Efficient Cipher-Only Attack of a Stream Cipher in RDHEI Based on Pixel Smoothness Estimation

Reversible data hiding in encrypted images (RDHEI) has recently emerged as an effective approach to protect the confidentiality of image content through encryption while assisting in the management of encrypted images by lossless hiding some additional data in encrypted images. In 2018, Khelifi proposed a cipher-only attack (COA) on the RDHEI methods where a bit-wise XOR encryption (also called as stream cipher) method was applied. In this work, we propose an efficiency COA method where the number of encrypted images can be effectively reduced for estimating the key stream with the same error rate. According to the given encrypted images, this work first estimates the smoothness probability of all pixel in the corresponding original images. And then, the weight coefficient of each pixel in the COA attack is assigned based on their estimated smoothness. The rationality of the proposed pixel smoothness estimation algorithm is discussed and analyzed from both theoretical and experimental aspects. Experimental results show that the error rate of the key stream estimated by the proposed COA method is less than 1/14 of that by the literature COA algorithm 10.