  One of the key challenges to the IoT's success is how to secure and anonymize
billions of IoT transactions and devices per day, an issue that still lingers
despite significant research efforts over the last few years. On the other
hand, technologies based on blockchain algorithms are disrupting today's
cryptocurrency markets and showing tremendous potential, since they provide a
distributed transaction ledger that cannot be tampered with or controlled by a
single entity. Although the blockchain may present itself as a cure-all for the
IoT's security and privacy challenges, significant research efforts still need
to be put forth to adapt the computation-intensive blockchain algorithms to the
stringent energy and processing constraints of today's IoT devices. In this
paper, we provide an overview of existing literature on the topic of blockchain
for IoT, and present a roadmap of research challenges that will need to be
addressed to enable the usage of blockchain technologies in the IoT.
