algorithm to crack steganography

what is the basic idea behind steganography?ie ,how do you get开发者_JAVA技巧 the hidden information? suppose if it is an image and some text is within that image...how do you get that text?..

Every stenography algorithm is different in that respect. Every algorithm hides the information differently and thus getting the information back is different.
A simple example goes like this - Each pixel of the image is composed of 3 bytes, one for red, green and blue. Most people can't detect a difference of one bit in the color in an image so one option is to use the least significant bit of each color channel for your data. This way you can store 3 bits of information in every pixel with very little effect on the general quality of the image.
To get the information back you'll need to read the first bit of every color channel of every pixel and gather all the bits together.

This is just a very simple and almost trivial way to do stenography. Real stenography algorithms are somewhat more involved. Like in cryptography, there is no way to generically "unhide" all stenography. you need to know which algorithm you're trying to decode.

The very basic idea is that images contain tons of redundant information that your eye cannot see. For instance if you changed the last bit on each pixel there would be no visible change as almost all of the information about the color is the other bits. So you can encode messages using the last bit (the most basic algorithm). The histogram however will be changed and a large message will easily be detectable. As far a decoding the message itself, well, the message itself is probably using public key encryption so you will never know what the actual payload was.

Steganography unlike cryptography is considered broken if Eve (who is eavesdropping and practising steganalysis) knows that there is a message at all. The assumptions are based on that Alice and Bob are being watched and any communication is sign that they are up to something (aka prisoners, restrictive governments, all governments in the future hehe ;-))

And of course the algorithms become much more complex that just flipping the last bits, but encoding data that will not affect the structure of image (and become vulnerable to statistical attacks.) :

I read this book last summer and I thought is was an excellent introduction (it has a lot a psuedocode of the algorithms used) http://www.amazon.com/Steganography-Digital-Media-Principles-Applications/dp/0521190193

Steganography, coming from greek Steganos(i'm greek :P) is the art of hiding messages. While cryptography is about scrambling a message, steganography is about a person not being able to locate the message.

There are many tools that do this procedure for you. Writing a tool like that can be a complicated procedure i think, though i haven't tried to do that. You would need to create a sophisticated approach of correctly using unused or seemingly not important image pixels or data, in order to add your own message, file etc. For some more information, please take a look at : http://www.symantec.com/connect/articles/steganography-revealed