ESTPE: An efficient and stable thumbnail-preserving encryption scheme

Abstract

Thumbnail-preserving encryption (TPE) can balance the usability and the privacy of images in the cloud. Most ideal TPE schemes obtain thumbnails by dividing the plaintext image into multiple blocks, and then perform substitution and permutation operations in each block. The size of partitioned blocks affect the usability and the privacy of the encrypted images. The size of the pixel groups in the blocks, i.e., the number of encrypted pixels per time, has experienced the development from 2-pixel to n-pixel in recent years. Existing solutions cannot promote the real implementation and commercial realization because of inefficiency and instability. In this paper, we propose an efficient and stable TPE scheme, which can select intelligently different sizes of pixel groups in each block. Theoretical analysis indicates that the encryption time is influenced by the sum and the number of pixels in a pixel group. The length of the pixel group and the sum of pixel values in a group influence each other. We try to determine the optimal combination of these two factors. Through the analysis of Markov model, the proposed scheme has the nonce-respecting security. The experimental results of comparison show that, for the images with size of 480 × 320, the average encryption time of the proposed scheme is 61.7% less than that of 3-pixel flexible TPE, which is the most effective scheme up to now. And the fluctuation value decreases by 34.8%.