Covert channel is a concealed communication path which is against organizational security policy. It often appears when two or more users are sharing files. Covert channel and legitimate channel attacks are often mixed up. For example, Steganography, the name of combination of methods which are used to hide the existence of data and messages, can be viewed as misuse of legitimate channel and can be protected by high secure operating system. But covert channel can not be avoided completely because it is a system feature , but the explicit design and analysis strategies can be used to weaken it. The concealment system which is similar to Steganography is to hide the information to some uncorrelated data in order to implement confidentiality. These two can be used to implement the covert channel, but they are more concentrated on the data and messages rather than the communication channel perspective. Tunneling can skip over the function of system and the access control mechanisms when accessing to the bottom facilities directly, but a covert channel can be hidden from access control mechanisms in a high secure OS , because it doesn’t use the legitimate commands in data transformation. Any two IPS protocol layers are often related to tunneling. For example, a TCP connection can be taken above SMTP as a covert channel to keep away from access control mechanisms when the TCP layer is applied by a security gateway below SMTP . Storage channel is one of the two kinds of covert channels which is used to communication by changing the stored entity. It keeps the confidentiality between two system entities’ communication by writing and reading process to the storage location.
Pseudo-anonymous remailer, also called pseudonymous remailer, is to hide the original mail address of sender by giving him a pseudonym address. Compared with anonymizer viewed as a proxy sever computer between user and internet which can protect user’s personal information by concealing information of client computer and the onion routing which is to provide confidentiality and anonymity for the source of packets, pseudonymous remailer is more like implementing the function of alias to replace the entity’s real name in order to conceal or masquerade. They all appear in communication process, act as intermediaries and can make the activities untraceable, but pseudonymous remailer is concentrated on implement in mail system with the anonymity of user address and received headers . Unlike anonymous remailer, such as cypherpunk anonymous remailers, it assigns a mapping between alias and real user name, and there are instructions in the database about the transformation. Anonymizer cares about the privacy and anonymity for client’s information and behaviors, such as IP, so that anyone including the server can not gather client’s information. Onion routing mainly makes messages encrypted repeatedly rather than use alias in anonymity. Messages are encapsulated with encrypted layered packets. When being sent through multiple onion routers, each onion router will decrypt one layer to get instructions and send messages to next router. In this way, any router could not know where the messages come from and go to, and the content. Anonymity in communication is showed in these three methods, all of them are aimed at keeping the confidentiality and avoiding traceable actions in communication process.
 “Glossary of Computer Security Terms”, NCSC-TG-004, version 1, 21 October 1988.
 Suranga Manage, ”iNet Guide”, email@example.com, Jan 02, 1999