Lecture Steganography: Steganographic channels introduce contents such as problem, steganography by the cover selection, steganography by cover synthesis, steganography by cover modification | Steganographic channels Ho Dac Hung 1 Contents Problem Steganography by cover selection Steganography by cover synthesis Steganography by cover modification 2 1. Problem 3 1. Problem 4 1. Problem Passive warden scenario Active warden scenario Malicious warden scenario 5 1. Problem The problem of steganography can thus be formulated as finding embedding and extraction algorithms for a given cover source that enable communication of reasonably large messages without introducing any embedding artifacts that could be detected by the warden. In other words the goal is to embed secret messages undetectably. 6 2. Steganography by cover selection In steganography by cover selection Alice has available a fixed database of images from which she selects one that communicates the desired message. 7 2. Steganography by cover selection The embedding algorithm can work simply by randomly drawing images from the database till an image is found that communicates the desired message. The stego key here is essentially the set of rules that tell Alice and Bob how to interpret the images. 8 2. Steganography by cover selection An important case of steganography by cover selection involves message-digest hash functions. h x x 1 mod 2 x 2 mod 2 x 3 mod 2 9 3. Steganography by cover synthesis In steganography by cover synthesis Alice creates the cover so that it conveys the desired message. Steganography by cover synthesis could be combined with steganography by cover selection to alleviate the exponential complexity of embedding by hashing. 10 3. Steganography by cover synthesis 11 3. Steganography by cover synthesis Let us assume that the images are 8-bit grayscale with xj i standing for the intensity of the ith pixel in the jth image i 1 . . . n j 1 . . . K. Alice will use a cryptographic hash function modified to return 4 bits when applied to 16 pixels. 12 3. Steganography by cover synthesis Alice divides every image into disjoint blocks of 4 4 pixels and assembles a new image in a .
