Abstract: With the popularity of mobile phones, computers, and tablets, photos are more easily available in daily life, and people are used to storing a large number of photos in the cloud. While enjoy- ing the convenience brought by cloud storage, users are also threatened by privacy leaks. Although scholars have devised many image encryption schemes to prevent privacy leakage, they often ignore the usability of images. Recently, Tajik et al. proposed an ideal thumbnail preservation encryption scheme to balance image privacy and usability well, but this scheme only used two pixels as a group during the process of encryption and thus leads to the low efficiency. Therefore, an image encryption scheme using a segmentation method is proposed. This scheme uses three pixels as a group to do encryption, so as to keep the thumbnail of the ciphertext image consistent with the thumbnail of the plaintext image. The proposed scheme has higher efficiency than the Tajiks. Experiments show that this scheme can make the ciphertext image accurately maintain the same thumbnail as the plaintext image, which balances privacy and usability.

Key words: thumbnail-preserving encryption (TPE), usability, privacy, image encryption