ABSTRACT
Modern cryptosystems work on the principle of exchanging keys and messages over an insecure network as compared to the classical cryptosystem which uses substitution of words or alphabets based on fixed displacement as in the vigenere cipher. This chapter shows that more powerful modern encryption techniques based upon Shannon theory. Image encryption techniques vary from region selection algorithms to spatial and frequency domain algorithms. Based on complexity, region selection algorithms are divided into two broad categories, one is block selection and other is the pixel-based selection. Block selection technique incorporates the edge detection method. Edge detection includes suitable filters or operators for identifying sharp edges and sudden changes in intensity of images. Pixel selection technique is one of the most important techniques for region selection. An image is divided into pixels, where each pixel is represented by an 8-bit code. Observation is an important aspect of any image encryption technique.
