【摘要】 随着图像传输技术日益发展，人们对图像安全的需求也在大幅提升。由传统图像加密方案获得的类噪声图像虽然可以保证内容安全，但无法直接用于预览和检索。一些学者基于排序后加密方法，设计了一种三像素缩略图保留加密方案（TPE2），用于平衡图像安全性和可用性，然而该方案的加密效率较低。为此，本文提出一种有效的精确缩略图保留加密方案。首先对明文图像进行分块和位平面置乱，然后采用Z字形置乱模型改变最低的4个位平面中比特的位置，随后介绍了用于改变最高的4个位平面中比特位置的操作（这是隐马尔科夫模型的一个扩展应用）。最后，根据每个位平面中比特的权重不同，设计了一种比特级分权扩散规则。至此生成的加密图像能保证块内像素和不变。仿真结果表明，该方案在平衡图像隐私性和可用性的同时，提高了加密效率。更多还原

【Abstract】 With the substantial increase in image transmission, the demand for image security is increasing. Noise-like images can be obtained by conventional encryption schemes, and although the security of the images can be guaranteed, the noise-like images cannot be directly previewed and retrieved. Based on the rank-then-encipher method, some researchers have designed a three-pixel exact thumbnail preserving encryption(TPE2) scheme, which can be applied to balance the security and availability of images, but this scheme has low encryption efficiency. In this paper, we introduce an efficient exact thumbnail preserving encryption scheme. First, blocking and bit-plane decomposition operations are performed on the plaintext image. The zigzag scrambling model is used to change the bit positions in the lower four bit planes. Subsequently, an operation is devised to permute the higher four bit planes, which is an extended application of the hidden Markov model. Finally, according to the difference in bit weights in each bit plane, a bit-level weighted diffusion rule is established to generate an encrypted image and still maintain the same sum of pixels within the block. Simulation results show that the proposed scheme improves the encryption efficiency and can guarantee the availability of images while protecting their privacy.更多还原