Covert channel
A covert channel is a way of passing information through something that was never meant to carry it, such as the timing of an action, the load on a machine, the name of a file folder or the order of a list. Butler Lampson described such paths in his 1973 paper on confinement. Because no file moves and nothing looks stolen, a covert channel can be hard to notice, and a channel nobody thought of is a channel nobody is watching.
