  In the age of Big Data, enabling task scheduling while protecting users'
privacy is critical for various decentralized applications in blockchain-based
smart contract platforms. Such a privacy-preserving task scheduler requires the
task input data to be secretly maintained until a prescribed task execution
time and be automatically recorded into the blockchain to enabling the
execution of the task at the execution time, even if the user goes offline.
While straight-forward centralized approaches provide a basic solution to the
problem, unfortunately they are limited to a single point of trust and involve
a single point of control. This paper presents decentralized techniques for
supporting privacy-preserving task scheduling using smart contracts in Ethereum
blockchain networks. We design a privacy-preserving task scheduling protocol
that is managed by a manager smart contract. The protocol requires a user to
schedule a task by deploying a proxy smart contract maintaining the
non-sensitive information of the task while creating decentralized secret trust
and selecting trustees from the network to maintain the sensitive information
of the task. With security techniques including secret sharing and layered
encryption as well as security deposit paid by trustees as economic deterrence,
the protocol can protect the sensitive information against possible attacks
including some trustees destroying the sensitive information (drop attack) or
secretly releasing the sensitive information before the execution time
(release-ahead attack). We demonstrate the attack-resilience of the proposed
protocol through rigorous analysis.Our implementation and experimental
evaluation on the Ethereum official test network demonstrate the low monetary
cost and the low time overhead associated with the proposed approach.
