  Improving transaction throughput is one of the main challenges in
decentralized payment systems. Attempts to improve transaction throughput in
cryptocurrencies are usually a trade-off between throughput and security or
introduce a central component.
  We propose Hydra, a decentralized protocol that improves transaction
throughput without the security trade-off and has no central component. Our
novel approach distributes blocks over multiple blockchains. Hydra makes a
trade-off between transaction throughput and finality, the time it takes to
stabilize the record of a transaction in the blockchain. We rigorously analyze
the double spend attack in a multiple-blockchain protocol. Our analysis shows
that the number of transactions per second can be increased significantly while
finality is within acceptable boundaries.
