行動裝置視覺密碼之研究

近年來隨著網際網路與硬體產品的發展迅速，許多的3C產品也越來越便利，科技可以讓使用者隨時隨地透過筆記型電腦、智慧型手機或平板電腦，將資料做分享或是儲存，這樣的方法大多是以網際網路做為媒介。由於數位資訊有能輕易地被儲存、複製、修改與傳遞等特性，於是產生了資安上的課題，為了使我們的資料在網路上得到保障，密碼學與視覺密碼等多樣性的保護方法因而被發展出來。　　本研究中，利用了智慧型手機或平板電腦所搭載的Android系統進行應用設計。由於智慧型手機有較好的便利性與功能性，讓使用者可以在照完相片後立即上傳到一個伺服器端，由擁有較強運算效能的伺服器進行影像加密的動作，而加密的方式有許多變化，本論文提出了三種方式，可以提供給用戶做選擇，這也使得安全機制方面多了一層的保護。本研究的三種影像加密方法，分別為(1)一個新的視覺密碼設計、(2)彩色影像分享及(3)改善(r, n)機密影像分享法。第一個方法著重在視覺密碼的防護上，讓影像可以得到多一層面的安全防護，這個方法在編碼端有著低運算量的特性，由結果可看出，提出的方法可幾乎近無失真的取回原圖，並消除視覺密碼的像素擴展問題；第二個方法是經由改善視覺密碼的一個新的演算法，本文將原先視覺密碼上的缺點加以改進，使得原先的像素擴展與影像失真等問題得以消除。現今大多的視覺密碼幾乎都是運用在黑白或是灰階圖像，本研究將改善學者所提出的演算法，運用在彩色圖像上，使視覺密碼在彩色圖像上也可毫無顧慮的使用，由實驗結果可看出，視覺密碼分享的確可以在彩色圖像上表達出一樣的效果；第三個方法為利用隨機網格搭配(r, n)-門檻的資料分享理論進行影像分享，並以簡化後的方程式來增加解碼時的效率。

關鍵字

行動裝置；隨機網格；視覺密碼；影像分享； (r, n)-門檻；資訊安全

並列摘要

With the rapid development of the Internet equipment, 3C products are increasingly produced. Digital technology allows users sharing data among laptops, smart phones and tablet computers, such sharing skills are mostly based on networks. Because digital information can easily be stored, copied, modified or transmitted, in order to make the information being protected, various cryptography methods are then developed. 　　The present study takes advantage of smart phone or tablet equipped with the Android system. Because modern hand-held devices are convenient and powerful, a user can take a photo and upload to a server immediately, the server can therefore be used to conduct the process of image encryption. In this thesis, three kinds of encryption methods are proposed. In terms of security, the proposed design is safer. Our image encryption process is divided into three different sharing methods, namely a new visual cryptography design, color image sharing, and a rapid secret image sharing method. The first method focuses on the visual cryptography, so that the image can be secure. This method has a characteristic of low computation on the encoding side. From the experimental results, the proposed method can recover a nearly distortion-free image, and also eliminate visual cryptography pixel expansion problem. The second method is to improve previous visual cryptography algorithms, the pixel expansion and image distortion problem can be eliminated. The experimental results show that the proposed visual cryptography sharing method can be used to color images. The third method is to combine random grid and (r, n) - threshold sharing methods for image sharing. Simplified equations in our method are derived to increase the decoding efficiency.