**2. Background**

Lampson was the first to introduce the idea of a covert channel [3]. Transferring data between two entities indirectly through a channel is called a covert channel. Before the client and server use the channel to transfer data, they must agree on a pre-agreement knowledge (e.g., shared memory, table). For example, a word containing "mm" means bit "0" other than this means bit "1." So, if the client wants to send "10" to the server indirectly, the client will send "secure communication" to the server. The attacker hardly breaks the covert channel and it is considered to be more secure if it is undetectable [3, 4].
