  Blockchain (BC) systems are highly distributed peer-to-peer networks that
offer an alternative to centralized services and promise robustness to
coordinated attacks. However, the resilience and overall security of a BC
system rests heavily on the structural properties of its underlying
peer-to-peer overlay. Despite their success, BC overlay networks' critical
design aspects, connectivity properties and network-layer inter-dependencies
are still poorly understood. In this work, we set out to fill this gap and
study the most important overlay network structural properties and robustness
to targeted attacks of seven distinct BC networks. In particular, we probe and
crawl these BC networks every two hours to gather information about all their
available peers, over a duration of 28 days. We analyze 335 network snapshots
per BC network, for a total of 2345 snapshots. We construct, at frequent
intervals, connectivity graphs for each BC network, consisting of all potential
connections between peers. We analyze the structural graph properties of these
networks and compare them across the seven BC networks. We also study how these
properties associate with the resilience of each network to partitioning
attacks, i.e., when peers are selected, attacked and taken offline, using
different selection strategies driven by the aforementioned structural
properties. In fact, we show that by targeting fewer than 10 highly-connected
peers, major BCs such as Bitcoin can be partitioned into disjoint, i.e.,
disconnected, components. Finally, we uncover a hidden interconnection between
different BC networks, where certain peers participate in more than one BC
network, which has serious implications for the robustness of the overall BC
network ecosystem.
