視覺密碼中不擴展的(k, n)門檻值秘密影像分享方法之研究

Abstract

Visual cryptography (VC, for short) encrypts the secret image into n shares (transparency). In this way, we cannot see any information from any one share, and decrypt the original image by stacking all of the shares. In this thesis, we extend it to the k out of n secret sharing scheme, (k, n)-threshold secret sharing scheme, which encrypts the secret image in the same way, but decrypts the original image by stacking at least k shares. If one stacks less than k shares, one cannot recognize the secret image. An important parameter when discussing a secret sharing scheme in VC is contrast 'a'. If 'a' is larger, the recoverd image is clearer. Another subject is progressive visual secret sharing, that means when more shares are stacked progressively, the combined share will be clearer. In this thesis, we construct a new (k, n)-threshold secret sharing scheme in VC for any positive integers n >= k >= 2 by using a method of combination, and the size of each share is as small as the original image. That is, there is no expansion needed while some of the previous scheme need. In the same time, our scheme has better contrast 'a' than previous method and it is also a (k, n)-threshold progressive visual secret sharing scheme.