Lampson [8, p. 614] informally specified a special type of communication being:
Covert channels, i.e. those not intended for information transfer at all, such as the service program's effect on the system load.
A more general definition can be found in [14, p. 110].
Covert channels often involve what is called timing channels and storage channels. An example of a timing channel is the start-time of a process. The modulation of disc space is an example of a storage channel. Methods that describe how covert channels can be fought can, e.g., be found in [9]. For more information about covert channels, see [1].
Simmons [10] introduced the research on covert channels to the cryptographic community by introducing a special type of channel, which he called a subliminal channel. (Simmons did not regard these channels as fitting the definition of a covert channel.) He observed that covert data could be hidden within the authenticator of an authentication code [10]. The capacity of this channel.
