Anonymity in wireless networks under capture or selective jamming: An admissible length study

Traffic analysis, where eavesdroppers retrieve networking information such as source-destination pairs and paths of data flow, severely compromises user privacy and can equip an adversary to launch more powerful network attacks. In wireless networks, such information can be easily obtained by using a radio receiver or merely an energy detector. Furthermore, active adversaries can intercept transmissions by capturing transmitted packets or time selectively jamming certain links thus distorting the timing of packet flows. By carefully selecting the packets to intercept, the adversary can use the distorted timing signature to improve his detection of the path of packet flows. Anonymous communication, where users exchange information without revealing the communicating parties is essential in any data network. On the Internet, anonymous communication is typically enabled using Chaum mixes-relay nodes or proxy servers which use cryptographic and batching strategies to mask source identities. In this work, a theoretical framework is proposed to investigate the ability of a mix to thwart timing analysis by active adversaries who can capture packets or selectively jam links. Specifically, using the detection time of an adversary as a metric, the maximum achievable anonymity of a mix is characterized as a function of its available memory and the number of packets Eve can capture. Although a closed form solution for the optimal admissible length is as yet intractable, upper and lower bounds are provided by studying an asymptotically optimal strategy and a clairvoyant adversary respectively.