Steganography is the science or art of hiding the very existence of a message . Steganography hides the existance of message inside media which could be image, sound, video or binary excutable files.
Whereas encryption conceals your message by making it unreadable to the outsider, the aim of steganography is to hide the message being communicated.
You may have heard of invisible ink or of writing a letter with lemon juice. Those are types of stenography. An early example of it is a secret message, sent from captivity by Herodotus circa 440 BC. He shaved the head of his favourite slave, tattooed the text on his scalp, and waited for the slave’s hair to re-grow thus obscuring the message from guards. The same method was used by the German army as recently as in the early 20th century.

Digital Steganography

  1. Steganography in image
Think of all those pixels in an image and each pixel has three color numbers , there are zillions of numbers in an image. If you were to change a few of these color numbers the resulting picture would probably look a lot like the original image; in fact, most people probably couldn’t tell that you had changed the image at all.
Steganography works by changing a few pixel color values; we will use selected pixel values to represent characters instead of a color value. Ofcourse, the resulting image will still look mostly like the original except that a few tiny ”blips” might seem a little out of place if you look very closely. We can then send the image to a buddy and they can extract the message if they know which pixels to decode.
Most famous algorithm is using  changing LSB :

Note : pixel consist of 24 bit (3 bytes ) the first byte for blue , second for green and the third for red

The original image  
notice the LSB of each byte of image stream bytes.

The text which will be hide in the image
Hiding text into image

To extract text from image  
humans eyes may not notice the difference between two image

2.Steganography in sound

Three algorithms can be used to hide text inside sound files.
  • LSB  usually used is low bit encoding which is some what similar to LSB that is generally used Images.
        The problem with low bit encoding is that it is usually noticeable to  the human ear, so it is a rather risky method for someone to use if they are trying to   mask  information inside of   an audio file. 
  • Echo data hiding  Embeds data into a carrier signal by introducing an echo.
       - The data are hidden by varying three parameters of the echo: initial amplitude, decay rate, and offsetEchos when done well can actually improve the aural quality of the sound rather than distort it.
       - When the time delay between the original sound and the echo decreases, it becomes less detectable to the human ear.
        - The timing of the delay signifies whether the bit of digital information is a 1 or 0. Hence this delays throughout the audio file will combine together to form the secret message when decoded.
 Spread Spectrum  Spread spectrum encoding is the method of hiding a narrow-band signal in a large cover signal .
       human ears can only hear sounds of a particular range of frequency.
       This is achieved by removing the audio frequency that the human ear cannot pick up.
       Natural audio, however, records a much larger frequency, and removing the excess sounds does not significantly change the quality of the audio (to our ears).
    For example, the MP3 format.
                                Here’s a frequency diagram of an audio transcript  
 The same piece of audio, with a message hidden within the frequency                     

3.Steganography in video
When information is hidden inside video the program or person hiding the information will usually use the DCT (Discrete Cosine Transform) method.
DCT works by slightly changing the each of the images in the video, only so much though so it’s isn’t noticeable by the human eye.
Steganography in Videos is similar to that of Steganography in Images, apart from information is hidden in each frame of video.
When only a small amount of information is hidden inside of video it generally isn’t noticeable at all, however the more information that is hidden the more noticeable it will become.
Here Frame 1 and Frame2 are highly correlated , so we can hide information in one of them such as hiding information in image, so it will not be noticeable by human eyes.
4.Steganography in binary excutable

The steganographic software takes a message and an executable as input , and outputs a functionally identical executable that contains the steganographically embedded message.
Here the inherent redundancy in the machine instruction set
For example:  (e.g., the i386 processor family instruction set) is used to encode the message, as several instructions can be expressed in more than one way.
For example, adding the value 50 to register eax can be represented as either

"add %eax, $50"
                                     "sub %
eax, $-50"

Using these two alternate forms, we can 
encode one bit of information.

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