In information theory, a covert channel is a communications channel that does a writing-between-the-lines form of communication.

Typically a covert channel is parasitic to its host channel; it reduces bandwidth of the host channel by reducing the signal-to-noise ratio in the host channel.

Observers are unaware that a covert message is being communicated. Only the sender and recipient of the message notice it. For example, in steganography hidden messages are encoded within pictures or other data in such a way that the picture does not appear to be altered. To an outside observer the picture would appear innocuous, but the recipient is able to extract the message from within the image.

A covert channel could be defined as a communications channel that transfers some kind of information using a method originally not intended to transfer this kind of information.

The term is used in the TCSEC specifically to refer to ways of transferring information from a higher classification compartment to a lower classification. There are two kinds of covert channels: storage channels, which communicate by modifying a stored object; and timing channels, which transmit information by affecting the relative timing of events.

External links

• Timing Channels (http://www.multicians.org/timing-chn.html) an early exploitation of a timing channel in Multics.