WWW/Internet - Portal do Software Público Brasileiro

ISBN: 978-972-8939-25-0 © 2010 IADIScoefficients of image transforms, The stenographer’s job is to make the secretly hidden information difficultto detect given the complete knowledge of the algorithm used to embed the information except the secretembedding key [2]. This so called Kerckhoff’s principle is the golden rule of cryptography and is oftenaccepted for steganography as well, in real world, however, it may be rather difficult if not impossible toobtain this knowledge. In one typical scenario, Alice and Bob exchange messages (images) and Eve wants tofind out if they use steganography. Alice and Bob draw images from a certain source, Eve may find out, forexample, that Alice uses a specific scanner, digital camera, a camcorder with a TV card, or some otherimaging device as a source of images. Eve could purchase the same type of imaging device and generate alarge database of images.3. RANDOM IMAGE GENERATION (RIG)Generally, each pixel in plane image can be represented by x and y coordinates, and the pixel value isevaluated from the three components Red, Green and Blue, which can be denoted as [R, G, B].The pixel components (red, green, blue) should also have a value between [-1, 1], since its value isdependent on a mathematical functions (such as sin, cos, etc.). For example, RGB [0,0,0] is black which isnormalized into RGB [-1,-1,-1] , RGB [255,0,0] is red which normalized into RGB [1,-1 –1] and so on, theequations below convert the default RGB values into values between [-1, 1]:( n 1) * 255Nb1..10..255, Where n is any number from [-1...1] (3.1)2n * 2Nb0..2551..1 1, Where n is any number from [0...255] (3.2)2553.1 The Grammar RulesThe used grammar rules are written using Backus-Naur Form (BNF). BNF is a notation for expressing thegrammar in the form of production rules [8]. BNF grammar consists of terminal{s} and non-terminals. InBNF notation, angle brackets are used to distinguish non-terminals, the symbol: : = stands for “ is definedas”, and | stands for “or” . The use of | is a shorthand for several productions, which have a concatenation ofzero and more symbols (terminals and non-terminals) on the right hand side [7]. The BNF grammar used inthe proposed RIG is shown in Table 1Table 1. The RIG grammarProductionrule number0123456789The Production rule::=::=::=X::=Y::=R::=Add(,)::=Mult(,)::=Div(,)::=Mod(,)::=Sin()Productionrule number101112131415161718The Production rule::=Tan()::=Arctan()::=Cos()::=Reverse()::=Bw()::=Expf()::=Rgb(,,)::=If(,,)::=Mix(,,,)Several mathematical functions are selected and used in the proposed system to generate the cover image.356