  Due to the rapid development of Internet of Things (IoT), a massive number of
devices are connected to the Internet. For these distributed devices in IoT
networks, how to ensure their security and privacy becomes a significant
challenge. The blockchain technology provides a promising solution to protect
the data integrity, provenance, privacy, and consistency for IoT networks. In
blockchains, communication is a prerequisite for participants, which are
distributed in the system, to reach consensus. However, in IoT networks, most
of the devices communicate through wireless links, which are not always
reliable. Hence, the communication reliability of IoT devices influences the
system security. In this article, we rethink the roles of communication and
computing in blockchains by accounting for communication reliability. We
analyze the tradeoff between communication reliability and computing power in
blockchain security, and present a lower bound to the computing power that is
needed to conduct an attack with a given communication reliability. Simulation
results show that adversarial nodes can succeed in tampering a block with less
computing power by hindering the propagation of blocks from other nodes.
