  Architectures relying on continuous authentication require a secure way to
challenge the user's identity without trusting that the Continuous
Authentication Subsystem (CAS) has not been compromised, i.e., that the
response to the layer which manages service/application access is not fake. In
this paper, we introduce the CALIPER protocol, in which a separate Continuous
Access Verification Entity (CAVE) directly challenges the user's identity in a
continuous authentication regime. Instead of simply returning authentication
probabilities or confidence scores, CALIPER's CAS uses live hard and soft
biometric samples from the user to extract a cryptographic private key embedded
in a challenge posed by the CAVE. The CAS then uses this key to sign a response
to the CAVE. CALIPER supports multiple modalities, key lengths, and security
levels and can be applied in two scenarios: One where the CAS must authenticate
its user to a CAVE running on a remote server (device-server) for access to
remote application data, and another where the CAS must authenticate its user
to a locally running trusted computing module (TCM) for access to local
application data (device-TCM). We further demonstrate that CALIPER can leverage
device hardware resources to enable privacy and security even when the device's
kernel is compromised, and we show how this authentication protocol can even be
expanded to obfuscate direct kernel object manipulation (DKOM) malwares.
