Steganography in Digital Media

The main goal of steganography is to communicate secret messages without making it apparent that a secret is being communicated. This can be achieved by hiding messages in ordinary-looking objects, which are then sent in an overt manner through some communication channel. In this chapter, we look at the individual elements that define steganographic communication.

Before Alice and Bob can start communicating secretly, they must agree on some basic communication protocol they will follow in the future. In particular, they need to select the type of cover objects they will use for sending secrets. Second, they need to design the message-hiding and message-extraction algorithms. For increased security, the prisoners should make both algorithms dependent on a secret key so that no one else besides them will be able to read their messages. Besides the type of covers and the inner workings of the steganographic algorithm, Eve's ability to detect that the prisoners are communicating secretly will also depend on the size of the messages that Alice and Bob will communicate. Finally, the prisoners will send their messages through a channel that is under the control of the warden, who may or may not interfere with the communication.

We recognize the following five basic elements of every steganographic channel (see Figure 4.1):