All-optical Encrypted Movie Based On Fractional Fourier Transform

Information age has seen the great development of acquisition, processing and applications of movies and videos. Movies/videos are highly related to people’s daily life. With wide use of the Internet, people have access to a great number of movies and videos via the Internet, which makes security a critical issue. Therefore, encryption for movies has given rise to intensive research interests in recent years. Traditionally, optical cryptographic techniques are used to enhance the confidentiality of movies. However, the existing optical cryptographic techniques are based on the double random phase method, which means that only double keys are used to encrypt the movies, so the risk that the contents of the movies could be leaked remains.In this thesis we propose an optical cryptographic scheme for movies/videos based on the fractional Fourier transform. The scheme consists of two phases:encryption and decryption. In the encryption phase, all frames of a movie are modulated by a cosine amplitude grating based on the θ modulation technology. Then all the modulated frames are superposed and encrypted into a single frame with the fractional Fourier transform and the double random phase method. In the decryption phase, the encrypted frame is recovered by using the Fourier transform, spatial filtering and the inverse Fourier transform. Thus, diffraction spots of all the modulated images can be seen in the Fourier plane. After the filtering of the diffraction spots one by one, the decrypted movie can be recovered by using the inverse Fourier transform.Both the simulations and experiments have been implemented to justify the proposed scheme. The results of both the simulations and experiments demonstrate the feasibility of our method. The frames of the sample movie are fully recovered and recognized by using the proposed scheme. Moreover, we find that sizes and shapes of the filter apertures would influence the visual quality of the recovered images, but the recognition rates of the frames of the movie are almost unaffected. The main contributions of this thesis are as follows. We have increased the number of keys from two to four, which enhance the security of movies significantly; we have optimized the algorithm by implementing the grating modulation process before the encryption. Compared with the traditional method which implements encryption and decryption for every frame of a movie, our method needs one encryption step and one decryption step only.