  Allowing a compromised device to receive privacy-sensitive sensor readings,
or to operate a safety-critical actuator, carries significant risk. Usually,
such risks are mitigated by validating the device's security state with remote
attestation, but current remote attestation protocols are not suitable when the
beneficiary of attestation, the relying party, is a constrained device such as
a small sensor or actuator. These devices typically lack the power and memory
to operate public-key cryptography needed by such protocols, and may only be
able to communicate with devices in their physical proximity, such as with the
controller whose security state they wish to evaluate. In this paper, we
present a remote platform attestation protocol suitable for relying parties
that are limited to symmetric-key cryptography and a single communication
channel. We show that our protocol, including the needed cryptography and
message processing, can be implemented with a code size of 6 KB and validate
its security via model checking with the ProVerif tool.
