There is a difference between integrity and security.
This is an important slide from Boeing 2012. Note that while the current state of ADSB is version 2, and that is what the US is mandating for 2020, AUS is mandating Version 0 with no upgrade requirements.
What should also be noted is that a ADSB and UAT aircraft cannot 'see' each other. A dual mode ADSB/UAT system?