Technologies that promise users anonymity on the Web might not be all they're cracked up to be, according to a dissertation published recently by a doctoral candidate at the University of Cambridge's Computer Laboratory.
The paper, written by Stephen Murdoch, takes a close look at "covert channels" -- particularly anonymity systems designed to protect a user's identity while working the Web. Murdoch suggests ways in which it may be possible to glean valuable information about users simply by observing their behavior.
At 140 pages, the paper is not a quick read, but it does show how an attacker or a defender could apply heavy scrutiny to the actions of a single user and gain enough information to understand that user's intentions -- or perhaps even his identity.
In the paper, Murdoch uses the example of a bridge game to show how card players can sometimes identify cards by observing the behavior of other players in the game. This process may actually become easier when two partners collude, he observes.
This notion can be applied to TCP/IP environments, in which an attacker might deposit two bits of seemingly innocuous or unrelated bits of malware on a single computer to track behavior and pinpoint the user's purpose or identity, Murdoch observes.
The paper also demonstrates how such concepts of behavior monitoring can be applied to simple traffic analysis of an "anonymous" network such as Tor, allowing researchers to determine the identity of a user. Murdoch's conclusions on this aspect of surveillance are similar to those presented by a group of researchers at the Black Hat conference last August. (See New Threat: Network Eavesdropping.)
The methods used to reduce anonymity in Tor could also be used to break down and at least partially expose the identities of users who employ other methods of anonymization as well, Murdoch postulates.
The bottom line of the paper is that while anonymizing technologies might protect a user from casual scans or monitoring, they generally would not hold up against intense and careful scrutiny by a truly dedicated attacker, researcher, or law enforcement officer.
"[There is] a wealth of practical experience in covert channel discovery that can be applied to find and exploit weaknesses in real-world anonymity systems," the paper says.
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