TruSat: Building Cyber Trust in Collaborative Spacecraft Networks

To protect a network of collaborating space assets from cybersecurity threats, it is crucial to provide authenticity, integrity, and confidentiality, in line with mutually agreed security policies and regulations. This can only be ensured if the deployed key management schemes are based on shared credentials. The IETF and CCSDS recognize this as a hard problem due to delay, disruptions, and limited on-board resources. In terrestrial networks, this is solved by using Public Key Infrastructures (PKIs). However, PKIs typically rely on a chain-of-trust of authorities that is rooted in a single trusted entity. Considering the space sector includes national space agencies and a growing count of commercial players, establishing a single trusted entity for all parties seems impractical. Further, verifying a public key at the given point in time remotely, which is a key aspect in PKI, becomes non-trivial in space. Thus, enabling PKI for spacecraft networks remains an open problem. In this work, we present TruSat, an extension for traditional PKIs that enables their use in collaborative spacecraft networks. We focus on two key aspects. (1) We propose a decentralized certificate authority scheme, which allows mutually distrusting parties to establish a common certificate authority. We leverage a distributed consensus protocol to allow the involved parties to collaborate in a secure manner, while avoiding single-points-of-failure as well as tolerating malicious behavior among the parties (e.g., due to compromise). (2) We consider several certificate validation schemes based on recent advances in the field to enable lightweight and delay-tolerant certificate validation. We compare and discuss the properties of these schemes and also demonstrate this with a deployed prototype on ESA's OPS-SAT in-orbit laboratory.