1. Introduction and motivation
Tor uses entry guards to prevent an attacker who controls some
a fraction of the network from observing a fraction of every user's
traffic. If users chose their entries and exits uniformly at
random from the list of servers every time they build a circuit,
then an adversary who had (k/N) of the network would deanonymize
F=(k/N)^2 of all circuits... and after a given user had built C
circuits, the attacker would see them at least once with
probability 1-(1-F)^C. With large C, the attacker would get a
sample of every user's traffic with probability 1.
To prevent this from happening, Tor clients choose a small number
of guard nodes (currently 3). These guard nodes are the only
nodes that the client will connect to directly. If they are not
compromised, the user's paths are not compromised.
But attacks remain. Consider an attacker who can run a firewall
between a target user and the Tor network, and make many of the
guards they don't control appear to be unreachable. Or consider
an attacker who can identify a user's guards, and mount
denial-of-service attacks on them until the user picks a guard
that the attacker controls.
