Enhancing Security for JPEG Image Against Mosaic Attack Using Inter-Block Shuffle Encryption

Inter-block shuffle has been proved to be able to enhance the security for JPEG image encryption. However, it does not change the numbers of non-zero coefficients in 8*8 blocks. This leads to information leakage. In this paper, the defects of inter-block shuffle are analyzed. An attack scheme is developed based on a combined application of chosen-plain-text attack and non-zero quantized coefficients counting attack. It is verified that the main content of the original image could be reconstructed without the knowledge of encryption algorithm details and keys. The attack results are visualized to be mosaic appearance images. An improvement of the attack is also provided, which is called a fast mosaic attack. In the improved attack, the chosen images are designed to have distinguished numbers of non-zero quantized DCT coefficients in different blocks. The designed images remarkably accelerate the attack processing. The fast mosaic attack generates the same results as the original scheme with the much smaller time cost.