  We define and analyze three mechanisms for getting common knowledge, a
posteriori truths about the world onto a blockchain in a decentralized setting.
We show that, when a reasonable economic condition is met, these mechanisms are
individually rational, incentive compatible, and decide the true outcome of
valid oracle queries in both the non-cooperative and cooperative settings.
These mechanisms are based upon repeated games with two classes of players:
queriers who desire to get common knowledge truths onto the blockchain and a
pool of reporters who posses such common knowledge. Presented with a new oracle
query, reporters have an opportunity to report the truth in return for a fee
provided by the querier. During subsequent oracle queries, the querier has an
opportunity to punish any reporters who did not report truthfully during
previous rounds. While the set of reporters has the power to cause the oracle
to lie, they are incentivized not to do so.
