【作者】 林润海；

【导师】 叶瑞松；

【作者基本信息】 汕头大学 ， 应用数学， 2009， 硕士

【摘要】 计算机网络的快速发展和多媒体技术的广泛应用使越来越多的数字图像通过各种媒介传输.由于数字图像数据所具有的大数据量特点,传统的加密方法如DES、AES和RSA并不完全适合直接对图像加密.近年来,基于混沌理论的数字图像加密方案引起了学者的重视并得到广泛研究.混沌系统对初始值和系统参数非常敏感,其产生的混沌序列具有遍历性、内在随机性、可精确再生和难以预测等特征.这些特性和密码学的很多要求相吻合,使得混沌在数字图像加密和保密通讯中具有实际的应用价值,可以借助混沌系统设计出容易控制和具有高安全性的图像加密系统.本文基于三维整数拟仿射变换和四维混沌系统对图像加密.首先把二维有限整数域上的拟仿射变换推广到三维上,重点研究三维有限整数域上的拟仿射变换的性质及构造方法,用这种新方法来置乱图像的像素位置(如果需要的话,灰度值也可以改变),并用混沌映射来生成变换的参数,从而增强了统计和差分攻击的抵抗力.其次利用一种四维混沌映射产生混沌二值序列,在此基础上,针对能体现混沌序列的几个重要特性进行分析、实验,得出此改进后的混沌序列具有较好的自/互相关特性、初值敏感性、频数检验和序列检验的特性.最后把本文方法产生的三维整数拟仿射变换和混沌伪随机二值序列用于图像加密,再将加密图像与目标图像进行线性融合得到最终的图像.仿真及分析结果表明,该算法密钥空间大,具有较强的隐藏效果和较好的统计特性,较强的抗干扰能力和较高的执行效率,加密效果对密钥敏感,是一种安全性高且加密速度快的新算法.更多还原

【Abstract】 With the rapid development of computer network technology and multimedia technology, more and more digital images go across the world over the transmission media. Due to some intrinsic features of images, such as bulk capacity of data, it is troublesome in image encryption for conventional cryptographic algorithms such as DES, AES and RSA. In recent years, digital image encryption schemes based on chaotic maps have drawn a lot of attention and have been extensively researched.The distinct properties of chaos, such as sensitivity dependence on initial conditions and system parameters, ergodicity, quasi-randomness, accurately regeneration and uncertain for forecasting, are fit for demands of cryptology, which grant chaotic systems have potential value in digital image encryption and security communication area. It is hopeful to scheme out an image encryption system which can be easily controlled and own high security.In this paper, the symmetric image encryption scheme based on 3-D quasi-affine transformation over limited integer grids and 4-D chaotic map. The two-dimensional quasi-affine transformations over limited integer grids are generalized to three-dimensional ones. The properties of three-dimensional quasi-affine transformations are analyzed and the corresponding construction scheme is proposed. The three-dimensional quasi-affine transformations are employed to shuffle the positions of images. Experimental results show that the proposed scheme owns good encryption effects and robustness against statistical and difference attacks. Secondly, an improved chaotic binary sequences algorithm based on a four-dimensional chaotic map is proposed. Through the analysis and experiments for certain important characteristics of chaotic sequences, we show that the improved chaotic sequences have better correlation, sensitivity with good frequency test and sequence test. The binary sequences generated by the proposed algorithm are applied to image encryption. The final image is obtained by linear image fusion of both the encrypted image and the object image. The simulation and analysis show that the algorithm has a large space of keys, good statistical characteristic, strong anti-noise ability and high efficiency, and the encryption effect is greatly sensitive to the keys as well.更多还原