![](bg7.png)
channels via malicious routing (up to potentially to-
tal isolation from the victim’s neighbors) and to deny
service to a victim via malicious HTLC construction.
Tochner et al. (Tochner et al., 2019) propose a denial
of service attack by creating low-fee channels to other
nodes, which are then naturally used to route pay-
ments for fee-minimizing network participants and
then dropping the payment packets, therefore forcing
the sender to await the expiration of the already set-up
HTLCs. (Herrera-Joancomart
´
ı et al., 2019) provides a
closer look into the privacy-performance trade-off in-
herent in LN routing. The authors also propose an
attack to discover channel balances within the net-
work. (Wang et al., 2019) examines the LN routing
process in more detail and proposes a split routing
approach, dividing payments into large size and small
size transactions. The authors show that by routing
large payments dynamically to avoid superfluous fees
and by routing small payments via a lookup mecha-
nism to reduce excessive probing, the overall success
rate can be maintained while significantly reducing
performance overhead. (B
´
eres et al., 2019) makes a a
case for most LN transactions not being truly private,
since their analysis has found that most payments oc-
cur via single-hop paths. As a remediation, the au-
thors propose partial route obfuscation/extension by
adding multiple low-fee hops. Currently still work
in progress, (Antonopoulos et al., 2019) is very close
to (Antonopoulos, 2014) in its approach and already
provides some insights into second-layer payments,
invoices and payment channels in general. The Light-
ning Network uses the Sphinx protocol to implement
onion routing, as specified in (Lightning Network,
2019b). The version used in current Lightning ver-
sions is based on (Danezis and Goldberg, 2009) and
(Kate and Goldberg, 2010), the latter of which also
provides performance comparisons between compet-
ing protocols.
6 CONCLUSION
This paper has shown that off-chain routing mecha-
nisms may be exploited to infer confidential informa-
tion about the network state. In particular, consider-
ing the LN as a case study, we set up a local infras-
tructure and proposed two ways in which the current
implementation c-lightning can be exploited to gain
knowledge about distant channel balances and trans-
actions to unconnected nodes: By deliberately failing
payment attempts, we were able to deduce the exact
amount of (milli-)satoshis on a channel located two
hops away. Using this technique repeatedly, we were
able to determine whether a transaction occurred be-
tween this node and another over the monitored chan-
nel. By timing the messages related to HTLC con-
struction and termination, we were able to infer the
remaining distance of a forwarded packet accurately.
Our work raises several interesting research ques-
tions. In particular, it remains to fine-tune our attacks
and conduct more systematic experiments including
more natural/interconnected network topologies, also
on other off-chain networks. More generally, it will
be interesting to explore further attacks on the con-
fidentiality of off-chain networks exploiting the rout-
ing mechanism and investigate countermeasures. Fur-
thermore, our work raises the question whether such
vulnerabilities are an inherent price of efficient off-
chain routing or if there exist rigorous solutions.
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