  Space-air-ground integrated networks (SAGINs), which have emerged as an
expansion of terrestrial networks, provide flexible access, ubiquitous
coverage, high-capacity backhaul, and emergency/disaster recovery for mobile
users (MUs). While the massive benefits brought by SAGIN may improve the
quality of service, unauthorized access to SAGIN entities is potentially
dangerous. At present, conventional crypto-based authentication is facing
challenges, such as the inability to provide continuous and transparent
protection for MUs. In this article, we propose an AI-oriented two-phase
multi-factor authentication scheme (ATMAS) by introducing intelligence to
authentication. The satellite and network control center collaborate on
continuous authentication, while unique spatial-temporal features, including
service features and geographic features, are utilized to enhance the system
security. Our further security analysis and performance evaluations show that
ATMAS has proper security characteristics which can meet various security
requirements. Moreover, we shed light on lightweight and efficient
authentication mechanism design through a proper combination of
spatial-temporal factors.
