<title>Overview of covert communications through networks</title>

Abstract

ABSTRACT Covert communications through digital networks are a small subset of covert channel communicationsthrough all other networks such as human contacts, postal, telephone, ham radio and other networks. Therepertoire of options available to anyone for covert communications through digital networks is limitedonly by imagination; covert communications are therefore inherently uncontrollable.Keywords: Covert communications, data hiding, anonymity, confidentiality, privacy, interception. 1. BACKGROUND Viewed as a potential pathway for covert communications, a network of computers is fundamentally nodifferent than any other network, be that a network of social or professional contacts, a postal network, atelephone network, an ATM cash-dispensing bank network, or any other.Covert communications can occur either through the commission of an act or through the omission of anexpected act; not saying “good night” when one usually says good night conveys the message “I am angrywith you” in an oblique way. The same goes for a multitude of time-honored social gestures such as thefirmness of a handshake, a lover’s longing look, pregnant pauses in speech, and the like. In a differentcontext, the popular literature is full of stories of individuals who communicate brief messages covertly bychalking marks on public road signs, of auction bidders who communicate their bid by scratching their ear,and of just about any profession by engaging in some pre-agreed signaling that is intended to be unnoticedby all other than the intended recipient of the message.Whereas it is easy to communicate pre-agreed messages, the process of covertly conveying extendedmessages requires a channel and a means of sending the extended message in a manner that will not alertanyone other than the intended recipient. This brings up the fundamental question that must be answeredspecifically for each requirement for covert communications, namely “hide exactly what and from whomand under what constraints and assumptions?”Covert channels have been extensively analyzed in the open literature; an excellent publication availableonline (www.radium.ncsc.mil/tpep/library/rainbow/NCSC-TG-030.html) is a 1993 unclassified publicationby NSA, “A Guide to Understanding Covert Channel Analysis of Trusted Systems”. A keyphrase searchfor “Covert Channels” or for “Covert Communications” on the Internet results in an extensive amount ofother papers on covert channels. The present paper will not repeat any of this extensive work; instead, thispaper will focus on the practical aspects of “so, now what?”1.1 Hide what and from whom?There are numerous elements of a communication that aspires to remain “covert”; depending on thespecifics of each situation, one may wish to hide one or more of these aspects. Each such aspect has its ownrepertoire of possible solutions. Does one want to hide:a) The content of the communication?