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Considering how important it is for everything from navigation to keeping clocks in sync, satellite navigation systems are surprisingly vulnerable to a variety of attacks, ranging from simple jamming to more sophisticated spoofing attacks. This may be changing, though, as Galileo, Europe’s GNSS, recently demonstrated its first cryptographically-secured position fix under spoofing conditions.
Most GNSS systems, including GPS, have no verification measures to keep an adversary from transmitting a false signal at a higher power and hijacking a receiver; since GNSS signals are extremely weak by the time they reach the ground, this presents no great difficulty to a moderately well-equipped attacker.
Galileo’s Signal Authentication System (SAS) aims to fix this. The Galileo ground station pre-selects signal spreading codes, which it then encrypts with a regularly-changing secret key and publishes. A receiver which anticipates needing a verified signal can then download these encrypted codes ahead of time and store them. Galileo satellites then transmit on the E6-C pilot signal, and the receiver records the signal. After transmitting a message block, it then transmits the decryption key on a separate signal, which the receiver uses to recover the spreading codes. The receiver then correlates these spreading codes with the recorded signal to find the satellite’s pseudorange.
It’s a rather complicated system, but it works: earlier this month in Andøya, Norway, the annual Jammertest GNSS testing event took place. For one week, a wide range of organizations tested the resilience of their GNSS systems against various attacks, including jamming, delayed retransmission, and spoofing. Using five Galileo satellites, the European Space Agency was able to obtain a stable lock on their receiver even during spoofing.
In principle, this method could be extended to other GNSS systems. There’s certainly motivation to do so; very large-scale attacks have been demonstrated recently.
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