  In multi-path routing schemes for payment-channel networks, Alice transfers
funds to Bob by splitting them into partial payments and routing them along
multiple paths. Undisclosed channel balances and mismatched transaction fees
cause delays and failures on some payment paths. For atomic transfer schemes,
these straggling paths stall the whole transfer. We show that the latency of
transfers reduces when redundant payment paths are added. This frees up
liquidity in payment channels and hence increases the throughput of the
network. We devise Boomerang, a generic technique to be used on top of
multi-path routing schemes to construct redundant payment paths free of
counterparty risk. In our experiments, applying Boomerang to a baseline routing
scheme leads to 40% latency reduction and 2x throughput increase. We build on
ideas from publicly verifiable secret sharing, such that Alice learns a secret
of Bob iff Bob overdraws funds from the redundant paths. Funds are forwarded
using Boomerang contracts, which allow Alice to revert the transfer iff she has
learned Bob's secret. We implement the Boomerang contract in Bitcoin Script.
