  While the large-scale Internet of Things (IoT) makes many new applications
feasible, like Smart Cities, IoT also brings new concerns on data reliability,
security, and privacy. The rapid evolution in blockchain technologies, which
relied on a decentralized, immutable and distributed ledger system for
transaction data auditing, provides a prospective solution to address the
issues in IoT. The blockchain and smart contract enabled security mechanism for
IoT applications have attracted increasing interests from both academia and
industry. However, integrating cryptocurrency-oriented blockchain technologies
into IoT systems meets tremendous challenges on scalability, storage capacity,
security, and privacy. Particularly, the performance of blockchain networks
significantly relies on the performance of consensus mechanisms, e.g., in terms
of data confidentiality, transaction throughput, and network scalability. In
this chapter, given an in-depth review of state-of-the-art blockchain networks,
the key matrix of designing consensus mechanism for IoT networks are identified
in terms of throughput, scalability, and security. To demonstrate a case study
on designing scalable, lightweight blockchain protocols for IoT systems, a
Microchain framework is introduced and a proof-of-concept prototype is
implemented in a physical network environment. The experimental results verify
the feasibility of integrating the Microchain into IoT systems.
