Key extraction via measuring a physical quantity is a class of information theoretic key exchange protocols that rely on the physical characteristics of the communication channel, to enable the computation of a shared key by two parties that share no prior secret information. The key is supposed to be information theoretically hidden to an eavesdropper. Despite the recent surge of research activity in the area, concrete claims about the security of the protocols typically rely on channel abstractions that are not fully experimentally substantiated. In this paper, we propose a novel methodology for the experimental security analysis of these protocols. The crux of our methodology is a falsifiable channel abstraction that is accompanied by an efficient experimental approximation algorithm of the conditional min-entropy available to the parties given the view of the eavesdropper. We focus on the signal strength between two wirelessly communicating transceivers as the measured quantity, and we use an experimental setup to compute the conditional min-entropy of the channel given the view of the attacker which we find to be linearly increasing. Armed with this understanding of the channel, we showcase the methodology by providing a general protocol for key extraction in this setting that is shown to be secure for a concrete parameter selection. In this way, we provide a comprehensively analyzed wireless key extraction protocol that is demonstrably secure against passive adversaries assuming our falsifiable channel abstraction. Our use of hidden Markov models as the channel model and a dynamic programming approach to approximate conditional min-entropy might be of independent interest, while other possible instantiations of our methodology can be feasible and may be motivated by this paper.

Tor is an anonymous communications network with thousands of router nodes worldwide. An intuition reflected in much of the literature on anonymous communications is that, as an anonymity network grows, it becomes more secure against a given observer because the observer will see less of the network. In particular, as the Tor network grows from volunteers operating relays all over the world, it becomes less and less likely for a single autonomous system (AS) to be able to observe both ends of an anonymous connection. Yet, as the network continues to grow significantly, no analysis has been done to determine if this intuition is correct. Further, modifications to Tor's path selection algorithm to help clients avoid an AS-level observer have not been proposed and analyzed.

In this paper we define a new metric for quantifying the degree of anonymity collectively afforded to users of an anonymous communication system. We show how our metric, based on the permanent of a matrix, can be useful in evaluating the amount of information needed by an observer to reveal the communication pattern as a whole. We also show how our model can be extended to include probabilistic information learned by an attacker about possible sender-recipient relationships. Our work is intended to serve as a complementary tool to existing information-theoretic metrics, which typically consider the anonymity of the system from the perspective of a single user or message.

Tor is a popular tool for online anonymity that currently does not have a standard graphical user interface. We present Vidalia, an open source, cross-platform GUI for Tor. We discuss some of the design decisions we have made in Vidalia, as well as what we have found users expect in a Tor GUI.