Abstract. Protocol steganography allows users who wish to communicate secretly to embed information within other messages and network control protocols used by common applications. This form of unobservable communication can be used as means to enhance privacy and anonymity as well as for many other purposes, ranging from entertainment to protected business communication or national defense. In this paper, we describe our approach to application-layer protocol steganography, describing how we can embed messages into a commonly used TCP/IP protocol. We also introduce the notions of syntax and semantics preservation, which ensure that messages after embedding still conform to the host protocol. Based on those concepts, we attempt to produce reasonably secure and robust stegosystems. To demonstrate the efficacy of our approach, we have implemented protocol steganography within the Secure Shell (SSH) protocol. Findings indicate that protocol steganographic system is reasonably secure if the statistical profile of the covermessages and the statistical profile of its traffic match their counterparts after embedding.