الفهرس 
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Abstract
Nowadays, many types of attacks and hacking algorithms imposed to information during the transmission have been appeared in a serious manner. So the need for securing multimedia information has been indeed for high robustness against different types of attacks. One of the most important methods of security was the encryption method. In this thesis it has been concerned on images encryption due to its vital rule in multimedia information transmission, especially in optical encryption. Optical nature of an image and high speed processing were the main reasons behind interesting in study and developing optical encryption techniques. Robustness against various types of attacks has represented the main target objectives. Many optical encryption techniques based on Double Random Phase Encoding (DRPE) have been introduced, on the other hand most of these techniques suffer from more than one weakness point as; the need for dealing with complex phases, the necessity of transmitting amplitudes as well as phases of random masks to the receiver, poor performance against most types of attacks, as well as poor performance in the case of color images encryption. In this thesis, eight proposed techniques based on modified DRPE will be introduced, the performance of these techniques will be evaluated and compared to the performance of the three traditional techniques for grayscale as well as color images against single and/or composite attacks. These attacks could be imposed to encrypted images; friendly attacks, or more realistic to original data. One of the proposed techniques will be applied for grayscale as well as color images encryption and other seven schemes have been developed specifically for color images encryption due to its importance over the grayscale ones. As a result of intensive comparative study based on the eight adopted performance metrics, it has been found that the proposed techniques have given better robustness against statistical as well as differential attacks with respect to conventional schemes. A composite type of attacks (severe attack) has been imposed to the original color image, the proposed technique number 6 provided the best performance metrics. Nine Chaotic maps; as random keys, were used to implement seven proposed techniques. Eight of these maps were predefined and one proposed for the first time which has given better performance than others. High enhancement ratios in performance against severe types of attacks has been achieved by using proposed algorithms with proposed map especially in the case of color images encryptionwhich represents high resolution case.