Zero-fee commitments for mobile wallets

The ongoing development to incorporate zero-fee commitments into lightning channels aims to enhance the functionality and security of mobile wallets in the cryptocurrency sphere. This initiative is paving the way for a more streamlined approach, especially tailored to the unique requirements and threat models associated with mobile wallet usage. Mobile wallets, by their nature, do not relay payments and thus have a smaller attack surface compared to routing nodes. However, they face specific challenges, notably in ensuring the safety of funds without relying on on-chain utxos for force-close transactions.

A critical aspect of safeguarding mobile wallet funds involves addressing the risk posed by revoked commitments. The zero-fee commitment scheme facilitates immediate spending of channel outputs to cover on-chain fees, thereby allowing for the publication of penalty transactions if a peer broadcasts a revoked commitment. This mechanism does not necessitate any modifications from the current BOLTs standards, maintaining simplicity in its implementation.

The handling of Hashed Time-Locked Contracts (HTLCs) underlines the nuanced security considerations for mobile wallets. For received HTLCs, the mobile wallet, being the final recipient, must ensure the timely broadcast and confirmation of commitment and HTLC-success transactions to prevent fund theft by unresponsive or malicious peers. Conversely, when sending HTLCs, mobile wallets bear no risk of fund loss upon successful payment, as they require the preimage from the peer to prove payment. Here, an interesting dynamic emerges where mobile wallets, unlike in the receiving scenario, are not pressured to rapidly claim HTLCs upon timeout, providing flexibility in dealing with uncooperative peers.

Furthermore, the proposal introduces a solution for additional signatures on HTLC transactions, addressing scenarios where peers may be uncooperative. By having peers sign two versions of HTLC transactions—one with no fee for standard operation and another at a higher feerate for use in unilateral close situations—mobile wallets gain the ability to force-close channels independently when facing unresponsive peers. This approach is particularly useful for managing HTLCs but does not extend to situations where there are no pending HTLCs, a limitation acknowledged within the proposed framework.

This proposal represents a significant step towards enhancing the resilience and user experience of mobile wallets within the Lightning Network ecosystem. It simplifies the operational requirements for dealing with various adversarial conditions, potentially increasing the adoption and utility of mobile wallets for cryptocurrency transactions. Feedback from the community is sought to refine this approach further, with the ultimate goal of formalizing these enhancements in a bLIP once a consensus is reached.