We introduce a reciprocity protocol, an innovative approach to coordinating and sharing rewards in blockchains. Inherently decentralized and easy to implement, it puts emphasis on incentives rather than forcing specic sharing rules from the outset. Analyzing the non-cooperative game the protocol induces, we identify a robust, strict, and Pareto-dominant symmetric equilibrium. In it, even self-centered participants show extensive reciprocity to one another. Thus, despite a setting
that is generally unfavorable to reciprocal behavior, the protocol manages to build trust between the users by taking on a role akin to a social contract.