Exploiting Size-Compatible-Match Block Technique for Arbitrary-Size Thumbnail-Preserving Encryption

In traditional image encryption, privacy is protected at the expense of all visual content resulting in poor usability. Recently, a novel image encryption concept, thumbnail preserving encryption (TPE), has been proposed to balance privacy and visual usability after encryption. However, the existing TPE schemes can only encrypt images with specific sizes (related to thumbnail block sizes). The thumbnail block, namely, the image sub-block of equal length and width, and specific means the image size can be divisible by the thumbnail block size. In fact, a little thought reveals that in reality the image size is arbitrary, and it is only by chance that images can be encrypted fully. To this end, we propose a generalized TPE scheme, and it realizes full encryption of images with arbitrary size. Specifically, first of all, a novel block technique called size-compatible-match is proposed. It can be used to accurately match and segment the portion of the image that cannot be encrypted by existing TPE schemes. Second, a chaotic system called 2D GM-OS is introduced to greatly reduce the time cost of the encryption and decryption process. Third, the block technique and 2D GM-OS chaotic system are combined with the TPE. The results have demonstrated that images with arbitrary size can be fully encrypted (no leakage of the original image) by the proposed scheme, and the encrypted image has useful visual meaning. Meanwhile, extensive experiments have been done that show the security and robustness of the proposed scheme.