BIP-352: Limiting the number of per-group recipients (K_max)

In recent developments regarding the Silent Payments module for libsecp256k1, it has been identified that the scanning method proposed in BIP-352 exhibits significant performance issues, particularly with transactions designed to be adversarial. This problem is notably prevalent when the transaction involves a large number of recipients sharing the same scan public key. To address this inefficiency, a new proposal suggests the implementation of a "K_max" protocol limit. This change aims to restrict the maximum number of per-group recipients in a single transaction. Although this adjustment could be seen as a departure from existing protocols, it is anticipated that current Silent Payment (SP) wallets would remain unaffected, assuming the K_max value is set at a sufficiently high threshold, such as the placeholder value of 1000.

The proposal for this protocol modification has been documented in a draft for a Bitcoin Improvement Proposal (BIP), which outlines the rationale and specifics behind the suggested change. The draft can be reviewed here. Furthermore, the discussion around this proposal continues to evolve, with ongoing dialogues accessible for review and contribution on GitHub at this issue.

This initiative has been communicated through various channels to ensure broad community engagement and feedback. In addition to direct notifications sent to SP wallet developers, the information has been shared across developer mailing lists to capture insights and concerns from a wider audience. Those interested in contributing to the conversation or voicing potential implications for existing or future wallet implementations are encouraged to participate in the discussion through the provided GitHub issue link. This outreach aims to consolidate diverse perspectives and expertise to refine and validate the proposed protocol enhancement effectively.