On the Security of Bitstream-level JPEG Encryption with Restart Markers

This paper aims to evaluate the safety of a bitstream-level JPEG encryption method with restart (RST) markers, in which encrypted data can preserve the JPEG file format with the same size as that without encryption. Data encrypted with the method can be decoded without any modification of header information by using a standard JPEG decoder. In addition, the use of RST markers allows us to define extended blocks separated by the RST markers, so spatially partial encryption and block-permutation-based encryption can be carried out. However, the security of the method was only evaluated in terms of the key space analysis for brute-force attacks and other limited attacks. Accordingly, in this paper, we perform the security evaluation of the method in terms of robustness against ciphertext-only attacks including state-of-the-art attacks. In experiments, the method is compared with conventional encryption methods, and it is verified to be robust against ciphertext-only attacks if parameters used for image encryption are carefully chosen.