  Micropayment channels are the most prominent solution to the limitation on
transaction throughput in current blockchain systems. However, in practice
channels are risky because participants have to be online constantly to avoid
fraud, and inefficient because participants have to open multiple channels and
lock funds in them. To address the security issue, we propose a novel mechanism
that involves watchtowers incentivized to watch the channels and reveal a
fraud. Our protocol does not require participants to be online constantly
watching the blockchain. The protocol is secure, incentive compatible and
lightweight in communication. Furthermore, we present an adaptation of our
protocol implementable on the Lightning protocol. Towards efficiency, we
examine specific topological structures in the blockchain transaction graph and
generalize the construction of channels to enable topologies better suited to
specific real-world needs. In these cases, our construction reduces the
required amount of signatures for a transaction and the total amount of locked
funds in the system.
