FTPE-BC: Fast thumbnail-preserving image encryption using block-churning

Abstract

With the boom in personal images, cloud services are beginning to take on a significant role in image storage. However, images uploaded to the cloud will face new privacy threats. Image privacy can be effectively protected using traditional encryption methods, but the encrypted images are snowflake-like noisy images, which makes the images unusable. Lately, Thumbnail Preserving Encryption (TPE) is proposed, which relieves this conflict between image availability and privacy in cloud platforms by retaining thumbnails unchanged after encryption. Existing TPE schemes for intra-block grouping usually utilize a lot of pseudo-random functions with high computational complexity, which makes encryption and decryption time too long. We propose a new model, block-churning, and propose a new TPE scheme based on this model. In this scheme, all pixels in the block are churned as a whole. This fundamentally solves the problem of low encryption efficiency caused by intra-block grouping, and greatly improves the encryption speed, while being able to change pixel values more significantly in blocks. The experiment shows that the encrypted image achieves a good balance between availability and privacy.