Relay Selection and Operation Control for Optimal Delay Anonymity Tradeoff in Anonymous Networks

Anonymous systems aim to protect users from revealing to an external unauthorized entity their identities and their network activities. Despite using layered encryption to protect the metadata in the packets, these systems are still vulnerable to timing analysis, wherein an eavesdropper can use traffic correlation to match a source stream with the corresponding destination. In this paper, an analytical framework is presented to integrate and control the link-padding mechanisms in the functioning of anonymous relays in these systems such that a desired degree of anonymity is achieved from timing analysis without adding significant latency. In particular, the optimal choices of relays and the degree of link padding are investigated to characterize the best tradeoff between anonymity from timing analysis, as measured by Shannon entropy of source destination pairs, and the average latency. The optimization required for the best tradeoff is shown to require exponential complexity, and a suboptimal algorithm is presented that is shown mathematically and numerically to perform close to the optimal, but only requires linear complexity. In addition, an incremental optimization is presented for a new user to be added optimally to an existing system without altering the prevalent routing scheme.