## Cryptology ePrint Archive: Report 2019/589

A${^2}$L: Anonymous Atomic Locks for Scalability and Interoperability in Payment Channel Hubs

Erkan Tairi and Pedro Moreno-Sanchez and Matteo Maffei

Abstract: The striking growth in cryptocurrencies is revealing several scalability issues that go beyond the growing size of the blockchain. Payment channel hubs (PCHs) constitute a promising scalability solution by performing off-chain payments between sender and receiver through an intermediary, called the tumbler. While currently proposed PCHs provide security and privacy guarantees against a malicious tumbler, they fall short of other fundamental properties, such as interoperability and fungibility.

In this work, we present A${^2}$L, the first secure, privacy-preserving, interoperable, and fungibility-preserving PCH. A${^2}$L builds on a novel cryptographic primitive that realizes a three-party protocol for conditional transactions, where the intermediary pays the receiver only if the latter solves a cryptographic challenge with the help of the sender. We prove the security and privacy guarantees of A${^2}$L in the Universal Composability framework and present two provably secure instantiations based on Schnorr and ECDSA signatures.

We implemented A${^2}$L and our evaluation shows that it outperforms TumbleBit, the state-of-the-art PCH in terms of interoperability, which is one of the central goals of this work. In particular, we show that in a commodity hardware as well as in a more realistic, distributed setting where sender, receiver and tumbler sit at different geographical locations worldwide, our ECDSA-based construction is 3x faster and requires 15x less bandwidth, while our Schnorr-based construction is 8x faster and requires 21x less bandwidth. These results demonstrate that A${^2}$L is the most efficient Bitcoin-compatible PCH.

Category / Keywords: applications / blockchain, mixing, bitcoin