TPE-AP: Thumbnail-Preserving Encryption Based on Adjustable Precision for JPEG Images

As the development of the Internet of Things (IoT), the security of images in social networks is attracting more and more attention. Among the various encryption methods, thumbnail-preserving encryption (TPE) has gained much attention and diverse study, for it has powerful capability of balancing the security and usability of cloud storage images by simultaneously securing privacy and preserving visual information of images. Unfortunately, most of the existing TPE schemes for JPEG images have the disadvantages of limited thumbnail precision or weak security or irreversibility, making them vulnerable to cryptanalysis. To address these issues, we propose a thumbnail-preserving encryption based on adjustable precision (TPE-AP) for JPEG images. Firstly, a joint adjustment strategy is introduced for encrypted quantized QC coefficient (QDCC) and quantized AC coefficient (QACC) of plain image, which makes the security and usability of the thumbnail controllable by exploiting numerical characteristics of QDCC and distribution features of QACC. Secondly, a time-varying encryption strategy based on international time is presented, characteristics of JPEG compression and quantized coefficients are combined to generate encryption sequences, improving the security of the whole encryption process. In addition, we explore the redundancy of QACC within DCT blocks and provide an improved embedding strategy to solve irreversibility problem. Experimental results show that the peak signal-to-noise ratio (PSNR) of thumbnail-preserving accuracy reaches 52 dB, the file expansion is minimally limited to 6.3%, the decryption time less than 0.13 seconds and the mean average precision (mAP) of retrieval attains 62%, it indicates that TPE-AP outperforms the state-of-the-art methods.