  Blockchain is rapidly emerging as an important class of network application,
with a unique set of trust, security and transparency properties. In a
blockchain system, participants record and update the `server-side' state of an
application as blocks of a replicated, immutable ledger using a consensus
protocol over the Internet. Mining blocks has become lucrative in recent years;
e.g., a miner receives over USD 200,000 per mined block in Bitcoin today. A key
factor affecting mining rewards, is the latency of broadcasting blocks over the
network. In this paper, we consider the problem of topology design for
optimizing mining rewards in a wide-area blockchain network that uses a
Proof-of-Work protocol for consensus. Contrary to general wisdom that a faster
network is always better for miners, we show a counter intuitive result where a
slower network is actually beneficial to some miners. This is because competing
miners must choose neighbors that not only decrease their own latency to
others, but also ensure that the latency between other miners do not decrease
because of itself. We formalize this problem, and provide both theoretical
analysis and experimental results to support our claim.
