  In this paper, we identify a new form of attack, called the Balance attack,
against proof-of-work blockchain systems. The novelty of this attack consists
of delaying network communications between multiple subgroups of nodes with
balanced mining power. Our theoretical analysis captures the precise tradeoff
between the network delay and the mining power of the attacker needed to double
spend in Ethereum with high probability.
  We quantify our probabilistic analysis with statistics taken from the R3
consortium, and show that a single machine needs 20 minutes to attack the
consortium. Finally, we run an Ethereum private chain in a distributed system
with similar settings as R3 to demonstrate the feasibility of the approach, and
discuss the application of the Balance attack to Bitcoin. Our results clearly
confirm that main proof-of-work blockchain protocols can be badly suited for
consortium blockchains.
