視覺密碼是一種依靠人眼解密的一種秘密分享方法，在無法使用電腦解密的情況下，它是一個很好的解決方案。大部分的視覺密碼方法由於像素擴展的關係，所以產生的分享影像大小會比原來的機密影像大上許多倍，尤其是應用在灰階和彩色影像上，其擴展的倍數更是驚人。本研究提出一種不需要像素擴展的灰階和彩色視覺密碼的技術；我們的方法利用多點同時加密的概念，每次取連續的m個點做為加密序列進行加密，我們的方法可以確保在m個具有b個黑點的加密序列中，一定會有b個加密序列是使用黑點基礎矩陣加密，另外的(m - b)個則使用白點基礎矩陣加密，因此在疊合影像上黑與白的變化很規律。因此，我們不但能達成像素不擴展的目標，而且也能確保重疊影像的視覺效果。針對彩色影像，我們應用色彩模型的原理，將影像分解成青、洋紅、黃三張不同色調的影像，將這三張單一色調的連續調影像，利用半色調技術轉換成二元影像，再利用本研究所提的多點加密法，來加密這些半色調影像。由實驗結果可以證明，在不作像素擴展的前提之下，分享影像仍然具有足夠的安全性，而且也能確保還原影像有很好的視覺效果，同時我們的方法可以很容易地延伸至任意的使用結構上。
Visual cryptography is a visual secret sharing method which encodes a secret into several shares and decodes a secret with human eyes; therefore, it is a good solution to decrypt secrets without computers. Most visual cryptographic methods need to expand pixels and hence enlarge the size of shares. This situation is more serious for gray-level and chromatic images. In this paper, we propose a new visual cryptographic method for gray-level and chromatic images without pixel expansion. We simultaneously encrypt m successive pixels (called an “encryption sequence”) each time in accordance with two basis matrices. In every m encryption sequences with b blacks, we make sure that b encryption sequences are encoded by black basis matrix, and the other (m - b) ones are encoded by white basis matrix. Therefore we can not only attain the aim of not expanding the pixels, but also ensure good visual effect of the stacked image. Additionally, we utilize the color model to decompose a chromatic image into three monochromatic images in tones of cyan, magenta, and yellow, respectively. These three images are transformed into halftone images and then encrypted by the proposed method. The experimental results show that the security of shares is still maintained though pixels are not expanded, and the visual effect of the stacked images is good. With appropriate basis matrices, our method can be easily extended to general access structure.
