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Issue 1.4

COLUMN

There's a file in my PICT!

Issue: 1.4 (February/March 2003)
Author: Didier Barbas
Article Description: No description available.
Article Length (in bytes): 12,546
Starting Page Number: 46
Article Number: 1425
Resource File(s):

Download Icon 1425.zip Updated: 2013-03-10 14:58:51

Related Link(s): None

Excerpt of article text...

We will today play with the concept of steganography. Steganography simply takes one piece of information (a file, text, anything) and hides it within another file. Computer files, especially images, contain unused or insignificant areas of data. Steganography takes advantage of these areas, replacing them with information (encrypted mail, for instance). The files can then be exchanged without anyone knowing what really lies inside of them. An image of the Namsan Tower in Seoul could contain the recipe of Rex Stout's 'Veal birds in Casserole' or my company's latest P+L (there actually is a relationship between my company and steganography: our annual meeting and the Fifth International Workshop on Information Hiding were both held in the same hotel, but, fortunately, not at the same time...). Steganography can also be used to place a hidden 'trademark' in images, music, and software; a technique known as watermarking. Users of Photoshop sometimes use such a function to assert their intellectual property rights on a graphic artwork.

The principle, basically, is to put to good use the insignificant data of a file (meaning that even if you modify this data, there won't be any appreciable difference to the casual, or not so casual, viewer) so you can store additional pieces of information. The simplest application of this principle is to use the least significant bit (hereafter LSB) of bitmap pictures' RGB color information to hold the information you want to hide. We will learn how to set up a basic steganographic engine based on PICT images.

Indeed, modifying the least significant bits of RGB colors in PICTs doesn't really affect the visual aspect of pictures, at least not to the human eye. It is considered a safe place to store data. True, a bit at a time is not much, but since PICT files are much bigger than their compressed colleagues, they are usually quite roomy, even at a 1/4 ratio.

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