  Bitcoin mining is a wasteful and resource-intensive process. To add a block
of transactions to the blockchain, miners spend a considerable amount of
energy. The Bitcoin protocol, named 'proof of work' (PoW), resembles a lottery
and the underlying computational work is not useful otherwise. In this paper,
we describe a novel 'proof of useful work' (PoUW) protocol based on training a
machine learning model on the blockchain. Miners get a chance to create new
coins after performing honest ML training work. Clients submit tasks and pay
all training contributors. This is an extra incentive to participate in the
network because the system does not rely only on the lottery procedure. Using
our consensus protocol, interested parties can order, complete, and verify
useful work in a distributed environment. We outline mechanisms to reward
useful work and punish malicious actors. We aim to build better AI systems
using the security of the blockchain.
