  Root of Trust Identification (RTI) refers to determining whether a given
security service or task is being performed by the particular root of trust
(e.g., a TEE) within a specific physical device. Despite its importance, this
problem has been mostly overlooked. We formalize the RTI problem and argue that
security of RTI protocols is especially challenging due to local adversaries,
cuckoo adversaries, and the combination thereof. To cope with this problem we
propose a simple and effective protocol based on biometrics. Unlike
biometric-based user authentication, our approach is not concerned with
verifying user identity, and requires neither pre-enrollment nor persistent
storage for biometric templates. Instead, it takes advantage of the difficulty
of cloning a biometric in real-time to securely identify the root of trust of a
given physical device, by using the biometric as a challenge. Security of the
proposed protocol is analyzed in the combined Local and Cuckoo adversarial
model. Also, a prototype implementation is used to demonstrate the protocol's
feasibility and practicality. We further propose a Proxy RTI protocol, wherein
a previously identified RoT assists a remote verifier in identifying new RoTs.
