  Verification of data generated by wearable sensors is increasingly becoming
of concern to health service providers and insurance companies. There is a need
for a verification framework that various authorities can request a
verification service for the local network data of a target IoT device. In this
paper, we leverage blockchain as a distributed platform to realize an on-demand
verification scheme. This allows authorities to automatically transact with
connected devices for witnessing services. A public request is made for witness
statements on the data of a target IoT that is transmitted on its local
network, and subsequently, devices (in close vicinity of the target IoT) offer
witnessing service.
  Our contributions are threefold: (1) We develop a system architecture based
on blockchain and smart contract that enables authorities to dynamically avail
a verification service for data of a subject device from a distributed set of
witnesses which are willing to provide (in a privacy-preserving manner) their
local wireless measurement in exchange of monetary return; (2) We then develop
a method to optimally select witnesses in such a way that the verification
error is minimized subject to monetary cost constraints; (3) Lastly, we
evaluate the efficacy of our scheme using real Wi-Fi session traces collected
from a five-storeyed building with more than thirty access points,
representative of a hospital. According to the current pricing schedule of the
Ethereum public blockchain, our scheme enables healthcare authorities to verify
data transmitted from a typical wearable device with the verification error of
the order 0.01% at cost of less than two dollars for one-hour witnessing
service.
