This chapter provides critical analysis of current state-of-the-art in steganography. First part of the this chapter provides the classification of steganography based on the underlying information hiding methodology used and covert-channel type, and desired features of the information hiding used for covert communication. This chapter also discusses various known steganalysis techniques developed to counteract the covert-communication and highlights limitations of existing steganographic techniques. Performance analysis of commonly used shareware/freeware steganographic tools and steganalysis tools is also provided in this chapter. Some open problems in covert-communication are also discussed.
TopIntroduction
The explosive growth of the Web, digital content generation, deployment of ultra high speed networks, and development of P2P technologies to the exchange information has created both tremendous opportunities and new security threats. Advances in digital content generation, manipulation, and distribution technologies have completely changed the way we used to sharing information, do business, market new products, make social networks, etc. Today, marketing a new product, sending pictures, sharing videos etc. is just a matter of few mouse-clicks, which was impossible few years ago. At the same time, however, advances in these technologies have also made covert-communication drag-and-drop easy, which poses new security threats; digital steganography is one of these treats.
Steganography deals with hiding information into a cover (host or original) signal such that no one other than the intended recipient can detect or extract the hidden message. The steganographic encoder embeds a message into the cover-signal using a secret key such that perceptual and other distortion constraints are satisfied. A statistical dissimilarity measure between the cover and the stego-signal is generally used to measure the security of a given steganographic method (Cachin, 1998; Zollner et al., 1998; Chandramouli & Memon, 2003).
Steganography can be modeled as a prisoner’s problem (Simmons, 1984). For example, consider two prisoners, Alice and Bob, who want to secretly exchange information regarding their escape plan. However, the warden, Wendy, examines every communication between Alice and Bob, and punishes them if steganographic covert communication is detected. In a standard steganographic framework, Alice sends a secrete message, M, to Bob by embedding her secret message into the cover-signal, S, to obtain the stego-signal, X. Alice then sends X to Bob using a public channel. The warden, who examines the communication channel between Alice and Bob, can be passive or active. A passive warden attempts only to detect a steganographic covert channel. An active warden, on the other hand, deliberately alters every signal exchanged between Alice and Bob, to foil any covert communication between them. The allowable distortion the warden can introduce in the stego-signal depends on the underlying model and the cover-signal used. Figure 1 illustrates secret key steganography for active and passive warden scenarios.
Figure 1. Secret key steganography in the presence of a passive warden (top) and an active warden (bottom)
Clearly, Alice and Bob attempt to design the steganographic channel (encoder, secret key, and decoder) such that the warden is unable to distinguish in any sense (statistically as well as perceptually) between the cover-signal and the stego-signal. On the other hand, Wendy tries to detect or estimating the hidden message, M, from the stego-signal, X, by using one or several steganalysis algorithms. In general, steganalysis must not make any assumption about the underlying steganographic algorithm used to embed a secret message (Chandramouli, 2002).
TopThere are number of ways to hide information in digital content, ranging from simple random bit replacement to sophisticated one based on content adaptation. All of them offer some stealth, but not all of them are equally strong. Exiting existing information hiding techniques exploit randomness, uncertainty, or “holes” in the cover (or host) file for information hiding. Existing information hiding techniques for steganographic applications can be classified into the following categories: