将交叉熵与图像的最优分块处理相结合，提出了一种基于分块交叉熵的数字图像置乱程度的评价方法。为了能用图像交叉熵反映出图像置乱效果，采用具有周期性的置乱变换进行实验，结果表明，该方法能够较好地刻画图像的置乱程度，反映了加密次数与置乱程度的关系，与人的视觉基本相符。而且对于不同的图像，该评价方法能在一定程度上反映了所用的置乱变换在各个加密阶段的效果。

By unifying the cross-entropy and the image optimal block processing, a digital image scrambling degree evaluation method based on block cross-entropy is proposed. To reflect the image scrambling effect by image cross-entropy, the scramble transformations with periods are simulated. The results show that: the proposed method is effective to describe the relationship between the enciphering effect and the number of iterations in the enciphering techniques, which largely consists with human vision. For different images, when some transformation is used, this appraisal method can reflect to some extent the scrambling effects in each encryption stage.