The spread of wide-scale Internet surveillance has spurred interest in anonymity systems that protect users’ privacy by restricting unauthorised access to their identity. This requirement can be considered as a flow control policy in the well established field of multilevel secure systems. I apply previous research on covert channels (unintended means to communicate in violation of a security policy) to analyse several anonymity systems in an innovative way.

One application for anonymity systems is to prevent collusion in competitions. I show how covert channels may be exploited to violate these protections and construct defences against such attacks, drawing from previous covert channel research and collusion-resistant voting systems.

In the military context, for which multilevel secure systems were designed, covert channels are increasingly eliminated by physical separation of interconnected single-role computers. Prior work on the remaining network covert channels has been solely based on protocol specifications. I examine some protocol implementations and show how the use of several covert channels can be detected and how channels can be modified to resist detection.

At cl.cam.ac.uk

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