Being a critical infrastructure of our society, the security of low Earth orbit (LEO) satellite networks has a high stake. A satellite network may be subject to excessive attacks launched by strongly motivated adversaries, some of whom are state sponsored with potentially unlimited computing and intelligence resources thus seriously challenging existing security measures, quite often under the assumption that attackers are resource constrained. Assuming this updated threat model, this project will explore new counter-measure defense approaches. The novelties of proposed efforts are to take considerations of the intrinsic resources possessed by a LEO satellite system at its network layer, such as rich and dense satellite connectivity and high satellite mobility, to develop a suite of physical-constrained, rather than computation-constrained, security methods that can improve LEO satellite network’s resilience against attacks. The project's broader significance and importance are to directly address the security threats faced by LEO systems in order to protect the social and economic benefits of this critical infrastructure.<br/><br/>This project will fundamentally advance the state of art LEO satellite network security research from several aspects. In particular, by exploiting the global spatial distribution of LEO satellites and their dense connectivity and high mobility, this project will propose physical-constrained randomized multi-path-routing-based secret key generation methods that are immune to resource-unconstrained attacks. This project will also study cyberattacks on the availability of the LEO networks by considering stealthy, intermittent, and selective packet dropping attacks. A novel detection method will be developed to accurately identify those stealthy packet droppers under harsh inter-satellite communication environment caused by the high mobility. Evaluation of all proposed methods will be performed on a scale-down snapshot-oriented static testbed, complemented by computer simulations on satellite-orbiting-trace data of real LEO systems such as Starlink and OneWeb.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.