Image Steganography Techniques

Topics: Bit, Discrete Fourier transform, Steganography Pages: 6 (2772 words) Published: April 21, 2014
Image Steganography Techniques: Hiding messages in Images

Introduction
The word steganography is derived from the Greek words “stegos” meaning “cover” and “grafia” meaning “writing” defining it as “covered writing”. There are 5 kinds of Steganography: Text, Image, Audio/Video and Protocol (Joshi, R. et al. 2013). The term Steganography refers to the art of covert communications. By implementing steganography, it is possible for Alice to send a secret message to Bob in such a way that no-one else will know that the message exists (Bateman, P. and Schaathun, H. 2008).

Images are considered as the most popular file formats used in steganography. They are known for constituting a non-causal medium, due to the possibility to access any pixel of the image at random. In addition, the hidden information could remain invisible to the eye. However, the image steganography techniques will exploit "holes" in the Human Visual System (Hamid, N. et al. 2012)

Scientific Review
As the computer, the internet and the Digital Signal Processing (DSP) started to boost, steganography has gone digital (Cheddad, A. and Condell, J. 2010). The popular images to try steganography are binary images and 3D images and steganography for such types of images have shown a lot of progress but researchers focus on using grey-scale and colour images to hide data. Grey-scale images are preferred over colour images as it is easy to detect the route of encryption because of the disturbances of correlations between colour components (Li, B. and He, J. 2011).

There are various techniques used for Image steganography which are performed on various formats of images. Some of the techniques include:

Least Significant Bit technique – In this method, the image is scanned row wise and it is encoded in binary. Then, the message is also encoded in binary. The image size and the message size are compared. Once done, a random pixel from the image is chosen and is divided into three parts i.e. Red, Green and Blue. The 2X2 bits of the secret message are stored in the two LSBs of each of the Red, Green and Blue parts (Juneja, M. and Sandhu, P. 2009) (Al-Shatnawi, A. 2012).

Mapping pixels to letters – There are 26 English alphabets in total which are used to write a sentence (a…z). There are some special characters as well which are commonly used which sums up the number of characters to 32. 32 = 25, therefore at least 5-bits are needed to represent the characters in a digital system. A grey scale uses 256 grey scales for its each pixel. Therefore, 8 bits per pixel are needed to produce 28=256 grey scales. Using the equation (Sequence + (32*0))….. (Sequence + (32*4)), each character is written down in 7 Most Significant (MSB) binary form for each case. This method operates by mapping all the 4-cases i.e. 27=128 of the 7 MSB in a pixel to any of the 32-cases of the alphabets and special characters. It is done in order to recognize the same pixels in the images that are mapped to a character or alphabet of the message (Al-Husainy, M. 2009).

JPEG quantization tables – The image to be worked upon is divided into blocks of 8X8 pixels which do not overlap each other. The messages is embedded in the Least Significant Bit (LSB) of the quantized Discrete Cosine Transform (DCT) coefficients of each block. However this method is not very efficient because it only embeds one bit in a quantized coefficient where the value is not 1, 0 or -1 (Almohammad, A. and Ghinea, G. 2009) (Kumar, M. and Newman, R. 2009).

Wavelet transformation – Firstly, the cover image is checked if it contains patterns that could not be recognized and if it passes the histogram test. Then, level correction, contrast correction and colour balance correction are

applied for each pixel. The cover image is then converted into Wavelet domain. The size of the message that can be input if found and if the value of the Discrete Wavelet Transform (DWT) coefficient < Threshold, then store the...


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