  The recent worldwide introduction of RemoteID (RID) regulations forces all
Unmanned Aircrafts (UAs), a.k.a. drones, to broadcast in plaintext on the
wireless channel their identity and real-time location, for accounting and
monitoring purposes. Although improving drones' monitoring and situational
awareness, the RID rule also generates significant privacy concerns for UAs'
operators, threatened by the ease of tracking of UAs and related
confidentiality and privacy concerns connected with the broadcasting of
plaintext identity information. In this paper, we propose $A^2RID$, a protocol
suite for anonymous direct authentication and remote identification of
heterogeneous commercial UAs. $A^2RID$ integrates and adapts protocols for
anonymous message signing to work in the UA domain, coping with the constraints
of commercial drones and the tight real-time requirements imposed by the RID
regulation. Overall, the protocols in the $A^2RID$ suite allow a UA
manufacturer to pick the configuration that best suits the capabilities and
constraints of the drone, i.e., either a processing-intensive but
memory-lightweight solution (namely, $CS-A^2RID$) or a computationally-friendly
but memory-hungry approach (namely, $DS-A^2RID$). Besides formally defining the
protocols and formally proving their security in our setting, we also implement
and test them on real heterogeneous hardware platforms, i.e., the Holybro X-500
and the ESPcopter, releasing open-source the produced code. For all the
protocols, we demonstrated experimentally the capability of generating
anonymous RemoteID messages well below the time bound of $1$ second required by
RID, while at the same time having quite a limited impact on the energy budget
of the drone.
