Many approaches and techniques are available in the literature for information hiding. Figure 1 shows a generic embedding and extracting scheme. The inputs to the embedding scheme are the hiding message, the cover data and an optional public or secret key K. The output is stego data, also called stego object. Inputs to the generic extracting scheme are the tested data, the secret or public key, and the original cover data besides information about steganographic scheme used. The output is the extracted message.
Generic embedding and extracting scheme
Steganographic techniques can be divided into various categories. The basic and most common approach in partitioning of hiding techniques is in the spatial domain, frequency domain. Another way for categorization of steganographic methods is based on the condition whether or not they use the original data for extraction of hiding message from tested data. The third method is based on with or without encryption. Three types of steganography can be identified based on their difference in the nature and combination of inputs and outputs (Cox, Miller, Boom, & Fridrich, 2008)
Pure Steganography
We call a steganography system pure when it doesn't require prior exchange of some secret information before sending message. The pure steganography can be defined as the quadruple (C, M, D, and E) where:
C: the set of possible covers.
M: the set of secret massage with |C| ≥ |M|.
E: C×M→C the embedding function.
D: C→M of the extraction function with the property that
(E (c, m)) = m for all m ∈ M and c ∈C
Secret key steganography
We call a steganographic system a shared-secret or shared-key or secret when it requires prior exchange of data like shared keys. Here the sender chooses a cover and embeds the secret message into the cover using a secret key. If the secret key used in the embedding process is known to the receiver, he can reverse the process and extract the secret message. The secret key steganography can be defined as the quintuple (C, M, K, DK, EK) where:
M: the set of secret message.
K: the set of secret keys.
EK: C×M×K→C with the property that
DK (EK(c, m, k), k) = m for all m ∈M, c∈ C and k ∈ K
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.