We introduce a reciprocity protocol, an innovative approach to coordinating and sharing rewards in blockchains. Inherently decentralized and implementable, it puts emphasis on incentives rather than forcing specific 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 systemic reciprocity. Thus, despite a setting that is generally unfavorable to reciprocal behavior, the protocol enables users to build trust between themselves by taking on a role akin to a social contract.