: how to ensure privacy while maintaining usability. Thumbnail‐Preserving Encryption (TPE) has emerged to strike a balance between these needs. However, most existing TPE methods for JPEG images modify the quantization table in the header, leading to decryption failures after JPEG compression. To overcome this limitation, this article proposes RCTPE, a compression‐resistant approximate TPE scheme designed for better JPEG compression compatibility. Instead of altering the quantization table, RCTPE directly processes DCT coefficients, fundamentally addressing the recovery quality issue after recompression. This work employs a dynamic cross‐channel fusion embedding strategy, encrypting and embedding multi‐channel DC residuals along with selected AC coefficients into the luminance channel, and employs a hierarchical key mechanism to enable dynamic encryption. This hierarchical key generation mechanism dynamically derives keys based on image content and block locations, achieving content‐dependent encryption. This design enhances resilience against compression artifacts. Experimental results show that RCTPE achieves superior performance in encryption/decryption speed and minimizes file size expansion. Even after JPEG recompression at a quality factor (QF) of 75, RCTPE achieves an average text recognition accuracy of 86.87%, demonstrating strong recovery capability. Security analyses further confirm the robustness of RCTPE against outline attacks and statistical analysis, offering strong privacy protection.