The FANS 1/A ADS-C connection has no such security. If a suitably equipped ground station, perhaps a research facility, knows the ACARS address and tail number of a particular aircraft, an ADS-C connection can be established from anywhere on the globe.
In the B747-400 implementation, the flight crew is only aware that the ADS-C application is active. In the B777 and later implementations, the flight crew is aware of the number of ADS-C connections established and the ICAO identification code of each ADS-C connected ATSU.