Introduction to Steganography

Steganography

"Steganography is the art and science of writing hidden messages in such a way that no one apart from the intended recipient knows of the existence of the message i.e covered writing”[2]

Steganography dates back to the ages, the most famous example being Herodotus who in his Histories[4][5] tells how Histiaeus shaved the head of his most trusted slave and tattooed it with a message which disappeared once the hair grew back again. There are many accounts in history of such deeds relating to steganography.

By far the most popular method for linguistic steganography was the acrostic. Among the countless examples of steganographic usage over the ages, two important books were the Polygraphia and Steganographia by Trithemius. As can be expected steganography in text was not the only use, in the old days musical notes were also used to hide messages by famous musicians like J.S.Bach and John Wilkins. The Cardan Grille is another example of using hiding techniques.

Types of steganography

Pure Steganography
We call a steganographic system pure when it does not require prior exchange of data like shared-keys. The definition can be mathematically described[4] as

The set (C,M,D,E) where C is the set of covers, M the set of messages with |M| <= |C|, E the embedding function which maps E:C x M -> C and D is the extracting function which maps from D:C -> M and the property D(E(c,m))= m for all (m E M, c E C) is a pure steganographic system
Shared-secret Steganography
We call a steganographic system a shared-secret or shared-key or secret when it does not require prior exchange of data like shared-keys. The definition can be mathematically described[4] as

The set (C,M,S,Ds,Es) where C is the set of covers, M the set of messages with |M| <= |C|, e the embedding function which maps C x M -> C and S is the set of shared-secrets; Ds: C x S -> M and Es: C x M x S -> C and the property Ds( Es(c,m,s) , s) = m for all (c E C, m E M, s E S) holds is called a shared-secret steganographic system
Public-key Steganography
This kind of steganography does not rely on shared-key exchange. Instead it is based on the public-key cryptography principle in which there are two keys, one being the public key which can be usually obtained from a public database and the other a private key. Usually in this case the public key is used in the embedding process and the private key in the decoding process.

The typical steganographic system is usually a shared-secret system which looks something like this[4]:

Alice uses the a cover to hide the stego object with the help of a key generated from a random source and the result is Alice sending a letter to Bob which he then decodes using the same shared-key and extracts the message from the cover