  We present Colordag, a blockchain protocol where following the prescribed
strategy is, with high probability, a best response as long as all miners have
less than 1/2 of the mining power. We prove the correctness of Colordag even if
there is an extremely powerful adversary who knows future actions of the
scheduler: specifically, when agents will generate blocks and when messages
will arrive. The state-of-the-art protocol, Fruitchain, is an epsilon-Nash
equilibrium as long as all miners have less than 1/2 of the mining power.
However, there is a simple deviation that guarantees that deviators are never
worse off than they would be by following Fruitchain, and can sometimes do
better. Thus, agents are motivated to deviate. Colordag implements a solution
concept that we call epsilon-sure Nash equilibrium and does not suffer from
this problem. Because it is an epsilon-sure Nash equilibrium, Colordag is an
epsilon Nash equilibrium and with probability (1 - epsilon) is a best response.
