printer steganography for details. Using puzzles The art of concealing data in a puzzle can take advantage of the degrees of freedom in stating the puzzle, using the starting information to encode a key within the puzzle/puzzle image. For instance, steganography using sudoku puzzles has as many keys as there are possible solutions of a Sudoku puzzle, which is . Network In 1977, Kent concisely described the potential for covert channel signaling in general network communication protocols, even if the traffic is encrypted (in a footnote) in "Encryption-Based Protection for Interactive User/Computer Communication," Proceedings of the Fifth Data Communications Symposium, September 1977. In 1987, Girling first studied covert channels on a local area network (LAN), identified and realised three obvious covert channels (two storage channels and one timing channel), and his research paper entitled “Covert channels in LAN’s” published in IEEE Transactions on Software Engineering, vol. SE-13 of 2, in February 1987. In 1989, Wolf implemented covert channels in LAN protocols, e.g. using the reserved fields, pad fields, and undefined fields in the TCP/IP protocol. In 1997, Rowland used the IP identification field, the TCP initial sequence number and acknowledge sequence number fields in TCP/IP headers to build covert channels. In 2002, Kamran Ahsan made an excellent summary of research on network steganography. In 2005, Steven J. Murdoch and Stephen Lewis contributed a chapter entitled "Embedding Covert Channels into TCP/IP" in the "Information Hiding" book published by Springer. All information hiding techniques that may be used to exchange steganograms in telecommunication networks can be classified under the general term of network steganography. This nomenclature was originally introduced by Krzysztof Szczypiorski in 2003. Contrary to typical steganographic methods that use digital media (images, audio and video files) to hide data, network steganography uses communication protocols' control elements and their intrinsic functionality. As a result, such methods can be harder to detect and eliminate. Typical network steganography methods involve modification of the properties of a single network protocol. Such modification can be applied to the protocol data unit (PDU), to the time relations between the exchanged PDUs, or both (hybrid methods). Moreover, it is feasible to utilize the relation between two or more different network protocols to enable secret communication. These applications fall under the term inter-protocol steganography. Alternatively, multiple network protocols can be used simultaneously to transfer hidden information and so-called control protocols can be embedded into steganographic communications to extend