Hide and code: Session anonymity in wireless line networks with coded packets

The broadcast nature of the communication channel enables a malicious eavesdropper to gain information about connectivity and active sessions in a multi-hop wireless network. This can be achieved simply by overhearing the transmitted signals over the ether and analyzing their timings. Focusing on techniques that can meet information-theoretic criteria for session anonymity under traffic analysis attacks, we rely on a judicious choice of transmission schedules to conceal multicast or bidirectional unicast sessions from a global eavesdropper at any given point in time. A systematic approach for constructing the aforementioned transmission schedules for arbitrary network topologies is derived from an equivalent coloring problem in an auxiliary conflict graph. Although this type of anonymity requires various nodes to send dummy transmissions to confuse the eavesdropper, our results show that the additional cost in terms of energy, delay and throughput can be alleviated using network coding. The key intuition is that dummy transmissions can be replaced by coded transmissions, which carry useful information. For the case of a line network with N nodes supporting coded flows, we derive closed-form expressions, which show that anonymity comes at no cost in terms of throughput if at least one of the destinations is two hops away. The average per packet delay is shown to increase by at most 50%.