March 2, 2021

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Blitz: Secure Multi-Hop Payments Without Two-Phase Commits, by Lukas Aumayr and Pedro Moreno-Sanchez and Aniket Kate and Matteo Maffei

Payment-channel networks (PCN) are the most prominent approach to tackle the scalability issues of current permissionless blockchains. A PCN reduces the load on-chain by allowing arbitrarily many off-chain multi-hop payments (MHP) between any two users connected through a path of payment channels. Unfortunately, current protocols for MHP are far from satisfactory. One round MHP (e.g., Interledger) are insecure as a malicious intermediary can steal the payment funds. Two-round MHP (e.g., Lightning Network (LN)) follow the 2-phase-commit paradigm as in databases to overcome this issue. However, when tied with economical incentives, 2-phase-commit brings other security threats (i.e., wormhole attacks), staggered collateral (i.e., funds are locked for a time proportional to the payment path length) and dependency on specific scripting language functionality (e.g., hash time lock contracts) that hinders a wider deployment in practice.

We present Blitz, a novel MHP protocol that demonstrates for the first time that we can achieve the best of the two worlds: a single round MHP where no malicious intermediary can steal coins. Moreover, Blitz provides privacy for sender and receiver, it is not prone to the wormhole attack and it requires only constant collateral. Additionally, we construct MHP using only digital signatures and a timelock functionality, both available at the core of virtually every cryptocurrency today. We provide the cryptographic details of Blitz and we formally prove its security. Furthermore, our experimental evaluation on an LN snapshot shows that the LN collateral results in between 4x and 33x more unsuccessful payments than the Blitz collateral (both compared to a baseline), reduce the size of the payment contract by 26% and prevent up to 0.3 BTC (3397 USD in October 2020) in fees being stolen over a three day period as it avoids wormhole attacks by design.