  This paper proposes SilentDelivery, a secure, scalable and cost-efficient
protocol for implementing timed information delivery service in a decentralized
blockchain network. SilentDelivery employs a novel combination of threshold
secret sharing and decentralized smart contracts. The protocol maintains shares
of the decryption key of the private information of an information sender using
a group of mailmen recruited in a blockchain network before the specified
future time-frame and restores the information to the information recipient at
the required time-frame. To tackle the key challenges that limit the security
and scalability of the protocol, SilentDelivery incorporates two novel
countermeasure strategies. The first strategy, namely silent recruitment,
enables a mailman to get recruited by a sender silently without the knowledge
of any third party. The second strategy, namely dual-mode execution, makes the
protocol run in a lightweight mode by default, where the cost of running smart
contracts is significantly reduced. We rigorously analyze the security of
SilentDelivery and implement the protocol over the Ethereum official test
network. The results demonstrate that SilentDelivery is more secure and
scalable compared to the state of the art and reduces the cost of running smart
contracts by 85%.
