  We present Handel, a Byzantine-tolerant aggregation protocol that allows for
the quick aggregation of cryptographic signatures over a WAN. Handel has
polylogarithmic time, communication and processing complexity. We implemented
Handel as an open source Go library with a flexible design to support any
associative and commutative aggregation function. We tested Handel on 2000 AWS
instances running two nodes per instance and located in 10 AWS regions. The
4000 signatures are aggregated in less than 900 milliseconds with an average
per-node communication cost of 56KB.
