Flexichain: Flexible Payment Channel Network to Defend Against Channel Exhaustion Attack

Payment Channel Network (PCN) is an effective off-chain scaling solution widely recognized for reducing operational costs on permissionless blockchains. However, it still faces challenges such as lack of flexibility, channel exhaustion, and poor sustainability. Currently, a separate deposit is required for each payment channel, which locks a significant amount of coins for a longer period. This restricts the ability to move locked coins across their channels off-chain. Additionally, unbalanced (unidirectional) transfers can lead to channel exhaustion, rendering the channel unsustainable. To address these issues, we propose a novel off-chain protocol called Flexible Payment Channel Networks (Flexichain). Unlike existing approaches, Flexichain allows users to deposit coins per user rather than per channel. This provides flexibility to move coins freely between channels without relying on the blockchain or disrupting the off-chain cycle. Flexichain is proven to be secure under the Universal Composability framework and resistant against channel exhaustion attacks. To assess the performance of Flexichain, we conduct experiments on both on-chain and off-chain operations using snapshots of the Lightning Network (LN). We evaluated the on-chain gas costs, success ratio and success amount of off-chain payments under uniform and skewed payment demands, as well as the computational and communication overheads of the off-chain contracts.