On 20 Feb 2020, at 06:20, Vianney Gomezgil Yaspik <vgomezg1@jhu.edu> wrote:

A group of students at Johns Hopkins University and I have been analyzing the circuit selection algorithm for TOR´s browser EXIT nodes. It is an exploratory project trying to discover how do the EXIT nodes are selected every time the circuit changes.

So far, we have discovered that the time that the browser is accessed, calendar date,

physical location, use of a bridge (or not), and the entry node do not change the pattern of the EXIT nodes.

The set of Tor exits changes over time, so the calendar

date will change Tor's path selection slightly.

Similarly, Tor clients try to avoid choosing paths that

are within the same network, or all controlled by the

same operator. So guard selection does have a slight

impact on the chosen exit.

For more details, see tor's path spec, particularly the

constraints section:

https://gitweb.torproject.org/torspec.git/tree/path-spec.txt#n230

Moreover, than approximately 80% of all exit nodes come from 10 specific countries, even though these 10 countries only account for approximately 50% of all available exit nodes.

How are you counting exit nodes?

Tor uses the bandwidth weights in the consensus, to

weight its random selection of exit nodes:

https://gitweb.torproject.org/torspec.git/tree/path-spec.txt#n92

These weights are limited by:

* any operator-configured bandwidth limits,

and scaled using:

* the relay's own observed bandwidth usage, and

* the capacities measured by the 6 Tor bandwidth

authorities.

All of which has led us to conclude that the selection of the EXIT nodes in the TOR browser is not random. We would like to further explore however, what are the factors that determine which relay becomes the exit node every time a circuit is changed. Is there anyone that we could speak to or that could give us further insight as to how the selection of the exit node in TOR´s circuit works?