  Miners play a key role in cryptocurrencies such as Bitcoin: they invest
substantial computational resources in processing transactions and minting new
currency units. It is well known that an attacker controlling more than half of
the network's mining power could manipulate the state of the system at will.
While the influence of large mining pools appears evenly split, the actual
distribution of mining power within these pools and their economic
relationships with other actors remain undisclosed. To this end, we conduct the
first in-depth analysis of mining reward distribution within three of the four
largest Bitcoin mining pools and examine their cross-pool economic
relationships. Our results suggest that individual miners are simultaneously
operating across all three pools and that in each analyzed pool a small number
of actors (<= 20) receives over 50% of all BTC payouts. While the extent of an
operator's control over the resources of a mining pool remains an open debate,
our findings are in line with previous research, pointing out centralization
tendencies in large mining pools and cryptocurrencies in general.
