The existing redundant-space transfer (RST) image encryption method adopted in the reversible data hiding in encrypted images scheme improved the embedding capacity and the ability against the existing known plaintext attacks. To address the RST image encryption

based on the analysis of three key characteristics of RST image encryption and the definition of ordered equivalence set (OES)

the ordered equivalence division-based known plaintext attack method is proposed. For the obtained plain-cipher image pair

i.e.

original image and its corresponding encrypted image

the bit-plane scrambling image (BPSI) of the original image is first reconstructed based on the histogram distance comparison. And then for each block in the BPSI

the block scrambling key (BSK) of it is searched and inferred in the OESs obtained by dividing all blocks of encrypted image. We make an analysis on the relation of the SK estimation accuracy with block size

image block number and ordered equivalence set number. Experimental results demonstrate that the BSK estimation accuracy is not less than 0.95 when block size is not less than 4×4 for the test images of 512×512 pixels. Also

the visual effect on the decrypted images obtained from the BSK estimation accuracy of 0.51 to 0.94 shows that the content information of the original image may be leaked even when the BSK estimation accuracy is as low as about 0.50.