基于有序等价划分的冗余空间转移图像加密安全性分析

Ordered Equivalence Division Based Cryptanalysis of Redundant-Space-Transfer Image Encryption

现有的冗余空间转移(RST)图像加密算法有效提高了加密图像可逆数据隐藏的隐藏容量和抵抗现有已知明文攻击的能力.在分析RST图像加密三个密钥特性和定义有序等价集的基础上,提出一种基于有序等价划分的已知明文攻击方法.对得到的明-密图像对(原始图像及其加密图像),首先基于直方图距离比较重建原始图像的位平面置乱图像(BPSI),然后对BPSI的每个图像块,在加密图像划分得到的有序等价集中查找并推断该图像块的块置乱密钥(BSK).推导给出了BSK估计准确率与分块大小、图像块个数和有序等价集个数的关系.实验结果表明,对512×512的测试图像,当分块大小不小于4×4时,BSK估计准确率超过0.95;不同BSK估计准确率得到解密图像的视觉效果表明,即使BSK估计准确率低至0.50,也可能导致原始图像的内容信息泄露.

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.