The Galileo Open Service Navigation Message Authentication (OSNMA) is a key security feature in the Galileo system, designed to protect against unauthorized use or manipulation of GPS signals.

The system embeds data within the Galileo I/ NAV message transmitted over the E1-B open service signal, which is susceptible to interference and counterfeit signals.

The data is broadcast within the reserved bits embedded in the odd page parts of a nominal Galileo E1-B I/Nav message, consisting of an 8-bit Header and Root Key (HKROOT) and a 32-bit Message Authentication Code and Key (MACK).