Spear - fast multi-path payment with redundancy.

In a payment network, like the Lightning Network, Alice can transfer a payment to Bob by splitting the payment into partial payments and transferring these partial payments through multiple paths. The transfer, however, delays if any of the partial payments fails or delays. To handle this, one can add redundant payment paths. The challenge in doing so is that Bob may now overdraw funds from the redundant paths. To address this, Bagaria, Neu, and Tse introduced Boomerang, a mechanism based on secret sharing and homomorphic one-way functions, which allows Alice to revert the transfer if Bob overdraws. In this work, we introduce Spear, a simple method with lower latency than Boomerang. In addition, Spear needs significantly less computation, and half the maximum locktime of Boomerang. Unlike Boomerang, Spear can be implemented using only a minor change to the Lightning Network. This minor change enables both Alice and Bob to have control over the release of partial payments. This prevents Bob from ever overdrawing. Another interesting feature of Spear is that it is more robust than Boomerang against malicious intermediate nodes who do not forward payments in an attempt to lock up funds. Finally, Spear trivially supports division of a payment into uneven partial payments. This gives Alice maximum flexibility in dividing her payment into partial payments.