A Study Of Authentication Algorithm For Digital Images

Accompanying the rapid development of Internet and Multimedia technology, Multimedia data becomes the information resource of people's life. But at the same time, multimedia information security becomes more important. With more and more image processing devices and software being widely used, it is easier to copy, edit and modify digital images. Unlawful tempering and transmitting will cause critical harm to society and people, so multimedia content protection and integrality authentication is becoming a very hot study topic in the world.Digital signature and digital watermarking are the two main methods to solve the problem of multimedia information security. Traditional digital signature technology extracts one kind of abstract, which is undeniable and encrypted, and uses it to authenticate. Digital watermarking is a comparatively new technology proposed in 90's last century, it embeds copyright labels or authentication information into multimedia data to protect the ownership and integrality. Digital watermarking has many good capabilities, and much research work has been done on it.In this dissertation, we mainly study on digital image authentication method in two aspects: complete authentication and content authentication. After analyzing the previous work in this field, a novel semi-fragile watermarking algorithm for image content authentication is proposed, and experimental result shows the algorithm has good performance and security. The main content and contributions of this dissertation are:(1) General survey the image authentication technology. It includes: the background of image authentication; the classification of image authentication: complete authentication and content authentication; the methods of image authentication: digital signature and digital watermarking.(2) Focus on complete authentication. After considering the characters of complete authentication, several algorithms based on digital signature and complete fragile watermarking are proposed, and then we compare the two kinds of methods, indicate the shortcomings and give some improvement measures.(3) Pointing out the limitation of the image authentication algorithm based on dividing blocks and give the reason why collage attack can be successful. Authentication chain technology is proposed, it increases the relativity of imageblocks and makes the authenticating processing is related to the location of image blocks, so it can foil collage attack effectively.(4) Focus on content authentication. After analyzing the characters and requirements of content authentication, a novel self-embedding semi-fragile watermarking algorithm for image content authentication is proposed. It takes advantage of the invariant property of JPEG and embeds the secure watermark based on the image feature into the DCT domain. This algorithm is robust to the JPEG compression, quality factor of which is higher than the pre-determined one, rejects malicious tempering, locates the changed position precisely, and it can foil collage attack effectively because of using the technology of authentication chain.(5) A survey to the problem about how to estimate watermarking capacity and energy. Two kinds of methods applying to spatial and frequency domain respectively are proposed, and analyzes the common features of current methods, refers to directions of future work.