关键词: 可视密码, 分块字典, 像素膨胀

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