Hidden channels

The notion of "hidden channel" is very important in terms of security. A channel is a place through which information passes. A telephone line or a network protocol are classic examples of communication media. When it's hidden, it means that it runs counter to the normal operation expected in the channel environment. In Alexandre Dumas' "The Count of Monte Cristo", Edmond Dantès finds himself incarcerated in the Château d'If in solitary confinement. Hearing another prisoner digging, he in turn sets out to meet the unfortunate man. Edmond Dantès meets Abbé Faria, who will reveal his secret to him, enabling him to escape before becoming the Count of Monte Cristo. The success of this venture hinges on the secret link between the two men: a tunnel dug between their cells, which the jailers discover too late.

In this article, we explain the context and consequences of a hidden channel. In the first part, we explain what is meant by "information flow", a term that is central to understanding the issues surrounding covert channels. Containment and isolation are intuitively familiar concepts, but we clarify them in an IT context. We then show where to find hidden channels, and some possible attacks on information systems linked to the existence of these channels. Finally, we present a few examples of channels in the specific environment of a network.