BlindHub: Bitcoin-Compatible Privacy-Preserving Payment Channel Hubs Supporting Variable Amounts
Channel: ConsensusLab
Date: 14/06/2023

Xianrui Qin (The University of Hong Kong), Shimin Pan (The University of Hong Kong), Arash Mirzaei (Monash University), Zhimei Sui (Monash University), Oguzhan Ersoy (Radboud University and Delft University of Technology), Amin Sakzad (Monash University), Muhammed F. Esgin (Monash University and CSIRO's Data61), Joseph K. Liu (Monash University), Jiangshan Yu (Monash University), Tsz Hon Yuen (The University of Hong Kong)

Payment Channel Hubs (PCHs) address scalability issues in first-generation blockchains like Bitcoin by facilitating off-chain payments between a sender and receiver via an intermediary (tumbler). Privacy-preserving PCHs aim for relationship anonymity and value privacy, preventing the tumbler from identifying sender-receiver pairs and payment amounts. Existing Bitcoin-compatible PCHs with relationship anonymity support only fixed payment amounts, making them impractical for varying amounts. In this paper, we propose the first Bitcoin-compatible PCH, achieving relationship anonymity and supporting variable payment amounts. Our technical constructions include BlindChannel, a novel bi-directional payment channel protocol for privacy-preserving payments, and BlindHub, a three-party protocol for private conditional payments, where the tumbler cannot link sender and receiver while allowing variable amounts. We introduce two cryptographic primitives, Blind Adaptor Signature (BAS) and Flexible Blind Conditional (FBCS), as building blocks for BlindHub. BAS is an adaptor signature protocol based on a blind signature scheme, while FBCS enables atomic and privacy-preserving PCHs. We instantiate both BlindChannel and BlindHub protocols, presenting implementation results to demonstrate their practicality.

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