The correct answer is the second and fourth options.
The first one is non-issue because the circuit identifiers
are remapped to between each onion router and so there
is no way for the beginning and ends onion routers to
collude. The second statement is an issue. And Section
7 of TOR, the second generation onion router paper, the
authors discussed this issue in talking about end-to-end
timing attacks and introducing timing into messages. The
third item is not an issue. Different keys are used
between each router. So there is no way for Melissa’s
router to identify and associate a key with Alice.
This fourth item is an issue. To see this, let’s take this
example network between A and B and we’ve chosen a
circuit in red. An attacker can try introducing congestion
on each connection, for example, here and see if it
causes any disruption on the flow of packets into B. In
this case, it won't. And so the attackers learned this
connection is not involved. You can track here and
observe and he should see disruption of packets into B
and the attackers learned that these two onion routers
are on the circuit. You can continue on to do this on all
the connections and learn which onion routers are involved.
