In recent years, space communications have undergone a significant transformation, driven by the rapid growth of satellite constellations and the involvement of multiple independent operators. This increasing complexity calls for secure and interoperable solutions capable of enabling collaboration between networks that do not share mutual trust. This thesis proposes a decentralized protocol for securely relaying encrypted messages between satellite constellations operated by different entities. The system addresses the challenge of cross-network message delivery, where the source and destination are covered by different satellite constellations that do not have direct communication paths. To bridge this gap, the protocol relies on intermediate nodes that act as relays and are economically incentivized to forward messages in a verifiable and secure manner.