AC-coefficient histogram-based retrieval for encrypted JPEG images

This paper proposes a novel retrieval scheme for encrypted JPEG images. With this scheme, the DC and AC coefficients of JPEG images are encrypted using a stream cipher and scrambling encryption, respectively. Then, the encrypted images are transmitted to and stored in a server, which can also provide retrieval service. When receiving an encrypted query image, the server without any knowledge of the plaintext content may acquire statistically its AC coefficients histogram. By calculating the distances between the histograms of encrypted query image and database image, the server may output the encrypted images closest to the query image to the authorized user.