Intuition tells us that steganographic capacity should perhaps be defined as the largest payload that Alice can embed in her cover image using a specific embedding method without introducing artifacts detectable by Eve. After all, knowledge of this secure payload appears to be fundamental for the prisoners to maintain the security of communication. Unfortunately, determining the secure payload for digital images is very difficult even for the simplest steganographic methods, such as LSB embedding. The reason is the lack of accurate statistical models for real images. Moreover, it is even a valid question whether capacity can be meaningfully defined for an individual image and a specific steganographic method. Indeed, capacity of noisy communication channels depends only on the channel and not on any specific communication scheme.

This chapter has two sections, each devoted to a different capacity concept. In Section 13.1, we study the steganographic capacity of perfectly secure stegosystems. Here, we are interested in the maximal relative payload (or rate) that can be securely embedded in the limit as the number of pixels in the image approaches infinity. Capacity defined in this way is a function of only the physical communication channel and the cover source rather than the steganographic scheme itself. It is the maximal relative payload that Alice can communicate if she uses the best possible stegosystem. The significant advantage of this definition is that we can leverage upon powerful tools and constructions previously developed for study of robust watermarking systems.