An Encryption Algorithm And Implementation For High-resolution Digital Images Based On Chaos System

Nowadays, with the rapid development of Internet, more and more date will be transferred by Internet and Internet has become the largest carrier of information. A lot of conveniences have been brought to our daily work and life apparently, but as an open exchange platform, the information that we transmits may be attacked by unauthorized users, so we should pay attention to the security of information transmission. If we can’t guarantee the security of information transmission, the troubles of information disclosure will greatly disturb us. So, whether we can guarantee the information security or not will become a key factor restricting the development of Internet.Because of digital images’intuitive features, it has been the main way of human being’s data storage. Digital images are different from plain text, the data mount of images are larger than text, so the traditional encryption algorithms such as DES and 3-DES are not proper for images encryption, or the real-time will become very poor and not benefit for the transmission of image in time.Digital images encryption algorithm based on chaos system and space domain transform is mainly researched in this thesis. On the basis of traditional algorithm, scrambling sequence and replacement sequence are generated by the improved one dimension Logistic chaotic map. Digital image algorithm is realized by scrambling encryption combined with replacement encryphon.Firstly, some basic concepts and composition of cryptography are introduced, and then we elaborate some classic algorithms of cryptography and cryptanalysis. The emergence, development and modern definition of chaos system are introduced as well. The improved chaos map---NCA and Arnold transform are highlighted and some advantages and disadvantages of NCA chaos maps are listed also.Secondly, because of the increasing proportion of high-resolution digital images in the Internet, two encryption algorithms based on different types of high-resolution are proposed, one is the encryption of the entire image(algorithm 1) and another is partical encryption of image(algorithm 2). These two algorithms both take pixel replacement encryption first, and then take image scrambling encryption. They use the same pixel replacement sequence, but in Algorithm 1, the scrambling encryption is based on NCA chaos map, and in Algorithm 2, the scrambling encryption is based on Arnold transform. The pixel replacement sequence in the literature [32] is improved, We choose to take 48 significant digits of chaos value to generate pixel replacement sequence instead of taking 15 significant digits. The simulation results show that the improved method has good advantages such as a better real-time characteristic, correlation and distribution properties. In the image encryption algorithm, encryption key and decryption key are the same, at last, we use RSA algorithm to encrypt the initial values in order to prevent the keys are attacked by unauthorized users.Finally, a series of simulations have been done on replacement sequence and scrambling sequence with MATLAB. Different types of high-resolution images are encrypted by Algorithm 1 and Algorithm 2 respectively. Several analysis are made on encryption images such as gray level histogram, correlation, sensitivity to initial values and real-time characteristic, simulation results show that the new algorithm can protected the image effectively on the premise of real-time.