  Payment channel networks (PCN) enable scalable blockchain transactions
without fundamentally changing the underlying distributed ledger algorithm.
However, routing a payment via multiple channels in a PCN requires locking
collateral for potentially long periods of time. Adversaries can abuse this
mechanism to conduct denial-of-service attacks. Previous work focused on source
routing, which is unlikely to remain a viable routing approach as these
networks grow.
  In this work, we examine the effectiveness of attacks in PCNs that use
routing algorithms based on local knowledge, where compromised intermediate
nodes can delay or drop transactions to create denial-of-service. We focus on
SpeedyMurmurs as a representative of such protocols. We identify two attacker
node selection strategies; one based on the position in the routing tree, and
the other on betweenness centrality. Our simulation-driven study shows that
while they are both effective, the centrality-based attack approaches
near-optimal effectiveness. We also show that the attacks are ineffective in
less centralized networks and discuss incentives for the participants in PCNs
to create less centralized topologies through the payment channels they
establish among themselves.
