  Micropayments are increasingly being adopted by a large number of
applications. However, processing micropayments individually can be expensive,
with transaction fees exceeding the payment value itself. By aggregating these
small transactions into a few larger ones, and using cryptocurrencies, today's
decentralized probabilistic micropayment schemes can reduce these fees.
Unfortunately, existing solutions force micropayments to be issued
sequentially, thus to support fast issuance rates a customer needs to create a
large number of escrows, which bloats the blockchain. Moreover, these schemes
incur a large computation and bandwidth overhead, which limit their
applicability in large-scale systems.
  In this paper, we propose MicroCash, the first decentralized probabilistic
framework that supports concurrent micropayments. MicroCash introduces a novel
escrow setup that enables a customer to concurrently issue payment tickets at a
fast rate using a single escrow. MicroCash is also cost effective because it
allows for ticket exchange using only one round of communication, and it
aggregates the micropayments using a lottery protocol that requires only secure
hashing. Our experiments show that MicroCash can process thousands of tickets
per second, which is around 1.7-4.2x times the rate of a state-of-the-art
sequential micropayment system. Moreover, MicroCash supports any ticket issue
rate over any period using only one escrow, while the sequential scheme would
need more than 1000 escrows per second to permit high rates. This enables our
system to further reduce transaction fees and data on the blockchain by around
50%.
