Abstract: In order to avoid pixel expansion and contrast of the gathered secret image compromise，a new visual cryptographic scheme，which is called visual cryptographic scheme with sub-block coding method，is presented.Instead of taking m subpixels to represent one pixel，the share image is divided into sub-blocks with same size，each of which is taken as a unit to encode and with same pixel size.Then sub-block coding method is adopted to convert each shadow image to numbers matrix，that is，every sub-block meets a number.After that，according to a secret key，these two number matrices are converted into a series of number.Last the series is encrypted to transport or do some other operations.This scheme is compared to the （k，n） visual secret sharing scheme in section 4 and it shows that there do not exist pixel expansion and contrast compromise in this new scheme.

Key words: visual cryptographic scheme, sub-block coding, pixel expansion

摘要： 针对目前可视密码术引起的像素膨张、与原密图相比，叠合图像的亮度、对比度降低问题，提出一种新的基于分块字典的可视密码改进方法。该方法首先根据确定大小的像素分块，列举每个子图各子块像素分布情况后建立相应的分块字典，并对分块字典条目进行编号；然后对原密图置乱后，将原密图分成两个子密图；再分别两个子密图进行均匀分割，将其分成若干子块，所有密图子块对应一个字典编号并进行一维化，则得出由两个子图所有子块编号组成的序列。由此，密图就由这个分块字典编号序列所确定，经过传输后，提取并组成字典编号序列，二维还原后与相应的分块字典条目对应，装配得出两个子图叠合后还原成原密图。与以前的可视密码算法，如（k，n）门限方案等相比，采用子图交错叠合、基于分块字典的可视密码改进方法，可减少算法复杂度，编码的比特序列长度是对原密图直接编码后的比特序列长度的两倍，从而减少像素膨胀，也不会引起叠合图像的亮度、对比度降低的问题。从而更具实用性。

关键词: 可视密码, 分块字典, 像素膨胀