Abstract: In this paper, a known plaintext attack method based on the root mean square (RMS) of image block is proposed for ATBEM. Firstly, the scrambling sequence of bit planes is estimated to restore the original pixel value by using the invariant distribution ratio of 0 and 1 of plaintext images before and after encryption. Then, according to the characteristics of block scrambling and intra-block scrambling, which keeps the pixel value constant, a root mean square (RMS) feature of image block is defined to find and estimate the block scrambling matrix. The security performance of image block scrambling encryption is analyzed and discussed. The analysis results show that the smoother the image is, the smaller the blocks are, the more plaintext logarithms are needed to crack, and the more difficult the known plaintext attack is. Experimental results verify that the more texture the image is, the better the attack effect is. Under the condition of 2×2 block size, the attacker can crack more than 50% of the ciphertext image content with only one pair of known plaintext-ciphertext pairs known. And ATBEM encryption algorithm is difficult to resist the proposed known plaintext attack.

Key words: reversible data hiding in encrypted image, image block root mean square (RMS), known plaintext attack, bit plane-block scrambling