Steganographic Data Embedding SECURITY Schemes Classification
The choice of embedding algorithm in the most cases is driven by the results of the steganographic channel robustness analysis . One of the areas that improve steganographic robustness is usage of a key scheme for embedding messages. Various key steganographic schemes have various levels of protection. Key scheme term means a procedure of how to use key steganographic system based on the extent of its use. However, when the steganographic robustness is increased a bandwidth of the whole embedding system is decreased. Therefore the task of a scheme selection for achieving the optimal values of the steganographic system is not trivial.
Embedding messages in steganographic system can be carried out without use of a key or with use of a key. To improve steganographic robustness key can be used as a verification option. It can make an impact on the distribution of bits of a message within a container, as well as an impact on the procedure of forming a sequence of embedded bits of a message.
The first level of protection is determined only by the choice of embedding algorithm. This may be the least significant bits modification algorithm, or algorithms for modifying the frequency or spatial-temporal characteristics of the container. The first level of protection is presented in any steganographic channel. Steganographic system in this case can be represented as shown below:
Figure Steganographic System - The First Protection Level Scheme
- c - is a container file;
- F - steganographic channel space (frequency or/and amplitude container part, that is available for steganographic modification and message signal transmission)
- SC - steganographic system;
- m - message to be embedded;
- E - embedding method;
- ĉ - modified container file.
The second protection level of the steganographic system, as well as all levels of protection of the higher orders, is characterized by the use of Key (password) via steganographic modification. An example of a simple key scheme, which provides a second level of protection, is to write the unmodified or modified password in the top or bottom of the message; or the distribution of the password sign on the entire length of the steganographic channel. Such key schemes do not affect the distribution of messages through the container and do not use a message preprocessing according to the defined key (see figure below):
Figure Steganographic System - The Second Protection Level Scheme
This kind of steganographic systems are used in such tasks as, for instance, adding a digital signature for proof of copyright. Data embedding performance is not changed in comparison with the fastest approach of the first protection level usage.
Steganographic data channels that use key schemes based distribution of a message through the container and or preprocessing of an embedded message for data hiding are more secure. When the third protection level key scheme is used it affects the distribution of a message through the container (see figure below). Accordingly, the performance of container processing will be lower than in the case of the first and the second key schemes.
Figure Steganographic System - The Third Protection Level Scheme
- F(P, L) – distribution function of a message within a container;
- P – minimum number of container samples that are needed to embed one message sample;
- L – step of a message distribution within a container.
Taking into account that P≥L, the simplest representation of the F(P, L) function could be as following:
F(P, L) = cycle*L + step*P,
where cycle is a number of the current L section and step is a number of the embedded message sample.
This could be visualized by the following figure:
Figure Distribution Function F(P, L) of a Message within a Container
The difference between the fourth protection level scheme and the third one is that in steganographic system there are two distribution functions of a message within a container are used. The first is responsible for a message samples selection according to some function G(Q, N), and the second function F(P, L) is responsible for position selection in a container for message sample hiding. Here:
- Q – the size of message block to be inserted;
- N – the size (in bits) of one sample of the message file.
Figure Steganographic System - The Fourth Protection Level Scheme
Data hidding process high level view could be visualized as following:
Figure Message Block Selection G(Q, N) and Distribution Function F(P, L) within a Container
Based on the above discussion it is possible to define a classification table of key steganographic schemes
Note: protection level schemes (key schemes) research has been published initially at 'Informatics' (scientific journal, Belarus) in the following issue
Cherniavsky A.F. Key-schemes steadfastness in steganographic information embedding / A.F. Cherniavsky, I.L. Chvarkova, V.S. Sadau // Informatics. – 2008. – № 2(18). – P. 119-128