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题名:
视觉密码方案的构造
作者: 刘峰
答辩日期: 2009-01-14
授予单位: 中国科学院软件研究所
授予地点: 软件研究所
学位: 博士
关键词: 视觉密码 ; 秘密共享 ; 图像处理 ; 一次一密 ; 数字水印 ; 纠错编码
其他题名: Designs of Visual Cryptography Schemes
摘要: 视觉密码学是在最近十余年间逐渐发展起来的新型密码学分支。近年从事视觉密码学研究的学者越来越多,研究结果频现于三大密码学会和Journal of Cryptology, Designs, codes and cryptography和IEEE Transactions on Image Processing等高水平期刊上。视觉密码方案(VCS)具有的针对图片加解密,视觉解密,秘密共享和无条件安全等特点,较之传统的秘密共享方案,它具有解密设备简单的优点,解密过程不需要使用者拥有任何密码学知识,可以直接观察到解密图片。而较之传统的秘密隐藏方案,他具有信息容量大的优点,可以加密整幅图片的秘密信息,而且由于其加解密的对象是图片,因此其信息内容可以灵活多变。视觉密码方案可以应用在秘密共享,信息隐藏,身份认证/鉴别以及版权保护等方面。随着具有更多特殊性质的视觉密码方案的提出,其应用范围也会越来越广。本文的工作主要集中在两个方面:视觉密码方案的构造和分析以及视觉密码相关问题的研究。 在视觉密码方案的构造和分析方面,本文所做的工作主要包括: 本文在第二章提出一个利用嵌入技术构造有意义分享图片视觉密码方案(EVCS)的方法。与已知的构造相比,其优点包括:(1) 可以直接处理灰度图片;(2) 具有较小的秘密图片像素扩张;(3) 可以针对一般存取结构构造,且总是无条件安全的;(4)每个参与者只分配一张分享图片;(5) 具有在分享图片像素扩张、分享图片视觉效果和秘密图片像素扩张之间相互取舍的灵活性。 本文在第三章提出了一种针对一般存取结构的VCS的递归构造方法,与过往的VCS相比,在大多数存取结构下,本文的方法在平均像素扩张,像素扩张和对比度方面都有所改进。根据本文的构造方法,可知针对一般存取结构的VCS的构造可以通过递归调用(2,2)-VCS来实现,而无论其所基于的是“OR”还是“XOR”运算。本文的方案最终可能分配给某个参与者多张分享图片,由于平均像素扩张的降低,因此这一点并不能称之为缺点。另外,利用本文中的方案可以构造出针对一般存取结构的基于“XOR”运算的VCS。 对于彩色视觉密码方案(CVCS),基于Naor-Shamir视觉密码模型,本文在第四章提出一个的没有像素扩张的(k,n)-CVCS,以及与对应的黑白(k,n)-EVCS具有相同像素扩张的(k,n)-CEVCS;基于Tuyls视觉密码模型,本文首先分别提出了黑白(k,n)-VCS和黑白(k,n)-EVCS,并以此为构件提出(k,n)-CVCS和(k,n)-CEVCS。仿真实验结果表明,本文提出的方案具有较好的视觉效果。 对于防止欺骗的视觉密码方案,本文在第五章首先分析了已知的几种防止欺骗的视觉密码方案(CIVCS)的缺陷,然后提出一个新的CIVCS来避免这些缺陷。本文所提出的CIVCS构建于已知的VCS之上,在只泄漏极少量秘密图片的信息的情况下达到了很高的安全性。并且可以针对一般存取结构。本文所提出的CIVCS也可以应用在底层运算为“XOR”运算的VCS之上。另外,本文所提出的CIVCS可以根据参与者携带验证信息的多少,可以检测出欺骗行为或者可以揪出具体的欺骗者。 本文在第六章研究了基于“XOR”运算的(2,n)-VCS的最优化问题。给出了基于“XOR”运算的(2,n)-VCS的最小像素扩张,以及具有最小像素扩张条件限制下的最大对比度和最大平均对比度。另外给出了基于“XOR”运算的(2,n)-VCS的最大对比度,以及具有最大对比度条件限制下的最小像素扩张。上述四类方案在本文中都给出了具体的构造方法,以及相关对比度,像素扩张和基矩阵的结构性质的证明。本文中的结果表明基于“XOR”运算下的(2,n)-VCS的各个参数都要比基于“OR”运算下的对应的参数要好。本文还证明了构造具有最大对比度的(2,n)-VCS的基矩阵等价于构造具有相应参数且达到最大容量的二元码。故根据本文的研究结果,便可以通过利用已知的构造达到最大容量的二元码的方法来构造具有最大对比度的(2,n)-VCS,最后对于n=2^k-1的情况,本文还给出了一个利用m-序列来构造具有最大对比度的(2,n)-VCS的方法。 本文在第七章证明了Droste提出的基于“OR”运算的(k,n)-VCS同样在“XOR”运算下也成立。同时基于“OR”运算和“XOR”运算的VCS会给参与者带来便利,他们可以根据不同的环境而选择不同的视觉密码模型。另外,本文还提出一个进一步降低VCS的像素扩张的方法,与Tuyls等人所提出的(k,n)-VCS比较,本文的方法可以显著的降低方案的像素扩张。本文在第七章还提出一个可以运行在Tuyls视觉密码模型之上的同色(k,n)-VCS的构造方法,其中每个分享图片都是同色的,而叠加k个分享图片可以得到黑白的秘密图片。本文证明了当k为奇数时,同色的(k,n)-VCS是不存在的,并给出了当k为偶数时,同色(k,n)-VCS的构造方法。同色(k,n)-VCS可以用来防止分享图片被隐蔽的监控摄像头窃取。 在视觉密码相关问题的研究方面,本文所做的工作主要包括: 本文在第八章研究了视觉密码方案的幻灯片对齐问题,证明了,在幻灯片没有精确对齐的情况下,人眼仍能观察到秘密图片,且其平均对比度满足-(m-r)e/(m2(m-1))。这项研究可以为确定幻灯片中像素的尺寸做参考。本研究也证明了视觉密码方案本身具有一定的纠错能力。 本文在第九章研究了视觉密码中的对比度定义的表示问题。首先给出了四个关于VCS对比度的观察结论,并分析了已知的几种对比度定义的缺陷,从而提出了一个新的对比度定义。本文在理论和实验上说明了,其所提出的对比度定义符合上述观察结论,可以更精确的衡量所恢复的秘密图片的视觉效果。本文最后还针对概率视觉密码方案的定义给出了针对概率视觉密码方案的对比度的定义。 最后我们对本文的工作进行了总结,并对今后的一些研究方向进行了展望。
英文摘要: Visual cryptography is a recently developed secret sharing technique which allows the encryption of a secret image among a number of participants. More and more researchers have focused their work on visual cryptography. Many of the papers about visual cryptography have been published in reputable international conferences and journals. The beauty of visual cryptography scheme (VCS) is that, the decryption of the secret image requires neither cryptography knowledge nor complex computation. Moreover, it provides unconditional security. Compared with the traditional secret sharing schemes, it can encrypt a large amount of secret information, i.e. an entire image where its content can be versatile. The visual cryptography scheme can be applied in secret sharing, information hiding, identification/authentication and copyright protection. Along with the introductions of VCS with new traits, new applications of VCS will be found in the future. The work of this paper is mainly focused on two aspects, the designs and analyses of VCS and the related problems about VCS. First, for the designs and analyses of VCS, the contributions of the paper are as follows. An embedded extended visual cryptography scheme (Embedded EVCS) is proposed where its shares are all meaningful images rather than noise like images. The Embedded EVCS applies the embedded technique and halftone technique. Compared with some of the known EVCS's, the proposed scheme has the following advantages, (1) It can deal with grey level input images. (2) It can have small pixel expansion. (3) It can generate a general access structure EVCS and is always unconditionally secure. (4) Each participant only receives one share. (5) It is flexible in the sense that there exist two trade-offs between the share pixel expansion and the visual quality of the shares and between the secret image pixel expansion and the visual quality of the shares. An construction of general access structure VCS by applying (2,2)-VCS recursively is proposed. Compared with many of the known VCS's, the proposed VCS has smaller pixel expansion and average pixel expansion, and larger contrast in most cases. According to the proposed construction, a general access structure VCS can be constructed by only applying (2,2)-VCS recursively, regardless whether the underlying operation is OR or XOR. This result is most interesting, because the construction of VCS under the operation XOR for general access structure has never been claimed to be possible before. Four types of color visual cryptography schemes (CVCS) are proposed. (1) A (k,n)-CVCS under the visual cryptography model of Naor and Shamir with no pixel expansion. (2) A (k,n)-CEVCS under the visual cryptography model of Naor and Shamir with pixel expansion the same as that of its corresponding black and white (k,n)-EVCS. (3) A (k,n)-CVCS under the visual cryptography model of Tuyls, where its corresponding black and white (k,n)-VCS is also proposed. (4) A (k,n)-CEVCS under the visual cryptography model of Tuyls, where its corresponding black and white (k,n)-EVCS is also proposed. The drawbacks of some known cheating immune visual cryptography schemes (CIVCS) are analyzed, and a new CIVCS which avoids all the previous drawbacks is proposed. The proposed CIVCS is constructed based on a known VCS, and can be applied to all VCS's for general access structure. Furthermore, the proposed CIVCS can detect the cheaters or only detect the existence of cheaters depending on the amount of the authentication information provided. The smallest pixel expansion and largest contrast of (2,n)-VCS under the XOR operation is analyzed, and the values of the smallest pixel expansion, the largest possible contrast for schemes given the smallest pixel expansion, the largest contrast, and smallest possible pixel expansion for schemes given the largest contrast are given, and concrete constructions are proposed as well. The results show that the properties of the (2,n)-VCS under the XOR operation are much better than that under the OR operation in the sense of larger contrast and smaller pixel expansion. The paper also shows that, the construction of the basis matrix of contrast optimal (2,n)-VCS is equivalent to the construction of maximum capacity binary codes with specific parameters, hence the known constructions of maximum capacity binary code (constant weight or not constant weight) can be applied to construct contrast optimal (2,n)-VCS. A construction of VCS with largest contrast under the XOR operation for n=2^{k-1 is proposed as well. The paper shows that the (k,n)-VCS proposed by Droste in 1996 is a (k,n)-VCS that works both under the OR and XOR operations. This trait can bring more convenience to the participants. Furthermore, a method to reduce the pixel expansion of (k,n)-VCS is proposed. The method can significantly reduce the pixel expansion compared with that of the (k,n)-VCS proposed by Tuyls et al.. A construction of concolorous (k,n)-VCS where the shares are concolorous is proposed. The paper proves that the concolorous (k,n)-VCS does not exist with odd k, and proposes a construction of concolorous (k,n)-VCS with even k. The concolorous (k,n)-VCS can be used to protect the shares from being stolen by hidden cameras. Second, for the related problems about VCS, the contributions of the paper are as follows. The alignment problem of the VCS is analyzed. The result of this paper shows that, the original secret image can be recovered visually when one of the transparencies is shifted by at most m-1 subpixels, and the average contrast becomes -(m-r)e/(m2(m-1)). The study is based on a deterministic visual cryptography scheme, and the shifted scheme is a probabilistic visual cryptography scheme with less average contrast but still visible. This result helps to determine the size of the subpixel printed on the transparencies. The paper examines the defects of some known definitions of contrast of the VCS, and proposed a new definition based on the new observations. It is shown both experimentally and theoretically that the new definition of the contrast agree with the new observations more appropriately. At last, we conclude the results of this paper and give the prospects of further research.
语种: 中文
内容类型: 学位论文
URI标识: http://ir.iscas.ac.cn/handle/311060/7512
Appears in Collections:信息安全国家重点实验室_学位论文

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刘峰. 视觉密码方案的构造[D]. 软件研究所. 中国科学院软件研究所. 2009-01-14.
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